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Semi-Conservative DNA Replication: Meselson and Stahl

This structure has novel features which are of considerable biological interest . . . It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material . —Watson & Crick (1953)

Perhaps the most significant aspect of Watson and Crick's discovery of DNA structure was not that it provided scientists with a three-dimensional model of this molecule , but rather that this structure seemed to reveal the way in which DNA was replicated. As noted in their 1953 paper, Watson and Crick strongly suspected that the specific base pairings within the DNA double helix existed in order to ensure a controlled system of DNA replication . However, it took several years of subsequent study, including a classic 1958 experiment by American geneticists Matthew Meselson and Franklin Stahl, before the exact relationship between DNA structure and replication was understood.

Three Proposed Models for DNA Replication

Replication is the process by which a cell copies its DNA prior to division. In humans, for example, each parent cell must copy its entire six billion base pairs of DNA before undergoing mitosis . The molecular details of DNA replication are described elsewhere, and they were not known until some time after Watson and Crick's discovery. In fact, before such details could be determined, scientists were faced with a more fundamental research concern. Specifically, they wanted to know the overall nature of the process by which DNA replication occurs.

Defining the Models

Semiconservative replication was not the only model of DNA replication proposed during the mid-1950s, however. In fact, two other prominent hypotheses were put also forth: conservative replication and dispersive replication. According to the conservative replication model, the entire original DNA double helix serves as a template for a new double helix, such that each round of cell division produces one daughter cell with a completely new DNA double helix and another daughter cell with a completely intact old (or original) DNA double helix. On the other hand, in the dispersive replication model, the original DNA double helix breaks apart into fragments, and each fragment then serves as a template for a new DNA fragment. As a result, every cell division produces two cells with varying amounts of old and new DNA (Figure 1).

Making Predictions Based on the Models

Meselson and stahl’s elegant experiment.

The duo thus began their experiment by choosing two isotopes of nitrogen—the common and lighter 14 N, and the rare and heavier 15 N (so-called "heavy" nitrogen)—as their labels and a technique known as cesium chloride (CsCl) equilibrium density gradient centrifugation as their sedimentation method. Meselson and Stahl opted for nitrogen because it is an essential chemical component of DNA; therefore, every time a cell divides and its DNA replicates, it incorporates new N atoms into the DNA of either one or both of its two daughter cells, depending on which model was correct. "If several different density species of DNA are present," they predicted, "each will form a band at the position where the density of the CsCl solution is equal to the buoyant density of that species. In this way, DNA labeled with heavy nitrogen ( 15 N) may be resolved from unlabeled DNA" (Meselson & Stahl, 1958).

The scientists then continued their experiment by growing a culture of E. coli bacteria in a medium that had the heavier 15 N (in the form of 15 N-labeled ammonium chloride) as its only source of nitrogen. In fact, they did this for 14 bacterial generations, which was long enough to create a population of bacterial cells that contained only the heavier isotope (all the original 14 N-containing cells had died by then). Next, they changed the medium to one containing only 14 N-labeled ammonium salts as the sole nitrogen source. So, from that point onward, every new strand of DNA would be built with 14 N rather than 15 N.

Just prior to the addition of 14 N and periodically thereafter, as the bacterial cells grew and replicated, Meselson and Stahl sampled DNA for use in equilibrium density gradient centrifugation to determine how much 15 N (from the original or old DNA) versus 14 N (from the new DNA) was present. For the centrifugation procedure, they mixed the DNA samples with a solution of cesium chloride and then centrifuged the samples for enough time to allow the heavier 15 N and lighter 14 N DNA to migrate to different positions in the centrifuge tube.

By way of centrifugation, the scientists found that DNA composed entirely of 15 N-labeled DNA (i.e., DNA collected just prior to changing the culture from one containing only 15 N to one containing only 14 N) formed a single distinct band, because both of its strands were made entirely in the "heavy" nitrogen medium. Following a single round of replication, the DNA again formed a single distinct band, but the band was located in a different position along the centrifugation gradient. Specifically, it was found midway between where all the 15 N and all the 14 N DNA would have migrated—in other words, halfway between "heavy" and "light" (Figure 2). Based on these findings, the scientists were immediately able to exclude the conservative model of replication as a possibility. After all, if DNA replicated conservatively, there should have been two distinct bands after a single round of replication; half of the new DNA would have migrated to the same position as it did before the culture was transferred to the 14 N-containing medium (i.e., to the "heavy" position), and only the other half would have migrated to the new position (i.e., to the "light" position). That left the scientists with only two options: either DNA replicated semiconservatively, as Watson and Crick had predicted, or it replicated dispersively.

Straight or Circular?

Following publication of Meselson and Stahl's results, many scientists confirmed that semiconservative replication was the rule, not just in E. coli , but in every other species studied as well. To date, no one has found any evidence for either conservative or dispersive DNA replication. Scientists have found, however, that semiconservative replication can occur in different ways—for example, it may proceed in either a circular or a linear fashion, depending on chromosome shape.

In fact, in the early 1960s, English molecular biologist John Cairns performed another remarkably elegant experiment to demonstrate that E. coli and other bacteria with circular chromosomes undergo what he termed " theta replication ," because the structure generated resembles the Greek letter theta (Θ). Specifically, Cairns grew E. coli bacteria in the presence of radioactive nucleotides such that, after replication, each new DNA molecule had one radioactive (hot) strand and one nonradioactive strand. He then isolated the newly replicated DNA and used it to produce an electron micrograph image of the Θ-shaped replication process (Figure 3; Cairns, 1961).

References and Recommended Reading

Cairns, J. The bacterial chromosome and its manner of replication as seen by autoradiography. Journal of Molecular Biology 6 , 208–213 (1961)

Meselson, M., & Stahl, F. The replication of DNA in Escherichia coli . Proceedings of the National Academy of Sciences 44 , 671–682 (1958)

Watson, J. D., & Crick, F. H. C. A structure for deoxyribose nucleic acid. Nature 171 , 737–738 (1953) ( link to article ).

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Dna Replication Experiment

DNA Replication and Meselson And Stahl's Experiment

Literally, replication means the process of duplication. In molecular biology, DNA replication is the primary stage of inheritance. Central dogma explains how the DNA makes its own copies through DNA replication, which then codes for the RNA in transcription and further, RNA codes for the proteins by the translation.

Let’s go through Meselson and Stahl Experiment and DNA replication.

Meselson and Stahl Experiment

Meselson and Stahl Experiment was an experimental proof for semiconservative DNA replication. In 1958, Matthew Meselson and Franklin Stahl conducted an experiment on E.coli which divides in 20 minutes, to study the replication of DNA.

Semi conservative DNA Replication through Meselson and Stahl’s Experiment

15 N (heavy) and 14 N (normal) are two isotopes of nitrogen, which can be distinguished based on their densities by centrifugation in Ca,esium chloride (CsCl). Meselson and Stahl cultured E.coli in a medium constituting 15 NH 4 Cl over many generations. As a result, 15 N was integrated into the bacterial DNA. Later, they revised the 15 NH 4 Cl medium to normal 14 NH 4 Cl. At a regular interval of time, they took the sample and checked for the density of DNA.

Observation

Sample no. 1 (after 20 minutes): The sample had bacterial DNA with an intermediate density. Sample no. 2 (after 40 minutes): The sample contained DNA with both intermediate and light densities in the same proportion.

Based on observations and experimental results, Meselson and Stahl concluded that DNA molecules can replicate semi-conservatively. Investigation of semi-conservative nature of replication of DNA or the copying of the cells , DNA didn’t end there. Followed by Meselson and Stahl experiment, Taylor and colleagues conducted another experiment on Vicia faba (fava beans) which again proved that replication of DNA is semi-conservative.

Frequently Asked Questions

Which mode of replication did the messelson and stahl’s experiment support.

Messelson and Stahl’s experiment supported the semi-conservative mode of replication. The DNA was first replicated in 14N medium which produced a band of 14N and 15N hybrid DNA. This eliminated the conservative mode of replication.

What are the different modes of replication of DNA?

The different modes of replication of DNA are:

Semiconservative
Conservative

How are semi-conservative and conservative modes of replication different?

Semi-conservative mode of replication produces two copies, each containing one original strand and one new strand. On the contrary, conservative replication produces two new strands and would leave two original template DNA strands in a double helix.

What is the result of DNA replication?

The result of DNA replication is one original strand and one new strand of nucleotides.

What happens if DNA replication goes wrong?

If DNA replication goes wrong, a mutation occurs. However, if any mismatch happens, it can be corrected during proofreading by DNA Polymerase.

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How DNA Replicates

Matthew Meselson Franklin W. Stahl

Matthew Meselson had a passion for physics and chemistry throughout his early life, often conducting science experiments in his family's garage. At the age of 16, he enrolled at the University of Chicago, beginning an academic career that led to doctoral studies at the California Institute of Technology under Linus Pauling. In addition to his widely known work demonstrating semi-conservative replication of DNA with Frank Stahl, Meselson has made many key discoveries in the molecular biology. He is also known for his work in limiting the proliferation of chemical and biological weapons. Meselson is a member of the National Academy of Science and a recipient of the Lasker Award. He continues to serve as a member of the faculty at Harvard University, where he has taught and conducted research since 1960.

Following a sheltered life in a Boston suburb (Needham), Frank stumbled his way through college (Harvard, 1951) before fleeing to a graduate school in biology (U Rochester) to avoid the military draft. While in the graduate school, Frank took a course taught by A. H. (Gus) Doermann, and, for the first time in his life, he had a goal. With Gus, he studied genetic recombination in phage. To meet a departmental requirement, Frank took a summer course at Woods Hole, where he met Matt Meselson and began the work described in this Key Experiment. In 1959, Frank joined the faculty at the University of Oregon, Eugene, in their new Institute of Molecular Biology. He has been there ever since. Frank is now an emeritus faculty member who still enjoys teaching as well as family life and the natural wonders of Oregon.

What's the Big Deal?

Some experiments have proven so influential that they have been christened with the names of the scientists who performed them. The "Meselson–Stahl experiment" is one of those. It has also been called "the most beautiful experiment in biology," a title that has seemed to stick over the years. Why was the Meselson and Stahl experiment so important? Their experiment provided the first critical test of the Watson–Crick models for the structure of DNA and its replication, which were not universally accepted at the time. The convincing results of the Meselson–Stahl experiment, however, dispelled all doubts. DNA was no longer just an imaginary model; it was a real molecule, and its replication could be followed in the form of visually compelling bands in an ultracentrifuge. Meselson and Stahl found that these DNA bands behaved in the ultracentrifuge exactly as Watson and Crick postulated they should. Why was the Meselson–Stahl experiment "beautiful"? Because it was conceptually simple and yet sufficiently powerful to differentiate between several competing hypotheses for how DNA might replicate. Taken together, the Watson–Crick model and the Meselson–Stahl experiment marked the transition to the modern era of molecular biology, a turning point as impactful as the theory of evolution. The story of the Meselson–Stahl experiment, as told here by its protagonists, also reveals how friendship and overcoming obstacles are as crucial to the scientific process as ideas themselves.

Learning Overview —

Big concepts.

Faithful replication of the genetic material (DNA) is the foundation of all life on earth. The experiment by Meselson and Stahl established that DNA replicates through a semi-conservative mechanism, as predicted by Watson and Crick, in which each strand of the double helix acts as a template for a new strand with which it remains associated, until the next replication.

Bio-Dictionary Terms Used

Bacteriophage (phage) , base , base pairing , chromosome , DNA , Hershey–Chase experiment , eukaryote , mutation , nucleotides , Prokaryote (bacteria) , recombination , RNA , ultraviolet light

Terms and Concepts Explained

Equilibrium density-gradient centrifugation , DNA replication , isotope , semi-conservative DNA replication

Introduction

Matthew Meselson and Franklin Stahl (both 24 years old) met at the Marine Biological Laboratory in Woods Hole in Massachusetts and decided to test the Watson–Crick model for DNA replication, which was unproven at the time.

What Events Preceded the Experiment?

Watson and Crick proposed a "Semi-Conservative" model for DNA replication in 1953, which derived from their model of the DNA double helix. In this proposal, the strands of the duplex separate and each strand serves as a template for the synthesis of a new complementary strand. Watson's and Crick's idea for DNA replication was a model, and they did not have data to support it. Some prominent scientists had doubts.

Two other models, "Conservative" and "Dispersive", for DNA replication were proposed.

Setting Up the Experiment

A method was needed to detect a difference between the parental and daughter (newly replicated) DNA strands. Then, one could follow the parent DNA molecule in the progeny. Meselson thought to distinguish between parental and newly synthesized DNA using a density difference in the building blocks (nucleotides) used to construct the DNA. The three models for DNA replication would predict different outcomes for the density of the replicated DNA in the first- and second-generation daughter cells.

The general experimental idea was first to grow bacteria in a chemical medium to make high-density DNA and then abruptly shift the bacteria to a low-density medium so that the bacteria would now synthesize lower density DNA during upcoming rounds of replication. The old and newly synthesized DNA would be distinguished by their density.

To measure a density difference in the DNA, Meselson and Stahl invented a method called equilibrium density gradient centrifugation. In this method, the DNA is centrifuged in a tube with a solution of cesium chloride. When centrifuged, the cesium chloride, being denser than water, forms a density gradient, reaching a stable equilibrium after a few hours. The DNA migrates to a point in the gradient where its density matches the density of the CsCl solution. Heavy and light DNA would come to different resting points and thus physically separated.

Doing the Key Experiment

Meselson and Stahl first decided to study the replication of DNA from a bacteriophage, a virus that replicates inside of bacteria, and used a density difference between two forms of the nucleobase thymine (normal thymine and 5-bromouracil). These experiments did not work.

The investigators changed their plans. They studied replication of the bacterial genome and used two isotopes of nitrogen (15N (heavy) and 14N (light)) to mark the parental and newly synthesized DNA.

When the population of bacteria doubled, Meselson and Stahl noted that the DNA was of an intermediate density, half-way between the dense and light DNA in the gradient. After two doublings, half of the DNA was fully light and the other half was of intermediate density. These results were predicted by the Semi-Conservative Model and are inconsistent with the Conservative and Dispersive Models.

Meselson and Stahl did another experiment in which they used heat to separate the two strands of the daughter DNA after one round of replication. They found that one strand was all heavy DNA and the other all light. This result was consistent with the Semi-Conservative model and provided additional evidence against the Dispersive Model.

Overall, the results provided proof of Semi-Conservative replication, consistent with the model proposed by Watson and Crick.

What Happened Next?

Within a couple of weeks after their key experiment, Meselson wrote a letter to Jim Watson to share news of their result (letter included).

Max Delbruck, the Caltech physicist and biologist who had proposed the dispersive model, was elated by the results, even though Meselson and Stahl disproved his replication hypothesis, and urged the young scientists to write up their results for publication and announce the important result to the world (1958).

Scientists now know a great deal about the protein machinery responsible for DNA replication.

Closing Thoughts

The Meselson–Stahl experiment had a powerful psychological effect on the field of genetics and molecular biology. It was the first experimental test of the Watson and Crick model, and the results clearly showed that DNA was behaving in cells exactly as Watson and Crick predicted.

In addition to having a good idea, the behind-the-scenes tour of the Meselson–Stahl experiment reveals that friendship and persistence in overcoming initial failures play important roles in the scientific discovery process. Also important was an atmosphere of freedom that allowed Meselson and Stahl, then very junior, to pursue their own ideas.

Guided Paper

Meselson, M. and Stahl, F.W. (1958). The replication of DNA in Escherichia coli. Proceedings of the National Academy of Sciences U.S.A., 44: 672–682.

This classic paper provides experimental evidence that the strands of the DNA double helix serve as templates to create a new copy of DNA. These results provide experimental evidence of The Watson and Crick model of DNA replication (‘semi-conservative replication’) demonstrating that genetic information is passed from one cell or organism to its progeny.

The first conversation between Matt Meselson and Frank Stahl, in the summer of 1954, began a collaboration that led to their Key Experiment on DNA replication and marked the beginning of a lifelong friendship. Matt and Frank describe the circumstances that brought them together below.

It was 1954, the year after Jim Watson and Francis Crick published their two great papers describing their double helical model of DNA and its implications for how it might replicate, mutate, and carry genetic information. Jim Watson (26 years old) and I (a 24-year-old first-year graduate student) were both at Caltech and living at the Athenaeum, the Caltech faculty club. We often talked while waiting for dinner. One day, Jim asked me to join him for the summer as his teaching assistant in the Physiology Course at the Marine Biological Laboratory (MBL) at Woods Hole, Massachusetts. He mainly wanted me to do experiments to see if RNA was a double helix. (As a side note, those experiments showed that RNA is not a double helix.) So in June 1954, I drove my 1941 Chevrolet coupe across the country from Cal Tech in Pasadena, California, to the MBL at Woods Hole, Massachusetts.

One day in Woods Hole, while planning student assignments for the Physiology Course, Jim went to the window of the course office upstairs in the MBL Lillie building and pointed towards a student sitting on the grass under a tree serving gin and tonics. That student was Frank Stahl. Jim said let's give him a really hard experiment to do all by himself – the Hershey–Chase Experiment; then, we'll see how good he really is. Two years earlier, Alfred Hershey and Martha Chase published an influential experiment that provided evidence implicating DNA, and not protein, as the substance conferring genetic inheritance in bacteriophage (see the whiteboard animation video on the Hershey–Chase experiment ).

I thought that this guy serving gin and tonics must be an interesting fellow, so I went downstairs to meet him and let him know what was being planned for him. Frank was then a biology graduate student at the University of Rochester. I sat down on the grass under the tree, and we hit it off right away. We found that we had much to discuss. I was very impressed with Frank's knowledge of phage genetics, a subject I knew nothing about, coming from the laboratory of Linus Pauling where I was doing X-ray crystallography. Frank can tell you more about our conversation under the "gin and tonic tree."

1954 was an exciting time for molecular biology. One year earlier, Watson and Crick published their model for the double helix structure of DNA (see the Narrative on DNA Structure by Vale ), which aroused much excitement as well as some serious disbelief. With Watson as an instructor and Crick and Sydney Brenner as visitors and several other founders of molecular biology, Woods Hole that summer became an epicenter for discussion of the great questions in molecular biology. Could the double helix model, as Watson and Crick proposed, explain the replication of the genetic information? What is the "code" for reading out the nucleotide sequence of DNA and turning that into the sequence of amino acids in protein? Would RNA have a similar structure to DNA? However, at the time of my arrival, I had no idea that Woods Hole was hosting anyone, but me, interested in the big problems of modern, i.e., molecular, biology.

I was a 24-year-old biology graduate student at the University of Rochester and had come to the MBL to take the Physiology Course. To beat the heat and, perchance, to meet someone interesting, I invested in a bottle of gin and a 6-pack of tonic, found some ice, a thermos jug, and a tree and sat myself down in the shade. Matt Meselson was one of my first catches.

Our conversation under the "gin and tonic tree" was life-changing. After Matt warned me of Watson's planned test of my experimental aptitude, we talked about the work we were doing. I explained to him the problem in phage genetics on which I was working. Eventually our conversation turned to DNA replication. Neither of us was working on the problem at the time, although we were both keenly interested in it. It was perhaps the most important contemporary question in molecular biology.

At Caltech, Matt had already come up with an idea for how the mechanism of DNA replication might be studied by density labeling. But, as a physical chemist, he was unfamiliar with the methods of phage and bacterial biology that would be needed to conduct the actual experiments. So we decided to collaborate. I was planning to be a postdoctoral fellow at Caltech starting that September. If we could develop a method for measuring the density of DNA molecules and successfully apply it to the problem of DNA replication, we could establish whether the Watson–Crick model for DNA replication, and even the model of the structure itself, was correct or not.

We did not begin our collaboration immediately because Matt needed to finish his X-ray crystallography and I had made plans to do my postdoctoral work on bacterial genetics with Joe Bertani.

You can also hear Matt Meselson describing the Meselson–Stahl experiment in Video 1 .

The genetic material of eukaryotic cells is organized in the form of chromosomes , each a single linear, double-stranded DNA molecule ( Figure 1 ). Most prokaryotes (bacteria) have a single, circular chromosome ( Figure 1 ). All forms of life must replicate their DNA and, except for recombination and infrequent mutations, pass identical copies of their genetic material to their progeny. (See also the Whiteboard Video on Keeping Track of Your DNA .)

Based upon their model for the structure of DNA, Watson and Crick proposed that DNA replicates in a "Semi-Conservative" manner ( Figure 2 ). In this model, the two strands of the DNA double helix unwind and separate, and each "parent" strand serves as a template for the synthesis of a new "daughter" strand. The Watson–Crick base pairing (see the Narrative on DNA Structure by Vale ) of adenine with thymine and guanine with cytosine ensures that, except for rare copying errors, mutations, the information of the original double-stranded DNA molecule is preserved during the synthesis of the daughter strands. In the Semi-Conservative model, each daughter cell in the first generation would inherit one of the original DNA strands from the parent and a recently synthesized DNA strand. In the second generation, two of the granddaughters would be composed of all newly synthesized DNA and two granddaughters would have hybrid DNA (one parental strand and one newly synthesized strand).

While (spoiler alert) the Semi-Conservative Model turned out to be correct, it was far from a foregone conclusion. Before our experiment, several leading scientists questioned the Semi-Conservative Model (see Dig Deeper 1 for more information on their reservations) and proposed alternate models discussed below.

Explorer's Question: What do you imagine are the pros and cons of this model?

Answer: The beauty of this model is that it provides a clear explanation of how a daughter strand is made from the template strand ( Figure 3 ). However, the model requires that the two long parental DNA strands unwind to become single-strand templates. This was seen as a weakness by many scientists at the time (see Dig Deeper 1 for more information).

The unwinding of the DNA helix and keeping the daughter and parental strands from becoming tangled posed problems for the Semi-Conservative model in the minds of many scientists. As a result, other models for DNA replication were imagined. One was a "Conservative Replication" model ( Figure 4 ). In this model, the parental double helix forms a template for a completely new double helix. The two original strands remain together, no unwinding occurs, and the daughter DNA is formed from newly synthesized DNA. In this model, in the first generation, one daughter DNA would inherit the original DNA double helix from the parent DNA and the other daughter DNA would inherit the newly synthesized DNA double helix. In the second generation, one of the four granddaughters would have the original parental DNA and the other three granddaughters would all be composed of newly synthesized DNA. While Conservative Replication was a logical possibility, it was not elaborated by any specific mechanism.

Answer: This model did not call for unwinding of the DNA strands, as in the Semi-Conservative Model, thus solving the concern about DNA unwinding. However, it was unclear how the copying machinery would read out the nucleotide sequence information buried in the core of the DNA double helix.

A third possibility was a model proposed by Max Delbrück (later called "Dispersive Replication") ( Figure 5 ). Delbrück doubted that the two strands of the double helix could be unwound or pulled apart to undergo Semi-Conservative replication and instead suggested that DNA strands broke at every half-turn of the helix during replication (discussed in more depth below and in Dig Deeper 1 ). According to Delbrück's Dispersive Replication Model, each helix of the replicated DNA consists of alternating parental and daughter DNA. Unlike the other two models, the progeny in subsequent generations would be indistinguishable with regard to their compositions of parental and newly synthesized DNA.

Answer: Fragmentation would create shorter templates for replication, which would minimize any unwinding or untangling problems faced by one very long DNA molecule. However, the reassembly of the fragments again into the intact chromosome could be problematic, especially if the breaks occur at every half-turn of the helix.

Explorer's Question: In the first generation, which model(s) would predict that the two daughter cells would receive approximately equal amounts of the original and newly synthesized DNA?

Answer: The Semi-Conservative Model and the Dispersive Model. However, differences in daughter composition arise in the second generation in the two models.

Matt and Frank learned about the models for DNA replication prior to their first meeting at the Physiology Course at the Marine Biological Laboratory through circumstances described below.

Sometime in 1953, while I was a graduate student of the great chemist Linus Pauling, I went to see Max Delbrück, a physicist and founder of the "phage group" who had become deeply interested in genetics and the basis of life (Max Delbrück, and his work with Salvador Luria, is featured in the Narrative on Mutations by Koshland ). I wanted to learn what problems in biology he thought were most important and what advice he might have for me about getting into biology. Almost as soon as I sat down in his office, he asked what I thought about the two papers by Watson and Crick that had been published in Nature earlier that year. I confessed that I had never heard of them.

Exasperated, Delbrück flung a little heap of reprints of the Watson–Crick papers at me and shouted "Get out and don't come back until you have read them." What I heard was "come back." So I did, but only after reading the papers.

When I came back, Delbrück said he did not believe in the Semi-Conservative mechanism of DNA replication proposed by Watson and Crick. Max had imagined that if replication is semi-conservative, the two daughter duplexes would become wound around each other turn-for-turn as the two chains of the mother molecule became unwound. To get around the supposed problem of untangling the daughter molecules, he proposed a model in which breaks are made to prevent interlocking when separating, and then joined back together (see Figure DD1 in Dig Deeper 1 ). This mechanism required rotation of only short lengths of duplex DNA, after which the chains would be rejoined. In the rejoining process, sections of the new chain would be fused to sections of the old chain, making all four of the chains mosaics of new and old DNA. Delbrück, in 1954, published a paper that questioned the Watson–Crick model and presented this new model (later referred to as "Dispersive Replication" as shown in Figure 5 and Dig Deeper 1 ). In some ways, the idea of Delbrück was ahead of its time. Transient breaks are now known to be made by an enzyme called topoisomerase, but in a manner that leaves the individual chains of the parent duplex intact (see Dig Deeper 1 ).

In addition to Max's reservations and model, several other scientists also posed their own concerns and solutions to the "unwinding problem" or alternatives to the Watson–Crick DNA structure itself (see Dig Deeper 1 ).

What I gathered from my conversations with Max and others was that not everyone believed the DNA replication model of Watson and Crick. It was only a hypothesis with no experimental evidence to support it. The key to solve this problem was to follow the parental DNA in the progeny. But how?

I was working on something very different for my PhD thesis with Linus Pauling, but earlier that year, I had an idea for labeling protein molecules with deuterium, a heavy isotope (2H) of hydrogen (1H) and separating them from unlabeled protein molecules in a centrifuge according to their density as a means to solve a quite different problem (see Dig Deeper 2 ). After that second meeting with Max, it occurred to me that density labeling and centrifugal separation might be used to solve the DNA replication problem. When I told this to Pauling, he urged me to get my X-ray crystallography done first. And when I proposed the density approach to Watson, one of those evenings waiting for dinner at the Athenaeum, he said I should go to Sweden to do it – where the ultracentrifuge had been invented (which I never did).

My entry point to thinking about DNA replication came when I was trying to understand how bacteriophage (viruses that infect bacteria) exchange pieces of DNA with one another. This process of DNA exchange between chromosomes is called recombination (see the whiteboard video on the experiments by Morgan and Sturtevant ). When did this recombination process occur? Did it occur when DNA replicates? Or perhaps recombination was an event that stimulated DNA replication? My intuition was that the processes of recombination and replication were somehow related. However, little was known about the mechanisms of either DNA replication or recombination at the time. Furthermore, I did not know how to pursue these questions in 1954. My ideas for experiments were lame and would have led to un-interpretable data.

Like many interesting questions in biology, often one has to be patient until either the right idea or technology emerges that allows one to answer them properly. In 1954, my awareness of a possible connection between replication and recombination primed my interest for the first gin and tonic conversation with Matt. However, it was several decades before I was sure that, in bacteriophage, DNA replication and recombination, in a large degree, depended upon each other. The convincing experiments were based on variations of a technique pioneered by Matt and Jean Weigle at Caltech. In this method, density-labeled, genetically marked parental phage infect the same bacteria. The densities and genetic makeup of progeny phage are determined by bioassay of the individual drops collected from a density gradient.

Matt and Frank

To distinguish between the Semi-Conservative, Conservative, and Dispersive Models of DNA replication described above, we needed a method that could tell the difference between the parental and daughter DNA strands. Figures 2 , 4 , and 5 illustrate the parent and newly synthesized strands with different colors. However, we needed to find a real physical difference that would serve the same function of distinguishing between the old and newly synthesized DNA. Matt had the idea of distinguishing old and new DNA by having the bacteria synthesize them with different isotopes and separating them in a centrifuge according to their density. If the original and the newly synthesized DNAs could be made of different density materials, then we could perhaps measure this physical difference. We will discuss the chemicals that were used to make the DNA heavier or lighter in the next section.

Our experimental idea was to grow an organism in a chemical medium that would make its DNA heavy. Then, while it was growing, we would switch to a new medium in which the newly synthesized DNA would be made of lighter material ( Figure 6 ). The density difference between the original and the newly replicated DNA could allow us to distinguish between models for DNA replication.

To separate DNA of different density, we invented a method, called "equilibrium density gradient centrifugation," and published it, together with Jerome Vinograd, a Caltech Senior Research Fellow who had taught us how to use the ultracentrifuge in his lab and provided advice. In this method, as applied to DNA, a special tube that has quartz windows so that ultraviolet light photos can be taken while the centrifuge is running is filled with a solution of cesium chloride and the DNA to be examined. Upon centrifugation at high speed (~45,000 revolutions per minute or 140,000 times gravity), the CsCl gradually forms a density gradient, becoming most concentrated at the bottom of the tube ( Figure 7 ). The CsCl solution toward the top of the tube is less dense than the DNA, while the CsCl solution at the bottom is denser than DNA. Thus, when a mixture of DNA in a CsCl solution is centrifuged, the DNA will eventually come to a resting point where its density matches that of the CsCl solution ( Figure 7 ). The DNA absorbs UV light, and its position along the tube was recorded by using a special camera while the centrifuge is running.

The method now seems straightforward, but in reality, it took a couple of years to develop. For example, we did not come to cesium chloride immediately. We looked at a periodic table for a dense monovalent atom that would not react with DNA; rubidium chloride (molecular weight of 121) was available in the Chemistry Department stockroom and we initially tried to use that but found that even concentrated solutions were not dense enough to float DNA to a banding point. We then moved one level down in the periodic table to cesium (the molecular weight of cesium chloride is 168) and that worked (for more details, see Dig Deeper 3 ).

Frank and Matt

In the fall of 1954, we were reunited in Cal Tech and lived for about eight months in the same house across the street from the lab. We finally could begin doing experiments to test models of DNA replication. It should be noted that DNA replication was our "side" project; we also had our "main" projects under the supervision of our respective professors. However, faculty at Cal Tech was kind in allowing two young scientists to venture forward with their own ideas.

While the general experimental approach that took form under the "gin and tonic tree" was straightforward, choices had to be made in how exactly to do the experiment. What organism should we use? Would a chemical trick of making DNA heavier or lighter work and could we measure a small density difference between the two? It took us a while to get the conditions right, about two years.

We first decided to examine the replication of the bacteriophage T4 inside of the bacterium Escherichia coli. Bacteriophages are viruses that invade and replicate inside of a bacterium; when new viruses are made, they will burst the bacterium and then spread to new hosts. Bacteriophage have small genomes and are therefore the smallest replicating systems. Frank's PhD thesis was on T4, so he knew how to work with this phage. Max Delbrück and others at Cal Tech were also actively studying phage (see the Narrative on Mutations by Koshland ). Thus, T4 seemed the obvious choice. To create DNA of heavier density than normal DNA, we decided to use the analogue, 5-bromouracil, of the base thymine, in which a heavier bromine atom replaces a lighter hydrogen atom. During replication, 5-bromouracil could be incorporated into DNA, instead of thymine.

However, while this approach seemed reasonable, it did not work in practice. Although we did not appreciate it at the time, during phage growth, the DNA molecules undergo recombination, joining parental DNA to newly synthesized DNA in a manner that after several generations would give no clear-cut distribution of old DNA among progeny molecules. Also, we learned from a recent paper that 5-bromouracil was mutagenic and made a detour into studying mutagenesis before coming back to our main project.

We clearly needed a new strategy.

Instead of phage, we decided to study the replication of the bacterial genome. This was a good choice – the bacterial DNA gave a very sharp and clear band when centrifuged in a solution of cesium chloride (to learn more about why we used cesium chloride to create a density gradient and the general use of this technique; see Dig Deeper 3 ).

We also switched our density label. DNA is made up of several elements – carbon, nitrogen, oxygen, phosphorus, and hydrogen. Some of these elements come in different stable isotopes, with atomic variations of molecular weight based upon different numbers of neutrons. Nitrogen-15 (15N) is a heavier isotope of nitrogen (the most common isotope, 14N, has a molecular weight of 14 Daltons). We could easily buy 15N in the form of ammonium chloride (15NH4Cl), which was the only source of nitrogen in our growth medium. The 15N in the medium then found its way into the bacterial DNA (as well as other molecules) in a harmless manner and did not impair bacterial growth.

We also had good luck in that Caltech bought a brand new type of ultracentrifuge called an analytical ultracentrifuge (Model E) developed by the Beckman Instrument Company. The Model E was a massive machine about the size of a small delivery truck (the current model is just a bit bigger than a dishwasher). Importantly, the Model E could shine a UV light beam on the tube while the centrifuge was spinning and detect and photograph the position of the DNA. The good news was that 15N-containing DNA and 14N-containing DNA could be clearly distinguished by their different density positions ( Figure 8 ).

Finally, we had everything in place to try our experiment properly. I decided to set up our first experiment in the following two ways:

1) Grow the bacteria in "light" nitrogen medium and then switch to "heavy."

2) Grow another culture of bacteria in "heavy" nitrogen for many generations and then switch to "light."

Frank was called to a job interview and could not perform this first experiment with me. But before leaving, he warned me – "Don't do the experiment in such a complicated way on your first try. You might mix up the tubes."

I ignored Frank's advice and did the experiment both ways.

In the first experiment after transferring bacteria grown in heavy nitrogen (15N) growth medium and then switched to "light" (14N) nitrogen medium, I saw three discrete bands corresponding to old, hybrid, and new DNA, as predicted by Semi-Conservative replication. Excited developing the photograph in the darkroom, I remember letting out a yelp that caused a young woman working nearby to leave in a hurry. But later I realized my mistake. Frank had been prophetic. I indeed had mixed tubes, combining two different samples, one taken before and the other taken after the first generation of bacterial growth in the light medium. As described for the correct experiment below, there is no time when old, hybrid, and fully new DNA are present at the same time.

When I came back from my trip, Matt and I performed what proved to be the decisive experiment. We grew bacteria in "heavy" nitrogen (15N; from 15NH4Cl) and then switched to "light" nitrogen (14N; 14NH4Cl) and, at different time points, collected the bacteria by centrifugation, added detergent to release the DNA, and combined this with concentrated CsCl solution to reach the desired density. After 20 hours of centrifugation and the final density positions of the DNAs had come into view, we knew that we had a clean answer ( Figure 9 ). The DNA from bacteria grown in heavy nitrogen formed a single band in the gradient. However, when the bacteria were shifted to a light nitrogen medium and then allowed to replicate their DNA and divide once (first generation), essentially all DNA had shifted to a new, "intermediate" density position in the gradient ( Figure 9 ). This intermediate position was half-way between the all heavy and all light DNA. At longer times of incubation in light nitrogen, after the cells had divided a second time (second generation), a DNA band at lighter density was seen and there were equal amounts of the intermediate and light DNA.

Explorer's Question: Which of the three models (Conservative, Semi-Conservative, or Dispersive) is most consistent with the results of this experiment?

Answer: The Semi-Conservative model. The Conservative model predicts a heavy and light band at the first generation, not an intermediate band. The Dispersive model predicts a single intermediate band at both the first and second generations (the band shifting toward lighter densities with more generation times).

Explorer's Question: Why are the two DNA bands at the 1.9 generation time point of approximately equal intensity?

Answer: After the first generation, each of the two heavy strands is partnered with a light strand. The bacterial DNA consists of one heavy strand and one light strand. When that heavy–light DNA replicates again in the light medium, the heavy strands are partnered with new light strands (intermediate density DNA) and the light strands are also partners with new light strands (creating all light density DNA).

The experiment that Frank described above took hardly any time at all (2 days) and yielded a clean result. We then repeated it without any problem. Once we knew how to set up the experiment, it was relatively easy. But it took us two years of trials before we got the experimental design and conditions right for the final ideal experiment.

The experiment clearly supported the Semi-Conservative Replication model for replication and, in doing so, also supported the double helical model of DNA itself. However, we wanted to do one more experiment that would examine whether the "intermediate" density band of DNA in the first generation was truly made of two and just two distinct subunits, as predicted by the Watson–Crick model. The model predicts that one complete strand of DNA is from the parent and should be heavy and the other complete DNA strand should be all newly replicated and therefore light ( Figure 10 ). We could test this hypothesis by separating the subunits with heat and then analyzing the density and molecular weight of the separated subunits by equilibrium density-gradient ultracentrifugation.

On the other hand, the Dispersive Model predicted that each DNA strand of first generation is an equal mixture of original and newly replicated DNAs ( Figure 11 ).

The results from the experiment were again clear ( Figure 12 ). The "intermediate density" DNA in the first generation split apart into a light and heavy component. From the width of the DNA band in the gradient (see Dig Deeper 3 ), we could also tell that the light and heavy DNA obtained after heating had each half of the molecular weight of the intermediate density DNA before heating. These results indicated that each parental strand remained intact during replication and produced a complete replica copy. This was decisive evidence against the Delbrück model for it predicted that both strands would be mosaics of heavy and light, not purely heavy and purely light. And the finding that the separated heavy and light subunits each had half the molecular weight of the intact molecule indicated that DNA was made up of two chains, as predicted by the Watson Crick model, and was not some multichain entity.

Based upon the results in Figure 9 and Figure 12 , we concluded that:

1) The nitrogen of a DNA molecule is divided equally between two subunits. The subunits remain intact through many generations.

2) Following replication, each daughter molecule receives one parental subunit and one newly synthesized subunit.

3) The replicative act results in a molecular doubling.

These conclusions precisely aligned with the Watson–Crick Semi-Conservative model for DNA replication. DNA, as a double-stranded helix, unwinds, and each strand serves as a template for the synthesis of a new strand.

When we had our result, Matt quickly shared the news with Jim Watson in a letter dated November 8, 1957 (available for the first time here ). It was common in those days to share results with colleagues through letters prior to a publication.

We also shared our results with Max Delbrück who took the news well that his Dispersive Replication model was incorrect. In fact, he wrote to a colleague that Meselson and Stahl had obtained a "world shaking result." But we were slow to get our work written up for publication. Once we knew the answer, we were keen to move onto new experiments rather than writing up our results.

Finally, Max had enough of our dallying and brought us down to the Caltech marine station at Corona del Mar. There, he quarantined us to a room in a tower, saying that we could not come out until we had written a draft of our paper. He was not being unkind, and we thought it great fun. Max's wife Manny Delbrück kindly came in occasionally to bring us delicious sandwiches, and Max also kept us company. We worked for 2 days straight and got him a draft.

Shortly thereafter, we completed our paper and Max communicated it in May 1958 to the Proceedings of the National Academies of Science, 4 years after our meeting at the Marine Biological Laboratory but less than a year after finally getting our experiments to work.

After our paper was published, we went separate ways in our lives. Frank got a job at the University of Missouri but soon thereafter moved to the University of Oregon in Eugene. Matt got promoted from a postdoctoral fellow to an assistant professor at Caltech and was teaching physical chemistry. However, the constant teaching limited time in the laboratory. Matt asked to be demoted from assistant professor back to senior postdoc, so he could get more work done in the lab. This is perhaps the only case in the history of Caltech in which a professor asked to move down the academic ladder. After a year as a senior postdoc, Matt then moved to Harvard to become an associate professor.

Decades have passed, and we now know much about the machinery that orchestrates DNA replication, including the unwinding of the strands and the synthesis of a new strand from the parental template. The details are beyond what can be discussed here, but you can view an animation of this process in Video 2 .

When we first discussed the use of density labeling to test models for DNA replication under the gin and tonic tree, we could not imagine the psychological effect our experiment would have on the field. Many scientists were not initially convinced by the Watson–Crick model for the structure of DNA or their proposal for its mode of replication. It was not clear if their model could explain heredity and the properties of genes. Some people seemed to think the model was too simple to be the gene. Others thought it too simple (meaning too beautiful) to be wrong!

However, after our experiment, the DNA model seemed very real. We could watch DNA with a camera; the visualization of DNA bands was simple and clear. Our results showed that the gene is made of two complementary halves, each a template for the other. Even the disbelievers, such as the deeply thoughtful Max Delbrück, acquiesced. DNA was no longer an imaginary molecule in the heads of Watson and Crick. It was a dynamic molecule; one could perform experiments on it, and it behaved in living cells as one might predict. Mendel's concept of a discrete "factor" that could determine a plant character and remain intact generation after generation and the physical reality of a gene as double-stranded DNA became intertwined from that moment on.

It is gratifying to think that our experiment, so simple by modern standards, is still valued and taught. But beyond the logic of how the experiment was performed, we hope that our story also conveys other important lessons about science.

• Every hypothesis needs to be rigorously tested with a clear experiment.

• An atmosphere of freedom is important. We were both very junior at the time of this experiment, but we were supported by senior scientists who encouraged us to pursue our own ideas.

• Success does not come immediately. Reading most scientific papers (including ours), everything seems straightforward and works right away. Our narrative shows that the so-called "most beautiful experiment in biology" had some unsuccessful excursions and two years of work to come to successful finish.

• Because success does not come immediately, it is valuable to be able to share difficult times with a friend. We kept each from getting discouraged. There was a certain gaiety in our work. We even had fun when things went wrong.

• Much of science is built upon collaboration and friendship. This Key Experiment could never have been the "Meselson Experiment" or the "Stahl Experiment." The "Meselson and Stahl Experiment" required both parties. We complemented each other scientifically and encouraged each other personally. Well more than a half-century has passed since this experiment was performed, and we remain good friends today.

Dig Deeper 1: Alternatives to Semi-Conservative replication

Max Delbrück, in his 1954 paper (PNAS 40: 783-788), said the following of the Watson and Crick Semi-Conservative replication mechanism:

"The principal difficulty of this mechanism lies in the fact that the two chains are wound around each other in a large number of turns and that, therefore, the daughter duplexes generated by the process just outlined are wound around each other with an equally large number of turns. There are three ways of separating the daughter duplexes: (a) by slipping them past each other longitudinally; (b) by unwinding the two duplexes from each other; (c) by breaks and reunions. We reject the first two possibilities as too inelegant to be efficient and propose to analyze the third possibility."

Max's solution was to break the single chains at regular intervals, allowing rotation about single bonds of the unbroken chain followed by joining in a way that dispersed short segments of parental DNA among the single chains of the daughter duplexes. This is the Dispersive Model presented in Figure 5 and presented in more detail in Figure DD1 . There was a germ of truth in Delbrück's idea of breakage. We now know of topoisomerases, enzymes that facilitate DNA replication by temporarily breaking, allowing unwinding, and then rejoining DNA. There are also enzymes that unwind DNA helixes called DNA helicases. Both enzymes use chemical energy derived from hydrolyzing adenosine triphosphate to perform work on the stable DNA double helix.

Another type of solution to the "unwinding problem," one that required no breaking and no entangling of the daughter molecules, was to imagine that the synthesis process would cause the entire parental duplex to rotate one turn for each turn of DNA synthesized. But this posed problems of its own – giving rise to a variety of long-forgotten proposed models, including evoking a motor at the growing point that would drive the rotation of the parent molecule, as proposed by John Cairns and Cedric Davern [J. Cellular Physiology, 70: S65–76 (1967)].

Alternative solutions questioned the DNA double helix model, but not semi-conservative replication. For example, one idea was to assume that the two chains are not wound around a common axis, but instead are simply pushed together (plectonemic coiling), which would require no unwinding and no rotation. This possibility, although it appeared remote, was not rigorously ruled out until much later in a paper by Crick, Wang, and Bauer in 1979 (J. Mol. Biol, 129: 449–461).

Dig Deeper 2: The idea for using density for separation

The idea for using density as a separation method came to me early in 1954 while I was a first-year graduate student at Caltech listening to a lecture by the great French scientist Jacques Monod. Monod was describing the problem of regulation of an enzyme called beta-galactosidase. If the bacteria were growing in a medium without lactose (a sugar), the enzyme activity was very low. When lactose or a chemical analogue of lactose was added, the enzyme activity was induced. The question was how? One model was that the enzyme was always there, but is inactive unless lactose is around. Another model (the correct one) was that the enzyme is synthesized de novo after the inducer is added. I thought that it might be possible to measure new enzyme and distinguish it from old enzyme if the enzyme was synthesized from heavier building blocks (amino acids). How could one make heavier building blocks? I thought that deuterium (a heavy isotope of hydrogen; 2 H) might be the answer. If one grew bacteria in heavy water ( 2 H 2 O) and switched to normal water ( 1 H 2 O) when one added inducer, then any newly synthesized beta-galactosidase would have had a greater density than the pre-existing beta-galactosidase. I never did the experiment, but the idea primed me for the DNA replication problem.

Dig Deeper 3: The role of the centrifuge in the Meselson–Stahl experiment

Matt describes briefly how this technique evolved

The first paper (see the reference list) that Frank and I wrote together (along with Jerome Vinograd) was on the method and theory of using centrifugation in an equilibrium density gradient, which showed that this method not only could separate molecules but could also be used as a tool to determine their molecular weights. This work was also part of my PhD thesis at Caltech. When I presented this work at my thesis defense, the great physicist Richard Feynman was on the examination committee, along with Pauling, Vinograd, and one of Pauling's post-docs who taught me X-ray crystallography. Feynman had not read the thesis but did so during the defense. I presented my rather long mathematical derivation showing that macromolecules in a density gradient in a centrifugal field would be distributed in a Gaussian manner about the position of neutral buoyancy with the width dependent upon the square root of the molecular weight. Feynman then went to the blackboard and, on the spot, produced a much shorter derivation of the same thing, modeled on the wave function for the quantum mechanical harmonic oscillator. Feynman writes about our experiment in his jolly book, Surely You're Joking, Mr. Feynman .

The way in which we found that CsCl forms a density gradient on its own was somewhat fortuitous. We initially thought that we needed to pour a CsCl gradient in the tube in advance. However, we found that just by centrifuging an initially homogeneous solution of cesium chloride produced a continuous gradient density on its own after several hours. From the width of the DNA band in the density gradient, we could also calculate the molecular weight of the DNA molecules in the gradient as 7 million Daltons. The chromosome of E. coli is much, much larger, but long molecules of DNA are fragile and had been broken up by shear forces while passing through the hypodermic needle with which we loaded the centrifuge cell. Subsequent to our result, CsCl equilibrium density-gradient centrifugation became a standard tool for isolating DNA from cells for decades and was used in important experiments such as the demonstration of messenger RNA by Brenner, Meselson, and Jacob and showing the mechanism of general recombination in phage lambda by Frank Stahl.

References and Resources

Matthew Meselson’s letter to James Watson from November 8, 1957, describing the results of their experiments on DNA replication. Download .

This paper describes the use of the centrifuge and density gradient to analyze biological molecules, a technique that was used in their 1958 paper but also very broadly used for many applications in biology. See also Dig Deeper 3 .

An outstanding resource for those wanting a detailed, accurate description of the Meselson–Stahl experiment.

A nice 7:30 min video describing the Meselson–Stahl experiment and its conclusions.

This film documents the discovery of the structure and replication of DNA including interviews with James Watson who, along with Crick, proposed the double helix model of DNA.

This activity is often used in conjunction with the short film The Double Helix. It introduces students to Meselson and Stahl experiment and helps them understand the concepts generated via those experimental results.

This collection of resources from HHMI Biointeractive addresses many of the major concepts surrounding DNA and its production, reading, and replication.

The Meselson-Stahl Experiment (1957–1958), by Matthew Meselson and Franklin Stahl

Illustration of the Meselson-Stahl Experiment

In an experiment later named for them, Matthew Stanley Meselson and Franklin William Stahl in the US demonstrated during the 1950s the semi-conservative replication of DNA, such that each daughter DNA molecule contains one new daughter subunit and one subunit conserved from the parental DNA molecule. The researchers conducted the experiment at California Institute of Technology (Caltech) in Pasadena, California, from October 1957 to January 1958. The experiment verified James Watson and Francis Crick´s model for the structure of DNA, which represented DNA as two helical strands wound together in a double helix that replicated semi-conservatively. The Watson-Crick Model for DNA later became the universally accepted DNA model. The Meselson-Stahl experiment enabled researchers to explain how DNA replicates, thereby providing a physical basis for the genetic phenomena of heredity and diseases.

The Meselson-Stahl experiment stemmed from a debate in the 1950s among scientists about how DNA replicated, or copied, itself. The debate began when James Watson and Francis Crick at the University of Cambridge in Cambridge, England, published a paper on the genetic implications of their proposed structure of DNA in May 1953. The Watson-Crick model represented DNA as two helical strands, each its own molecule, wound tightly together in a double helix. The scientists claimed that the two strands were complementary, which meant certain components of one strand matched with certain components of the other strand in the double helix.

With that model of DNA, scientists aimed to explain how organisms preserved and transferred the genetic information of DNA to their offspring. Watson and Crick suggested a method of self-replication for the movement of genetic information, later termed semi-conservative replication, in which DNA strands unwound and separated, so that each strand could serve as a template for a newly replicated strand. According to Watson and Crick, after DNA replicated itself, each new double helix contained one parent strand and one new daughter strand of DNA, thereby conserving one strand of the original double helix. While Watson and Crick proposed the semi-conservative model in 1953, the Meselson-Stahl experiment confirmed the model in 1957.

In 1954, Max Delbrück at Caltech published a paper that challenged the Watson-Crick Model for DNA replication. In his paper, Delbrück argued that the replication process suggested by Watson and Crick was unlikely because of the difficulty associated with unwinding the tightly-wound DNA structure. As an alternative, Delbrück proposed that instead of the entire structure breaking apart or unwinding, small segments of DNA broke from the parent helix. New DNA, Delbrück claimed, formed using the small segments as templates, and the segments then rejoined to form a new hybrid double helix, with parent and daughter segments interspersed throughout the structure.

After the release of Delbrück´s paper, many scientists sought to determine experimentally the mechanism of DNA replication, which yielded a variety of theories on the subject by 1956. Delbrück and Gunther Stent, a professor at the University of California, Berkeley, in Berkeley, California, presented a paper in June 1956 at a symposium at Johns Hopkins University in Baltimore, Maryland, which named and summarized the three prevailing theories regarding DNA replication at the time: semi-conservative, dispersive, and conservative. Delbrück and Stent defined conservative replication as a replication mechanism in which a completely new double helix replicated from the parent helix, with no part of the parent double helix incorporated into the daughter double helix. They described the semi-conservative process as Watson and Crick suggested, with half of the parental DNA molecule conserved in the daughter molecule. Lastly, Delbrück and Stent summarized Delbrück´s dispersive model, in which parental DNA segments distribute throughout the daughter DNA molecule. Delbrück and Stent´s paper provided the background for the Meselson-Stahl experiment.

In 1954, prior to publication of Delbrück´s initial challenge of the Watson-Crick model, Matthew Meselson and Franklin Stahl had joined the DNA replication discussion. During the spring of 1954, Meselson, a graduate student studying chemistry at Caltech, visited Delbrück´s office to discuss DNA replication. According to historian of science Frederic Holmes, during that meeting Meselson began brainstorming ways to determine how DNA replicated. In the summer of 1954, Meselson met Stahl at the Marine Biological Laboratory in Woods Hole, Massachusetts. Stahl, a graduate student studying biology at the University of Rochester in Rochester, New York, agreed to study DNA replication with Meselson the following year at Caltech.

Meselson and Stahl began their collaboration in late 1956. By that time, Stahl had completed his PhD and Meselson had completed the experiments for his PhD, which he received in 1957. They worked on a variety of projects, including DNA replication. All of their projects, however, involved a method first devised by Meselson in 1954, called density-gradient centrifugation. Density-gradient centrifugation separates molecules based on their densities, which depend on the molecular weights of the molecules.

Meselson and Stahl used density-gradient centrifugation to separate different molecules in a solution, a method they later used to separate DNA molecules in a solution. In density gradient centrifugation, a solution is placed in an ultracentrifuge, a machine that spins the samples very fast on the order of 140,000 times the force of gravity or 44,770 revolutions per minute (rpm). As the samples spin, denser substances are pushed toward the bottom, while less dense substances distribute according to their weight in the centrifuge tube. By the end of centrifugation, the molecules reach a position called equilibrium, in which the molecules stop moving and remain in a gradient. The position of the molecules at equilibrium is dependent on the density of the molecule. Meselson and Stahl measured the areas in which DNA was at the highest concentration. Higher concentrations were represented by darker bands of DNA in the centrifuged sample. Stahl represented those bands on a graph, so that the peaks represented locations in the gradient where there was the highest concentration of molecules. Multiple peaks meant that molecules of different densities separated out of the solution.

To describe how DNA replicated, Meselson and Stahl needed to distinguish between parental and daughter DNA. They achieved that by modifying the molecules so each kind had a different density. Then Meselson and Stahl could separate the molecules using density-gradient centrifugation and analyze how much parental DNA was in the new daughter helices after every replication cycle. First they tried to alter the density of parental DNA by substituting a one nucleotide base, thymidine, with a heaver but similar DNA nucleotide base, 5-bromouriacil (5-BU). However, Meselson and Stahl struggled to substitute enough units of 5-BU into the DNA molecules to make the parental DNA significantly denser than normal DNA.

By July 1957, Meselson and Stahl successfully incorporated the heavy substitution in parental DNA, but the type of DNA they used still caused problems. Meselson and Stahl first used DNA from a specific type of virus that infects bacteria, called a bacteriophage. However, bacteriophage DNA not only broke apart in solution during centrifugation, but also replicated too quickly for the distribution of DNA to be adequately measured after each cycle. Consequently, Meselson and Stahl struggled to see clear locations within the density gradient with the highest concentration of bacterial DNA. Therefore, in September 1957, Meselson and Stahl switched to using the DNA from the bacteria Escherichia coli (E. coli) . E. coli DNA formed clearer concentration peaks during density gradient centrifugation.

At around the same time, in addition to changing the source DNA, Meselson and Stahl also changed the type of density label they used, from substitution labels to isotope labels. An isotope of an element is an atom with the same number of positive charged nuclear particles or protons, and a different number of uncharged particles, called neutrons. A difference in neutrons, for the most part, does not affect the chemical properties of the atom, but it alters the weight of the atom, thereby altering the density. Meselson and Stahl incorporated non-radioactive isotopes of nitrogen with different weights into the DNA of E. coli . As DNA contains a large amount of nitrogen, so long as the bacteria grew in a medium containing nitrogen of a specified isotope, the bacteria would use that nitrogen to build DNA. Therefore, depending on the medium in which E. coli grew, daughter strands of newly replicated DNA would vary by weight, and could be separated by density-gradient centrifugation.

Starting in October 1957, Meselson and Stahl conducted what later researches called the Meselson-Stahl experiment. They grew E. coli in a medium containing only the heavy isotope of nitrogen ( 15 N) to give the parental DNA a higher than normal density. As bacteria grow, they duplicate, thereby replicating their DNA in the process. The researchers then added an excess of light isotopes of nitrogen ( 14 N) to the heavy nitrogen environment.

Meselson and Stahl grew E. coli in the 14 N isotope environment for all subsequent bacterial generations, so that any new DNA strands produced were of a lower density than the original parent DNA. Before adding 14 N nitrogen, and for intervals of several bacterial generations after adding light nitrogen, Meselson and Stahl pulled samples of E. coli out of the growth medium for testing. They centrifuged each sample for initial separation, and then they added salt to the bacteria so that the bacteria released its DNA contents, allowing Meselson and Stahl to analyze the samples.

Next, Meselson and Stahl conducted density gradient centrifugation for each DNA sample to see how the parental and daughter DNA distributed according to their densities over multiple replications. They added a small amount of each sample of bacterial DNA to a cesium chloride solution, which when centrifuged had densities within the range of the bacterial DNA densities so that the DNA separated by density. The researchers centrifuged the DNA in an ultracentrifuge for twenth hours until the DNA reached equilibrium. Using ultraviolet light (UV), the researchers photographed the resulting DNA bands, which represented peaks of DNA concentrations at different densities. The density of the DNA depended on the amount of 15 N or 14 N nitrogen present. The more 15 N nitrogen atoms present, the denser the DNA.

For the bacterial DNA collected before Meselson and Stahl added 14 N nitrogen, the UV photographs showed only one band for DNA with 15 N nitrogen isotopes. That result occurred because the DNA from the first sample grew in an environment with only 15 N nitrogen isotopes. For samples pulled during the first replication cycle, the UV photographs showed fainter the 15 N DNA bands, and a new DNA band formed, which represented half 15 N DNA nitrogen isotopes and half 14 N DNA nitrogen isotopes. By the end of the first replication cycle, the heavy DNA band disappeared, and only a dark half 15 N and half 14 N DNA band remained. The half 15 N half 14 N DNA contained one subunit of 15 N nitrogen DNA and one subunit of 14 N nitrogen DNA. The data from the first replication cycle indicated some distribution of parental DNA, therefore ruled out conservative replication, because only parental DNA contained 15 N nitrogen isotopes and only parental DNA could represent the 15 N nitrogen isotopes in daughter DNA.

The same trends continued in future DNA replication cycles. As the bacteria continued to replicate and the bacterial DNA replicated, UV photographs showed that the band representing half 15 N half 14 N DNA depleted. A new band, representing DNA containing only 14 N nitrogen isotopes or light DNA, became the prevalent DNA band in the sample. The depletion of the half 15 N half 14 N band occurred because Meselson and Stahl never re-introduced 15 N nitrogen, so the relative amount of 15 N nitrogen DNA decreased. Meselson and Stahl then mixed the samples pulled from different replication cycles and centrifuged them together. The UV photograph from that run showed three bands of DNA with the half 15 N half 14 N DNA band at the midpoint between the 15 N DNA band and 14 N DNA band, making it an intermediate band. The result indicated that the half 15 N half 14 N DNA band had a density exactly between the 15 N and 14 N nitrogen DNA, showing that the DNA in the central band contained half of the 15 N nitrogen and half of the 14 N nitrogen isotopes, just as predicted by the Watson and Crick model. The exact split between heavy and light nitrogen characterized semi-conservative DNA replication.

Meselson and Stahl made three conclusions based on their results. First, they concluded that the nitrogen in each DNA molecule divided evenly between the two subunits of DNA, and that the subunits stayed intact throughout the observed replication cycles. Meselson and Stahl made that conclusion because the intermediate band had a density halfway between the heavy and light DNA bands. That conclusion made by Meselson and Stahl challenged the dispersive mechanism suggested by Delbrück, which involved breaking the DNA subunits into smaller pieces.

Meselson´s and Stahl´s second conclusion stated that each new DNA double helix contained one parental subunit, which supported semi-conservative replication. Assuming that DNA consists of two subunits, if a parent passes on one subunit of DNA to its offspring, then half of the parental DNA is conserved in the offspring DNA, and half of the parental DNA is not. The researchers made that conclusion because if parental DNA did not replicate in that way, then after the first replication, some DNA double helices would have contained only parental heavy nitrogen subunits or only daughter light nitrogen subunits. That type of replication would have indicated that that some parental DNA subunits did not separate in the semi-conservative fashion, and instead would have supported conservative replication. The presence of one parental subunit for each daughter DNA double helix supported semi-conservative replication.

The third conclusion made by Meselson and Stahl stated that for every parental DNA molecule, two new molecules were made. Therefore, the amount of DNA after each replication increased by a factor of two. Meselson and Stahl related their findings to the structure of DNA and replication mechanism proposed by Watson and Crick.

Before Meselson and Stahl published their findings, word of the Meselson-Stahl results spread throughout Caltech and the scientific community. According to Holmes, Delbrück, who had strongly opposed the semi-conservative method of DNA replication, immediately accepted DNA replication as semi-conservative after seeing the results from the Meselson-Stahl experiment. Some experiments earlier that year had pointed towards semi-conservative replication, and the Meselson-Stahl experiment served to further support semi-conservative replication.

Despite the positive reception of the Meselson-Stahl experiment, years passed before scientists fully accepted the Watson-Crick Model for DNA based on the findings from the Meselson-Stahl experiment. The Meselson-Stahl experiment did not clearly identify the exact subunits that replicated in DNA. In the Watson and Crick model, DNA consisted of two one-stranded DNA subunits, but the Meselson-Stahl experiment also supported models of DNA as having more than two strands. In 1959, Liebe Cavalieri, a scientist at the Sloan-Kettering Institute for Cancer research in New York City, New York, and his research team had produced evidence supporting the theory that DNA consisted of two two-stranded subunits, making DNA a quadruple helix. Cavalieri´s proposal did not contradict the Meselson-Stahl experiment, because the Meselson-Stahl experiment did not define DNA subunits. However, later experiments performed by Meselson on bacteriophage DNA from 1959 to 1961, and experiments performed by John Cairns on E. coli DNA in 1962, settled the debate and showed that each subunit of DNA was a single strand.

As described by Holmes, many scientists highly regarded the Meselson-Stahl experiment. Scientists including John Cairns, Gunther Stent, and James Watson all described the experiment as beautiful in both its performance and simplicity. Holmes also described the academic paper published by Meselson and Stahl on their experiment as beautiful because of its concise descriptions, diagrams, and conclusions. The Meselson-Stahl experiment appeared in textbooks decades after Meselson and Stahl performed the experiment. In 2001, Holmes published Meselson, Stahl, and the Replication of DNA: A History of "The Most Beautiful Experiment in Biology," which told the history of the experiment.

The Meselson-Stahl experiment gave a physical explanation for the genetic observations made before it. According to Holmes, for scientists who already believed that DNA replicated semi-conservatively, the Meselson-Stahl experiment provided concrete evidence for that theory. Holmes stated that, for scientists who contested semi-conservative replication as proposed by Watson and Crick, the Meselson-Stahl experiment eventually changed their opinions. Either way, the experiment helped scientists´ explain inheritance by showing how DNA conserves genetic information throughout successive DNA replication cycles as a cell grows, develops, and reproduces.

Cairns, John. "A Minimum Estimate for the Length of the DNA of Escherichia coli Obtained by Autoradiography." Journal of Molecular Biology 4 (1962): 407–9.
Cavalieri, Liebe F., Barbara Hatch Rosenberg, and Joan F. Deutsch. "The Subunit of Deoxyribonucleic Acid." Biochemical and Biophysical Research Communications 1 (1959): 124–8.
Davis, Tinsley H. "Meselson and Stahl: The Art of DNA Replication." Proceedings of the National Academy of Sciences 101 (2004): 17895–6. http://www.pnas.org/content/101/52/17895.long (Accessed April 18, 2017).
Delbrück, Max. "On the Replication of Deoxyribonucleic Acid (DNA)." Proceedings of the National Academy of Sciences 40 (1954): 783–8. http://www.pnas.org/content/40/9/783.short (Accessed April 18, 2017).
Delbrück, Max and Gunther S. Stent. "On the Mechanism of DNA Replication." In McCollum-Pratt Symposium on the Chemical Basis of Heredity , eds. William D. McElroy and Bentley Glass, 699–736. Baltimore: Johns Hopkins University Press, 1956.
Holmes, Frederic L. Meselson, Stahl, and the Replication of DNA: a History of "The Most Beautiful Experiment in Biology." New Haven: Yale University Press, 2001.
"Interview with Matthew Meselson." Bioessays 25 (2003): 1236–46.
Judson, Horace Freeland. The Eighth Day of Creation: Makers of the Revolution in Biology . Cold Spring Harbor: Cold Spring Harbor Laboratory Press, 1996.
Levinthal, Cyrus. "The Mechanism of DNA Replication and Genetic Recombination in Phage." Proceedings of the National Academy of Sciences 42 (1956): 394–404. http://www.pnas.org/content/42/7/394.short (Accessed April 18, 2017).
Litman, Rose M. and Arthur B. Pardee. "Production of Bacteriophage Mutants by a Disturbance of Deoxyribonucleic Acid Metabolism." Nature 178 (1956): 529–31.
Meselson, Matthew. "The Semi-Conservative Replication of DNA." iBioMagazine 5 (2011). https://www.ibiology.org/ibiomagazine/issue-5/matthew-meselson-the-semi-conservative-replication-of-dna.html (Accessed April 18, 2017).
Meselson, Matthew, and Franklin W. Stahl. "The Replication of DNA in Escherichia Coli." Proceedings of the National Academy of Sciences 44 (1958): 671–82. http://www.pnas.org/content/44/7/671.long (Accessed April 18, 2017).
Meselson, Matthew, and Jean Weigle. "Chromosome Breakage Accompanying Genetic Recombination in Bacteriophage." Proceedings of the National Academy of Sciences 47 (1961): 857–68. http://www.pnas.org/content/47/6/857.short (Accessed April 18, 2017).
Meselson, Matthew, Franklin W. Stahl, and Jerome Vinograd. "Equilibrium Sedimentation of Macromolecules in Density Gradients." Proceedings of the National Academy of Sciences 43 (1957): 581–8. http://www.pnas.org/content/43/7/581.short (Accessed April 18, 2017).
Taylor, J. Herbert, Philip S. Woods, and Walter L. Hughes. "The Organization and Duplication of Chromosomes as Revealed by Autoradiographic Studies Using Tritium-Labeled Thymidine." Proceedings of the National Academy of Sciences 43 (1957): 122–8. http://www.pnas.org/content/43/1/122.short (Accessed April 18, 2017).
Watson, James D., and Francis H C Crick. "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid." Nature 171 (1953): 737–8. https://profiles.nlm.nih.gov/ps/access/SCBBYW.pdf (Accessed April 18, 2017).
Watson, James D., and Francis H C Crick. "Genetical Implications of the Structure of Deoxyribonucleic Acid." Nature 171 (1953): 964–7. https://profiles.nlm.nih.gov/ps/access/SCBBYX.pdf (Accessed April 18, 2017).
Weigle, Jean, and Matthew Meselson. "Density Alterations Associated with Transducing Ability in the Bacteriophage Lambda." Journal of Molecular Biology 1 (1959): 379–86.

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Today we know that this is the pattern used by living cells, but the experimental evidence in support of semiconservative replication was not published until 1958 . In the 5 years between Watson and Crick's suggestion and the definitive experiment, semiconservative replication was controversial and other patterns were considered.

Three hypothesized patterns were proposed:

Semiconservative - The original double strand of DNA separates and each strand acts as a template for the synthesis of a complimentary strand.
Conservative replication - the original double strand of DNA remains intact and is used as a template to create a new double stranded molecule.
Dispersive replication - similar to conservative replication in that the original double strand is used as a template without being separated, but prior to cell division, the strands recombine such that each daughter cell gets a mix of new and old DNA. With each round of replication, the original DNA gets cut up and dispersed evenly between each copy.

The methods Meselson and Stahl developed allowed them to distinguish existing DNA from newly synthesized DNA and to track new and old DNA over several rounds of replication.

They accomplished this by labeling cells with different stable isotopes of nitrogen. First, a culture of bacterial cell were grown for several generations in a media containing only 15 N ( a stable, heavy isotope of Nitrogen). After this period * of growth, all of the DNA in the cells contained 15 N. These cells were then rinsed and put into a media containing only the more common, lighter isotope of nitrogen ( 14 N). As the cells grew and divided in this fresh media, all newly synthesized DNA would contain only the lighter nitrogen isotope, while DNA from the original cells would still contain 15 N. In this illustration above, 15 N labeled DNA is shown in orange and 14 N labeled in green.

The 15 N and 14 N labeled DNA was then tracked using high speed centrifugation and a density * gradient created with cesium chloride (CsCl).

During centrifugation in a CsCl gradient, DNA accumulates in bands along the gradient based on its density. Since 15 N is more dense than 14 N, 15 N enriched DNA accumulates lower down in the centrifuge tube than the 14 N DNA. DNA containing a mixture of 15 N and 14 N ends up in an intermediate position between the two extremes.

By spinning DNA extracted at different times during the experiment, Meselson and Stahl were able to see how new and old DNA interacted during each round of replication.

The beauty of this experiment was that it allowed them to distinguish between the three different hypothesized replication patterns. The key result occurs at the second generation when all three proposed replication patterns give different results in the CsCl gradient.

That Meselson and Stahl's experiment showed the pattern predicted by the semiconservative hypothesis provided the definitive experimental evidence in support of the process proposed by Watson and Crick.

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v.101(52); 2004 Dec 28

Perspective

Density matters: the semiconservative replication of dna.

The semiconservative mode of DNA replication was originally documented through the classic density labeling experiments of Matthew Meselson and Franklin W. Stahl, as communicated to PNAS by Max Delbrück in May 1958. The ultimate value of their novel approach has extended far beyond the initial implications from that elegant study, through more than four decades of research on DNA replication, recombination, and repair. I provide here a short historical commentary and then an account of some developments in the field of DNA replication, which closely followed the Meselson–Stahl experiment. These developments include the application of density labeling to discover the repair replication of damaged DNA, a “nonconservative” mode of synthesis in which faulty sections of DNA are replaced.

DNA replication is arguably the most fundamental process required for the proliferation of all living cells. During cell division, each daughter cell must receive essentially the same genetic information that was encoded in the DNA of the parent cell. This conclusion means that DNA replication must generate a perfect copy of the genomic DNA complement. Convincing experimental evidence for a “semiconservative” mode of DNA replication was first provided by the elegant experiments of Matt Meselson and Frank Stahl ( 1 ), in which differential labeling with nitrogen-15 ( 15 N) and nitrogen-14 ( 14 N) was used to resolve parental and daughter DNA molecules by equilibrium sedimentation in a CsCl density gradient. By “semiconservative,” it is meant that the parental DNA subunits are conserved but that they become equally distributed into daughter molecules as replication proceeds. It was originally thought, and is now known to be true, that these “subunits” are the complementary single strands of the double-helical DNA duplex.

A comprehensive historical description of the collaboration between Meselson and Stahl, the milieu in which they worked, and their remarkable path to success was prepared by the late Frederic Lawrence Holmes and titled Meselson, Stahl, and the Replication of DNA : A History of “ the Most Beautiful Experiment in Biology ” ( 2 ). This account highlights the personalized side of the story and provides a wonderful example of how seminal research is actually done. The crisp rendition of experiments and their clear-cut interpretations in the published journal article cannot begin to reveal the tortuous path of the research, from the germination of ideas, through the disappointments and surprises as the experimental results appear, to the ultimate success of the project.

Speculation about how DNA might replicate directly followed the proposal by James Watson and Francis Crick for its double-helical structure, in which the pairing of bases through hydrogen bonds and stereochemistry ensured that the two strands would be complementary ( 3 ). A thymine in one strand is always paired with an adenine in the other, and correspondingly, cytosine is always paired with guanine. That part of the model incorporated Erwin Chargaff's “rules” ( 4 ), based on the relative frequencies of these bases in DNA. Reflecting on their duplex DNA model, Watson and Crick stated, “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material” ( 3 ). Thus, the strands might separate and then serve as templates for the synthesis of the respective complementary strands—a semiconservative mode of replication, in which each daughter DNA molecule would consist of one “old” strand and one “new” one.

Whereas the suggested mechanism seemed plausible, it was not immediately apparent how it might be rigorously tested. Furthermore, there were some rather vexing topological problems with which to contend. The DNA strands in the Watson–Crick helix are wound about each other in a “plectonemic” manner—which means that “for any winding number greater than zero, the `braid' consisting of the two chains cannot be combed” as Max Delbrück and Gunther Stent ( 5 ) pointed out in their early review on the subject. The Watson–Crick scheme assumed that unwinding and replication must proceed pari passu , with all three arms of the duplex DNA rotating at a replication fork. Another model suggested that periodic double-strand breaks would permit short sections of the duplex DNA to spin and then rejoin with the respective strand terminals of the same polarity. Although we now appreciate that an unprotected double-strand break in DNA is a very serious threat to cell viability, it has turned out that transient strand breaks are indeed the means by which the topological problem is resolved. As is often the case, when we are unable to explain how a plausible biochemical model might work, it may be because we have yet to discover an essential enzyme, in this case, topoisomerase. Topoisomerases are DNA “nicking-closing” enzymes and the type II topoisomerases, such as gyrase, in particular, are designed to pump negative supertwists into the DNA ahead of an advancing replication fork, thus relieving the unwinding stress and facilitating processive separation of the two strands ( 6 ). Otherwise, positive supertwists would accumulate ahead of the replication fork during replication as the parental strands are separated behind it. The topological problems of unwinding parental DNA strands and segregation of daughter DNA duplexes were resolved many years after the basic mechanism of DNA replication was revealed ( 7 ). Provocative, and perhaps clairvoyant, was the statement of Delbrück and Stent ( 5 ) regarding the putative semiconservative mode of DNA replication—“if it were possible to label differently the new material synthesized in each generation, then one could read off in each duplex the ages of the two chains.”

In an exemplary set of experiments (of which Max Delbrück was surely aware) in late 1956, J. Herbert Taylor et al. ( 8 ) labeled the chromosomes of Vicia faba (English broad bean) with 3 H-thymidine, and then followed the distribution of the tritium label through successive generations of duplication in nonradioactive medium, by using autoradiography. The remarkable conclusions from this study were “that the thymidine built into the DNA of a chromosome is part of a physical entity that remains intact during succeeding replications...” and “that a chromosome is composed of two such entities probably complementary to each other,” and “that after replication of each to form a chromosome with four entities, the chromosome divided so that each chromatid (daughter chromosome) regularly receives an `original' and a `new unit.”' Taylor appreciated, of course, that the “chromosome is several orders of magnitude larger than the proposed double-helix of DNA.” Nevertheless, he had demonstrated that eukaryotic chromosomes divide semiconservatively, in accordance with the predictions of the Watson–Crick model, if a chromosome contained a duplex DNA molecule.

The Germination of an Idea: Meselson Meets Stahl

Matt Meselson began his graduate study in chemistry at the California Institute of Technology (Caltech, Pasadena, CA) in 1953. He joined the laboratory of Linus Pauling and ultimately completed part II of his Ph.D. thesis ( 9 ) on The Crystal Structure of N, N′ dimethyl malonamide , to determine whether the peptide groups contained in this molecule were planar, and thus in accord with Pauling's resonance theory. That chapter of his thesis is less widely known than is part I, which was titled Equilibrium Sedimentation of Macromolecules in Density Gradients with Application to the Study of DNA. As a student in Pauling's course on the chemical bond, Matt became interested in the comparative strength of hydrogen bonds when the natural hydrogen was replaced with the heavy isotope, deuterium. While developing his interest in how living organisms might fare if they incorporated deuterium into their molecules, Meselson happened to attend a seminar at Caltech by Jacques Monod, who raised the question of whether the then-poorly understood phenomenon of induced-enzyme synthesis really involved new protein synthesis. Meselson speculated that if bacteria could be grown up in deuterium (heavy) water, and then transferred to ordinary water at the same instant that an “inducer” was added, any new proteins should be of “normal” density and the density difference between “old” and “new” proteins might permit their separation. In fact, if one centrifuged the proteins in a solution of intermediate density, then the “old” protein might sink whereas the newly synthesized protein should float. Later that year, he turned his attention to the DNA replication problem, after a lively discussion with Max Delbrück about the Watson–Crick double helix and possible modes for its duplication. It occurred to Matt that the same approach he had envisioned for protein synthesis might also be applied to study DNA replication. He decided at that point that he wanted to devote his energies to determine whether DNA, indeed, replicated in the manner predicted by Watson and Crick. Unrelated to that goal, he spent the summer of 1954 at the Marine Biological Laboratory at Woods Hole, MA, assisting Jim Watson with some titration experiments that were designed to provide possible support for a double-helical structure of RNA, analogous to the structure of DNA. Frank Stahl, then a graduate student in biology from the University of Rochester (Rochester, NY), was also at Woods Hole for the summer to take a physiology course. They met while Frank was sitting under a tree working on a problem in bacteriophage genetics. Fig. 1 is a photo of Matt and Frank 42 years later, standing at the same place. Whereas Matt was still quite naïve about bacteriophage genetics, he possessed the skills in calculus to help Frank solve the problem. As they became acquainted, Matt then raised the possibility that they might work in collaboration on the DNA replication problem—using phage DNA, to take advantage of Stahl's expertise. He also suggested using deuterium as a heavy label in a one-step growth experiment (by analogy with his earlier thoughts about density-labeling proteins), and to then centrifuge the sample in a solution of an appropriate density to separate the “light” DNA at the top of the tube from the “heavy” DNA at the bottom. However, they both soon recognized the complicating problem of doing their replication experiment with phage, because of the extensive recombination known to occur, which might be expected to reshuffle parental and daughter DNA, and thereby confuse the analysis. Fortunately, Stahl was already planning to go to Caltech for his postdoc, so they would be able to continue their discussion and work together there, perhaps to develop their strategy by using a “simple” cell system, the bacterium Escherichia coli .

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Photograph taken by F. L. Holmes of Matt Meselson and Frank Stahl in 1996, standing at the site where they met at Woods Hole 42 years earlier (figure 14.1 in ref. 2 ). Courtesy of the Holmes family.

Matt wanted to find out more about the chemical nature of the monomer precursors for DNA, and in the course of that literature search he learned about 5-bromouracil (5BU), an analog of thymine, which bacteria could incorporate during DNA synthesis in place of thymine. 5BU is equivalent to thymine except that bromine is substituted for the methyl group at the C5 position: the bromine conveniently has nearly the same van der Waals radius as a methyl group. Because of the different degree of ionization between 5BU and thymine, Matt considered that he might be able to separate 5BU-labeled molecules from those containing thymine by electrophoresis. However, more importantly, he appreciated the fact that 5BU would make the DNA containing it significantly heavier than normal thymine-containing DNA. He then considered using 5BU as a density label for DNA to follow its replication by the scheme considered earlier.

Matt became acquainted with Jerry Vinograd, who was the ultracentrifugation “guru” at Caltech, and he learned to operate the state-of-the-art Beckman Spinco Model E analytical ultracentrifuge ( Fig. 2 ). With Vinograd's initial tutelage, Matt tried sedimentation of DNA in a 7-molal solution of the heavy salt, CsCl—his idea was still that an experiment could be performed with a density label and that “light” DNA should float and that the density-labeled heavy DNA would sink in a solvent of the appropriate density. However, they were both amazed at how rapidly a salt gradient formed during the high-speed centrifugation and, furthermore, that the DNA migrated to a narrow band within the gradient. The band formed at the position of the buoyant density of the DNA in that stable salt gradient.

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Photographs of Jerome Vinograd and Matt Meselson. ( a ) Jerome Vinograd by “his” Spinco Model E analytical ultracentrifuge, serial no. 186. (Courtesy of the Caltech Archives.) ( b ) Matt Meselson at the controls for the UV optics and photography system of Model E no. 186 used for the classic experiment. (Courtesy of the Caltech Archives.)

The concept of equilibrium sedimentation in density gradients generated during the approach to equilibrium of a low molecular weight solute (e.g., CsCl) was elaborated by Meselson et al. ( 10 ) in a paper communicated to PNAS by Linus Pauling in May 1957. The figures in that paper and the theoretical calculations are essentially part I of Meselson's Ph.D. thesis, which, interestingly, provides no preview of the intent to apply density labeling to the study of DNA replication. The paper focuses instead on the nature of the band structure and the fact that the concentration distribution of a single macromolecular species in a constant density gradient should be Gaussian, and that the standard deviation of that band is then inversely proportional to the square root of the macromolecular weight. The model was remarkably correct, as tested with homogeneous DNA of known molecular weight from bacteriophage T4. This paper also documents the first analysis of the density distribution of DNA containing 5BU, obtained from T4-infected cultures of E. coli grown in media with this thymine analog. The 5BU fully substituted DNA molecules banded at a density of 1.8 g/cm 2 , whereas those of normal thymine-containing T4 bacteriophage DNA were well separated from these at 1.7 g/cm 2 . Although there was no mention of using this approach to study DNA replication, the application to study intact viruses and smaller molecules like proteins is discussed in this pioneering report on density gradient sedimentation.

The Classic Experiment

Matt and Frank were well on their way to design their landmark experiment on DNA replication. They might have used 5BU as the density label but they became concerned about the deleterious effects of its mutagenicity and cellular toxicity, as well as problems in obtaining uniform labeling, so they decided instead to use a synthetic growth medium in which the sole source of nitrogen was 15 NH 4 Cl.

The bacterium E. coli was grown for many generations in 15 NH 4 Cl medium so that the DNA would be essentially fully labeled with the heavy isotope 15 N. Then, the medium was diluted with a 10-fold excess of 14 NH 4 Cl as exponential growth continued. Samples were taken from the growing bacterial culture at various times to analyze the distribution of DNA densities in a CsCl gradient. There was initially a single band at the 15 N heavy DNA position, and then a second band began to appear at a position half way between the density of 15 N DNA and that of 14 N DNA. The parental 15 N band disappeared with time as this “hybrid” band formed. At precisely one generation (or division cycle), only the intermediate density hybrid band was present. It was then important, indeed essential, that the experiment was continued for a second generation, thereby to establish that when the hybrid DNA replicated in the 14 N medium, equal amounts of “light” and hybrid DNA were present at the completion of that second cycle. Thus, the hybrid DNA was continuously regenerated during replication and the amount of light DNA increased with each round of replication. There was the profound implication that the constant amount of hybrid molecules will be maintained “forever” as successive cell divisions continue.

At this point it was clear that “the nitrogen of a DNA molecule is divided equally between two subunits which remain intact through many generations,”... “the subunits are conserved,” and that each daughter molecule receives one parental subunit, according to the scheme shown in Fig. 3 [which is figure 5 in the Meselson–Stahl paper ( 1 )]. An essential requirement of the model is that the two subunits must separate. Meselson and Stahl ( 1 ) provided convincing evidence of this through thermal denaturation studies in which the DNA samples were kept at 100°C for 30 min in the CsCl before centrifugation. The hybrid DNA clearly resolved into two bands at the respective positions of heat-denatured 15 N-DNA and 14 N-DNA under these conditions. Furthermore, the broader Gaussian bands of the denatured DNA indicated a reduction of roughly one-half in the molecular weight from that of duplex DNA, as consistent with the view that the subunits are single DNA strands. Nevertheless, the conclusions were stated with cautious restraint, leaving open the questions about the nature of the molecular structures of the subunits and the relationship of these subunits to each other in a DNA molecule ( 1 ).

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Interpretation of what the density labeling data actually confirm in terms of a model for DNA replication (figure 5, from p. 677 of ref. 1 ). [Reproduced with permission from ref. 1 (Copyright 1958, PNAS).]

I first learned of the Meselson–Stahl experiment while I was a graduate student at Yale, when I attended the second annual meeting of the Biophysical Society, in Cambridge, MA, in early 1958. In a contributed-paper session, Matt was accorded two successive 15-min slots for his talk, as I recall, as the Chair announced that this next presentation was going to be of very special significance. It was indeed an exciting and generally convincing presentation: clearly the highlight of the meeting for me and for most others.

Caveats About the Proof

In Meselson's talk and in their PNAS paper, as noted above, Meselson and Stahl ( 1 ) were very careful about what they could actually claim from their experiment. Figure 5 in their paper implies no more and no less about what can be concluded, even though the most nonobvious and straightforward assumption is that the conserved “subunits” must be single DNA strands. Was the Meselson–Stahl experiment definitive proof for semiconservative replication of DNA? In principle, the answer is yes, but there were additional important controls to be carried out. Although it was by no means a favored interpretation of the results, it was technically possible that the conserved “subunits” of DNA constituted an end-to-end association of parental DNA with newly synthesized daughter duplex DNA “subunits,” rather than lateral association of parent and daughter DNA strands. This unlikely scenario was ruled out by Meselson's graduate student, Ron Rolfe, who showed that sonication to intentionally break the linear hybrid DNA into shorter lengths, did not alter the density of the DNA ( 11 ). Another unlikely scenario was promoted by Liebe Cavalieri et al. ( 12 ), who argued that the conserved “subunits” might be double-stranded DNA and that the hybrid DNA would then consist of the lateral association of two duplex DNA helices to form a four-stranded structure. Following up on several years of heated debate, definitive exclusion of the Cavalieri model was ultimately provided from the work of Robert Baldwin and Eric Shooter ( 13 ), who studied the melting of hybrid DNA, in which one subunit was labeled with 5BU. The melting profile was that expected for DNA in which the subunits were single strands rather than double helices. Meselson's graduate student at Harvard, John Menninger ( 14 ), had shown by low-angle x-ray scattering that the linear density of E. coli DNA corresponded to two chains rather than four.

Essential to the success of the Meselson–Stahl experiment was the fragility of the rigid linear DNA molecule, and the effect of extensive shearing of the DNA when it was handled; particularly as the sample was injected into the ultracentrifuge cell through a hypodermic syringe, now known to impose high shear. The molecular mass of the DNA fragments studied was only ≈7 × 10 6 Da. If the entire bacterial chromosome could have been isolated intact in these gradients, the interpretation of the results might have been more complicated, because there would have been a gradual shift with time of DNA from the parental to hybrid density, as sequential replication proceeded around the circular genome.

A few years after the classic replication paper was published, Meselson and Jon Weigle ( 15 ) used the combination of 15 N and 13 C to prepare heavily density-labeled λ phage DNA to determine whether recombination (with normal density λ phage) involved a “copy choice” mode or one of “breakage and reunion.” In other words, was there any parental DNA in recombinant phage? The answer was that both chromosomal subunits are broken during recombination and that recombination occurs by chromosome breakage (although other mechanisms were not excluded). These studies used preparative CsCl density gradient ultracentrifugation and the enhanced resolution afforded by using two density labels. However, the procedure to prepare the 13 C-labeled precursors was extremely tedious, until 13 C-labeled glucose became commercially available some years later. Stahl and colleagues ( 16 ) then used 15 N 13 C double labeling in a series of important studies to elucidate relationships between the processes of recombination and replication in λ phage. Their initial paper in this series ruled out the so-called master-strand model for replication, another unlikely alternative to the Watson–Crick scheme. It eventually became apparent that 5BU is a very convenient choice for density labeling DNA—for many reasons, including the fact that 5BU (fully replacing thymine) achieves a density shift roughly equivalent to the combined use of 15 N, 13 C, and deuterium, and at lower cost.

During my postdoc with Ole Maaløe in Copenhagen, in 1959, we found that if protein synthesis was inhibited in growing E. coli , then only a limited amount of DNA synthesis could occur, and we postulated that this constituted completion of those cycles of replication underway, without initiation of any new ones. The definitive proof of that hypothesis came from density labeling studies with 5BU—in which we showed that in the absence of protein synthesis only hybrid density DNA appeared during replication—thus, no second round could have been initiated to yield DNA molecules with 5BU in both strands ( 17 ). My studies with 5BU labeling prompted additional speculation about the detailed mode of DNA replication—why did one not observe DNA molecules in which replication forks had been caught midway? These molecules would be predicted to appear in the density gradient somewhere between the parental DNA density and that of the hybrid band.

Meeting Meselson

When I arrived at Caltech in September 1960 for my second postdoc (with Robert Sinsheimer), I immediately sought out Matt Meselson—and fortunately caught him for several short discussions before he departed in early 1961 for his faculty position at Harvard. We discussed the nature of the E. coli chromosome and Matt speculated that it might consist of short segments of DNA held together by some sort of protein “linkers” that could help with the topological unwinding problem. John Cairns ( 18 ) used tritium autoradiography several years later to provide evidence that the bacterial chromosome consisted of one intact closed circular molecule of DNA, and that DNA replication proceeded around the circle from one (or at most two) growing points. The conclusion that the chromosome consisted of double-stranded DNA was based on the contour length of the circle, compared with the cellular DNA content. The possibility of “linkers” between DNA segments could not be excluded, however, because of the low resolution of the technique.

I thought that a possible explanation for the lack of “intermediate” density DNA between parental and hybrid bands in the Meselson–Stahl experiment could be that the replication of a DNA “segment” was essentially “all or none”—it happened so rapidly that only a negligible fraction of the DNA segments might be caught in the act. However, my student, Dan Ray, and I ( 19 ) were able to isolate partially replicated DNA fragments from growing E. coli , by using 32 P pulse labeling along with 5BU incorporation, and a very gentle cell lysis procedure before preparative CsCl equilibrium sedimentation. After mild shearing of those fragments, the labeled DNA was resolved into hybrid and parental density bands, suggesting that the replication fork DNA might be unusually sensitive to breakage. The intermediate density 32 P pulse-labeled DNA fragments could also be chased into the hybrid band when excess 31 P was added to the growing cells ( 19 ). I then reasoned that if we could stall replication forks at obstructions in the template, we might stabilize and recover those partially replicated molecules for further analysis. My student, David Pettijohn, and I ( 20 ) examined the density distribution of DNA during labeling with radioactive 5BU in the period immediately after UV irradiation of the bacteria, to introduce cyclobutane pyrimidine dimers known to arrest DNA synthesis. We did indeed find a substantial amount of intermediate-density DNA but, curiously, there was also a significant amount of nascent DNA label at the parental density. Rebanding the parental density DNA in a second CsCl gradient verified the presence of 5BU-containing DNA with little or no evident density shift. The plausible explanation became apparent when I discussed our experiments with my former graduate mentor, Richard Setlow ( 21 ), who had just discovered that cyclobutane pyrimidine dimers are released from the chromosomal DNA in UV-resistant bacteria: he postulated an excision-repair scheme for damaged DNA. We were evidently observing the patching step in this putative process of excision repair, and the lack of a density shift was because of the fact that the patches synthesized by repair replication were too short to appreciably shift the density of the DNA fragments containing them. ( Fig. 4 ) Thus, the approach developed by Meselson and Stahl ( 1 ) to demonstrate semiconservative DNA replication was used to first document the “nonconservative” repair replication of damaged DNA ( 20 ). Intentional shearing of the “repaired” DNA by sonication did result in a measurable density shift, which, when combined with molecular weight determinations, could be used to estimate the patch size.

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Distinguishing semiconservative replication from nonconservative repair replication by using density labeling with 5BU.

As with the excision repair of damage (like cyclobutane pyrimidine dimers), the heteroduplex regions generated during genetic recombination were thought to provoke localized excision of a tract of nucleotides from one strand followed by repair synthesis to fill the gap. The excision repair of mismatched bases was also postulated, and Wagner and Meselson ( 22 ) obtained genetic evidence that, although well separated mismatches were repaired independently, sometimes those separated by <2,000 nt could be repaired by a single event, if these were on the same DNA strand.

The approach pioneered by Meselson and Stahl ( 1 ) continues to be widely used for research in the fields of DNA replication, recombination, and repair. It is the method of choice when one wishes to physically separate the newly synthesized DNA from DNA existing before an appropriate density label is introduced into a culture of growing cells or a replication system in vitro . It has become a classic approach for the biochemical detection of DNA strand exchange in recombination, although it does not approach the sensitivity of genetic analysis. Also, it is still used for the quantification of nucleotide excision repair in a variety of prokaryotic and eukaryotic cell systems. In a 1959 letter to Frank Stahl, Matt wrote that “CsCl has an inexhaustible number of golden eggs to lay.” That statement indeed has proved to be true.

This Perspective is published as part of a series highlighting landmark papers published in PNAS. Read more about this classic PNAS article online at www.pnas.org/misc/classics.shtml .

Abbreviation: 5BU, 5-bromouracil.

The Meselson-Stahl Experiment: “the Most Beautiful Experiment in Biology”

First Online: 24 November 2020

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Allan Franklin 3 &
Ronald Laymon 4

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In this chapter we will examine the Meselson-Stahl experiment which decided the issue between two models of DNA replication. The experimental result was held not to require replication. A second experiment of a rather different nature was used to eliminate a third possibility, and whereby contrast that experiment was further refined so as to yield a more accurate result. These experiments set the stage for the creation of new competing accounts of DNA replication all of which—given the framework established by the Meselson-Stahl experiments—were eliminated by experimental tests leaving only the Watson-Crick model in play.

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For an excellent history of this complex episode and complete references see Holmes ( 2001 ).

See Delbruck and Stent ( 1957 ) and Holmes ( 2001 , pp. 15–29, 393–394).

Often referred in textbooks as being N 14 labeled as opposed to unlabeled.

It should be noted that the consistent locations of the various bands as a function of their association with the labeling was determined and validated by the calibration procedure of superimposing the data images at generation points 0 and 1.9 as shown in the penultimate data row.

To see how this goes trace the replication paths indicated in Fig. 2.5 and note that at the end of the first generation there are now (as per the first part of the first conclusion) twice as many subunits are there were parental subunits to begin with. And then at the next generation there will again (as per the second part of the first conclusion) be twice as many progeny but where only one half will be half labeled. Thus, the number of half-labeled subunits at the end will be twice the number of parental subunits at the initial stage of the process.

In a later chapter on the discovery of the positron we’ll review another instance where the theoretical results were deduced from the experimental data (augmented by non-controversial theoretical assumptions) when Patrick Blackett made such a derivation in order to clearly demarcate what his experimental results demonstrated from their role in confirming Dirac’s hole theory of the production and nature of the positron.

Younger readers may need to be reminded that email was not always available.

For an extensive analysis of null experiments and the sense in which they satisfy demands for replication see Franklin and Laymon ( 2019 ).

There is, though, a caveat that we need to make. There emerged on the basis of further experimentation and theoretical development a realization that the sharpness of the separation bands reported by Meselson and Stahl was due in part to an unrecognized but uniformly consistent fragmentation of the DNA molecule that occurred during the preparation of the DNA samples. But the defect was not claimed in any way to lend experimental support for conservative replication, and moreover new experiments continued to support semi-conservative replication. For the details see Holmes ( 2001, pp. 395–397 ). As Thomas Kuhn would have described it, these further developments were part of normal science conducted in response to the Meselson and Stahl experiment understood as a paradigm (Kuhn 1962 , pp. 23–42).

There is an argument to be made even if it’s true that dispersive replication is not formally inconsistent with semi-conservative replication, the constraints imposed by semi-conservative replication on dispersive replication are so severe and restrictive that dispersive replication is effectively disconfirmed because of the evident difficulties that would be involved in creating a developed and specifically testable version of dispersive replication. Alternatively stated, once was enough for the Meselson and Stahl experiment to have effectively discouraged any further pursuit of dispersive replication as a promising approach to understanding the replication of DNA. Viewing dispersive replication as having been effectively disconfirmed in this way provides a charitable explanation why many of the textbooks claim that the Meselson and Stahl experiment disconfirmed not only conservative replication but also dispersive replication. See, for example (Lehninger 1975 , pp. 659–61) where it is claimed that the results of the Meselson and Stahl experiment “are exactly those expected from the hypothesis of semiconservative replication proposed by Watson and Crick; whereas they are not consistent with the alternative hypotheses of conservative or dispersive replication.”.

One can make the representation more realistic (in the sense of representing dispersion at multiple locations) by simply stacking the rectangular modules and making them of different height (and internal proportionality) to accommodate different dispersion locations. In which case, the argument for consistency with semi-conservative replication carries through as before.

Meselson and Stahl were here relying on a method of density and weight determination that they had developed along with Jerome Vinograd which was reported in Meselson et al. ( 1957 ).

For a brief review of this development see Holmes ( 2001, 394–395 ).

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Franklin, A., Laymon, R. (2021). The Meselson-Stahl Experiment: “the Most Beautiful Experiment in Biology”. In: Once Can Be Enough. Springer, Cham. https://doi.org/10.1007/978-3-030-62565-8_2

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Meselson and Stahl Experiment

Meselson and Stahl experiment gave the experimental evidence of DNA replication to be semi-conservative type . It was introduced by the Matthew Meselson and Franklin Stahl in the year 1958 . Matthew Meselson and Franklin Stahl have used E.coli as the “ Model organism ” to explain the semiconservative mode of replication.

There are three modes of replication introduced during the 1950s like conservative, semi-conservative and dispersive. The researchers were confused between these three that what could be the actual pattern of DNA replication. In 1958, Matthew Meselson and Franklin Stahl presented their research, where they concluded that the replication of DNA is semiconservative type .

Matthew Meselson and Franklin Stahl have conducted several experiments after the discovery of DNA structure (by the two scientists Watson and Crick ). Watson and Crick’s model is widely accepted to demonstrate the replicative model of DNA. We will discuss the definition, steps and observation of the Meselson and Stahl experiment along with the semi-conservative model of DNA.

Content: Meselson and Stahl Experiment

Semi conservation model of dna, meselson and stahl experiment steps, observation, definition of meselson and stahl experiment.

Meselson and Stahl Experiment gave us the theory of semi-conservative replication of DNA. They have taken E.coli as the model organism and two different isotopes, N-15 and N-14 . The N-15 is the heavier isotope, whereas N-14 is the lighter or common isotope of nitrogen. Meselson and Stahl performed their experiment by first growing the E.coli in the medium containing 15 NH 4 Cl for several generations. They observed that the heavy isotope has incorporated in the genome of E.coli and the cells become more substantial due to 15 N heavy isotope.

Meselson and Stahl then transferred the E.coli cells incorporated with 15 N isotope to the medium containing 14 NH 4 Cl for several generations. After every twenty minutes, the E.coli cells multiply. For the processing of DNA, the cells were centrifuged by the addition of Caesium chloride, resulting in the formation of the concentration gradient. As a result, light, intermediate and heavy DNA strands will get separated.

After completing their experiment, Meselson and Stahl concluded that after each cell division, half of the DNA would be conserved for every next generation. Therefore, this experiment proves that the DNA replication obeys the semi-conservative mode of replication in which 50% of the DNA conserve for every next generation in a way like 100%, 50%, 25%, and 12.5% and so on.

It is the type of DNA replication. The term semi means “ Half ” and conservative means “ To store ”. The semi-conservative DNA replication results in the two daughter DNAs after the parent DNA replication.

In the two daughter DNA’s, each strand will contain a mixture of the parent DNA’s template strand, and the other with a newly synthesized strand (in F-1 gen ). When the parental DNA replicates, half of the 100%, i.e. 50% of the DNA is conserved by having parent strand and the remaining 50% will produce newly synthesized strands.

After the F-1 gen, the multiplication of the cell will get double, which will produce four DNA strands (in F-2 gen ). In F-2 gen half of 50%, i.e. only 25% of DNA is conserved by having parental strands, and the remaining 75% will produce newly synthesized strands.

Growth of E.coli : First, the E.coli were grown in the medium containing 15 NH 4 Cl for several generations. NH 4 provides the nitrogen as well as a protein source for the growth of the E.coli. Here, the 15 N is the heavy isotope of nitrogen.
Incorporation of 15 N : After several generations of E.coli, Meselson and Stahl observed that the 15 N heavy isotope has incorporated between the DNA nucleotides in E.coli.
Transfer of E.coli cells : The DNA of E.coli labelled with 15 N isotope were transferred to the medium containing 14 NH 4 Cl . Here, the 14 N is the light isotope of nitrogen. The E.coli cells were again allowed to multiply for several generations. The E.coli cells will multiply every 20 minutes for several generations.
Processing of DNA : For the processing or separation of DNA, the E.coli cells were transferred to the Eppendorf tubes. After that, caesium chloride is added, having a density of 1.71 g/cm 3 (the same of DNA). Finally, the tubes were subjected to high-speed centrifugation 140,000 X g for 20 hours.

The result, after two generations of E.coli, the following results were obtained:

predictions for f1 gen made by meselson and stahl

In the F-1 generation : According to the actual observations, two DNA strands (with a mixture of both 15 N and 14 N isotopes) will produce in F-1 gen. The above diagram shows that the semiconservative and dispersive model obeys the pattern of growth explained by Meselson and Stahl.

Thus, it is clear that the DNA does not replicate via “Conservative mode”. According to the conservative model, the DNA replicates to produce one newly synthesized DNA and one parental DNA. Therefore, the conservative model was disapproved, as it does not produce hybrid DNA in the F-1 generation.

predictions for f2 gen made by meselson and stahl

In the F-2 generation : According to the actual observation, four DNA strands ( two with hybrid and the remaining two with light DNA ) will produce in the F-2 generation. The hybrid DNA includes a mixture of 15 N and 14 N. The light DNA strands contain a pure 14 N. The diagram shows that only semi-conservative type of replication gave similar results conducted by Meselson and Stahl. Thus, both the conservative and dispersive modes of replication were disapproved.

Therefore, we can conclude that the type of replication in DNA is “ Semi conservative ”. The offsprings have a hybrid DNA containing a mixture of both template and newly synthesized DNA in the semi-conservative model. After each multiplication, the number of offspring will double, and half of the parental DNA will be conserved for the next generation.

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An Elegant Experiment to Test the Process of DNA Replication: The work of Meselsohn and Stahl

by Nathan H Lents, Ph.D.

Did you know that many classic experiments in science are famous not because of their complexity but because of their simplicity? This is the case with Meselson and Stahl’s classic experiment in DNA replication. In one of the most famous experiments in molecular biology, Meselson and Stahl elegantly tested three scientific hypotheses with one simple design.

Around the world, there are thousands of scientists performing experiments at any given moment. Every once in a while, an experiment is performed and published that appears so clever, so important, and so successful in its goals, that it is destined to be cheered by scientists far and wide and taught in science classrooms for decades to come. However, with the passage of time, these so-called "classic experiments" seem more dramatic, ingenious, and clear than they were in their contemporary timing as the memories of complications, contradictions, and controversy fade. Nevertheless, what often sets apart these key experiments is not that they are especially complex, but that they are elegantly simple. The power of simplicity in an experiment is that it reduces the chance of alternative explanations for the results.

As explained in our module Theories Hypotheses , and Laws , a key feature of the modern scientific method is that valid scientific hypotheses make predictions that can be tested. Thus, the testing of predictions is a major part of scientific research , and part of the historic nature of many classic experiments is that they tested the predictions of a key scientific hypothesis in a way that provided a clear answer. The 1958 experiment by Matthew Meselson and Franklin Stahl is an example of such an experiment, and is one of the most famous in all of molecular biology. With one cleverly designed experiment, they tested the predictions of three different scientific hypotheses simultaneously, and the field of DNA biology was changed forever.

DNA replication

Following the discovery of DNA as the genetic material (see DNA I ), the new field of molecular biology focused intently on how DNA functions. One of the most important features of DNA is its ability to be copied accurately. When a cell , whether it is a yeast , a bacterium, or a human cell, divides in two, both resulting cells are genetically identical to each other and to the original parent cell. Thus, prior to division, a cell must somehow copy all of its DNA so that both resulting cells have the full complement of genetic material. Indeed, scientists such as Edwin Chargaff and others had observed that the amount of DNA in a cell doubles prior to cell division. The pool of DNA is then split equally between the two daughter cells, so that both have the same amount of DNA as the original parent cell had. But how exactly this DNA doubling takes place was at first a mystery, and scientists began to propose several possible mechanisms, or "models" of DNA replication?

Following the proposal and eventual acceptance of the Watson-Crick model of DNA structure, molecular biologists believed that each strand of DNA somehow served as a copy-template for the synthesis of a new DNA molecule (see our DNA II module for more information). However, conundrums remained. Most importantly, scientists had a difficult time envisioning how two strands of DNA that are immensely long and twisted around each other could separate from each other without resulting in the breakage of the strands or them becoming hopelessly entangled. In addition, scientists wondered how the two strands could be pulled apart given the enormous number of hydrogen bonds holding them together. Some envisioned replication as proceeding in short stretches, while others imagined a continuous process much like a zipper. This paradox even caused some prominent scientists of the day to doubt the double-helix structure of DNA altogether. Nevertheless, scientists began working on possible theoretical solutions to the separation of two intertwined DNA strands and by the late 1950s, three hypothetical models for DNA synthesis were being hotly debated: the conservative model, the semi-conservative model, and the dispersive model. Figure 1 below provides a diagram of each of these mechanisms.

Figure 1: Three competing models of DNA replication. This diagram shows the three competing models of DNA replication in the 1950s and 1960s.

Briefly stated, the conservative model of DNA replication holds that when DNA is replicated prior to cell division, one of the DNA double-strands receives all newly replicated DNA in both strands, while the other receives only the two original DNA strands in the parent cell. The semi-conservative model (also called the "zipper model" by James Watson), however, holds that the original two strands of DNA are split from each other and that the two daughter molecules are each comprised of one "old" strand of DNA from the parent cell, and one newly replicated strand. Finally, the dispersive model holds that the DNA is copied in short stretches and that both daughter DNA strands will receive a mixture of the original parental DNA and newly replicated DNA. Each of these three possible models had been proposed by different scientists and each had certain advantages in explaining the separation of the intertwined parental DNA. However, evidence to disprove or support any of the models was scarce.

Comprehension Checkpoint

Design of the Meselson and Stahl experiment

This changed when Matthew Meselson and Franklin Stahl, two scientists working at the California Institute of Technology (CalTech), constructed an ingenious experiment that tested all three models at the same time. To understand how this experiment worked, it is important to remember how atomic isotopes behave. Although a heavier isotope of a given atom behaves in a completely normal manner in chemical reactions , the presence of an extra neutron (or more) gives the atom a slightly higher atomic mass . As a result, molecules that contain these "heavy" isotopes are more dense . This small difference in density allows scientists to physically separate molecules with different isotopes based on the differences in their density.

For their experiment , Meselson and Stahl used a special form of nitrogen: 15 N. Normally, almost all of the nitrogen in any given cell is 14 N and thus contains seven neutrons in addition to its seven protons . So, 15 N, with eight neutrons, is considered "heavy nitrogen" (but it is not radioactive). When growing cells are fed heavy nitrogen, the 15 N isotope enters the cells' metabolism and significant amounts of it will be incorporated into the nitrogen-rich nucleotides and DNA . Thus, the DNA of cells grown with 15 N in their food source would be more dense than that of normal cells. The power of having DNA of different densities is that they can be separated by centrifugation .

For this procedure, cells were first broken open; then the cellular contents (called the "crude extract") were mixed with a solution of the heavy salt cesium chloride and placed in a centrifuge cell with clear quartz walls that allowed the solution to be photographed while spinning. The cell was then spun in a centrifuge at very high speeds for many hours and the heavy cesium ions were pulled towards the bottom of the cell by centrifugal force . Eventually, equilibrium was reached and a "density gradient" was established in the cell with the bottom containing the highest concentration of cesium and the top of the tube containing the lowest. Inside a density gradient like this, all the molecules from the cell extract, including the DNA , will "float" or "sink," migrating to the spot in the gradient that corresponds to their density. The most dense molecules will be pulled toward the bottom of the cell, while less dense molecules will settle higher in the cell, as shown in Figure 2.

Figure 2: The principle of density gradient centrifugation. When a liquid solution containing many large protein and DNA components is placed into a test tube or centrifuge cell and spun at high speed over many hours, the individual molecules separate based on their density. The most dense molecules fall to the bottom, the least dense remain at the top.

Before beginning their analysis of DNA replication , Meselson and Stahl first showed that DNA made with regular 14 N could be separated from DNA containing heavy 15 N. They accomplished this by growing two separate batches of Escherichia coli bacteria , feeding each batch a different nitrogen isotope . Then, they broke the bacterial cells open, mixed the extracts from both batches into one centrifuge cell, and spun it to establish the density gradient. To detect the DNA, they shined ultraviolet (UV) light on the spinning centrifuge cell because DNA absorbs UV light and thus casts a shadow during exposure of photographic film. Below, in Figure 3, is a black-and-white image of their data , and you can clearly see two bands of DNA, one lower in the cell and thus, more dense than the other.

Figure 3: Density gradient centrifugation of a mixture of 15 N-DNA and 14 N-DNA. Meselson and Stahl first showed that they can separate a mixture of DNA of the two different densities. The picture on the left is a UV photograph showing the banding of DNA of different densities following centrifugation. The graph on the right is a trace of the intensity of the bands in the picture.

How the experiment tested all three DNA replication hypotheses

Next, Meselson and Stahl did something interesting. They grew a large batch of bacteria in heavy nitrogen ( 15 N) and then switched the bacteria to a diet that contained only regular nitrogen ( 14 N). This allowed them to distinguish between pre-existing DNA from the parental cells and newly synthesized DNA, because any newly synthesized DNA strands would contain 14 N and be less dense . They used this experimental set up to put the three possible models of DNA replication to the test.

Like all proper scientific hypotheses , the three models of DNA replication each make certain predictions, and testing hypothetical predictions is a key part of scientific research . In the case of Meselson and Stahl's experiment , the predictions that each of these models makes are as follows. If the conservative model of DNA replication is true, then one would predict that the bacterial cells grown for one generation (20 minutes) with 14 N would have two different kinds of DNA: the original DNA would be the density of DNA grown with only 15 N nitrogen, while both strands of the new DNA molecules would be the lighter 14 N DNA band. However, if either the semi-conservative or the dispersive models of DNA replication are correct, the double-stranded DNA inside the bacteria after one generation would be a mixture of old and new DNA, and thus, one strand would be made of 15 N and one of 14 N DNA. Thus, this "hybrid" DNA would be an intermediate density halfway between the 14 N and 15 N bands of DNA. In Figure 4 below, you can see what the three models of DNA replication predict will happen in the Meselson and Stahl experiment, followed by what they actually observed.

Figure 4: Density gradient centrifugation of E. coli DNA after one cell division. Top panel: the three experimental predictions of three competing models of DNA replication. Bottom panel: The actual data. E. coli grown in 15 N DNA were switched to 14 N and then harvested at five different time points. The DNA was centrifuged resulting in the banding pattern shown here.

As you can see from their results above, after one generation of cell division, the total DNA of the growing bacterial cells had an intermediate density , halfway between that of 14 N and 15 N DNA. This strongly disproved the conservative model of DNA, which held that the original two strands of DNA would persist and remain bound to each other and a wholly new copy of two DNA strands would be synthesized. In other words, the conservative model of DNA replication would predict that, after one generation, half of the DNA molecules would have only 15 N DNA and half would have 14 N DNA, which would appear as two distinct bands of DNA density. But as seen above, all DNA molecules were of intermediate density. However, the two remaining models, semi-conservative and dispersive, were still consistent with these results. In order to examine the two remaining models of DNA replication, we must examine additional generations of bacterial growth with the normal 14 N isotope of nitrogen. In Figure 5 below are the predictions of all three models of DNA replication, over multiple generations:

Figure 5: Experimental predictions of three competing models of DNA replication over three generations.

And now, here are the actual observations of Meselson and Stahl (Figure 6):

Figure 6: Density gradient centrifugation of E. coli DNA over multiple generations. E. coli grown in 15 N DNA were switched to 14 N and then harvested at nine different time points. The DNA was centrifuged resulting in the banding pattern shown here.

As they let the bacterial cells grow and divide further, Meselson and Stahl observed that the 15 N DNA band disappeared, a band of 14 N DNA appeared and then got progressively darker, and a band of intermediate density appeared and persisted at about the same intensity. This strongly discredited the dispersive model of DNA replication , which predicted that only one band of DNA would exist and would get progressively less dense as the amount of 14 N DNA in the dispersive mixture increased with each generation . Thus, in one simple experiment with very clear results, Meselson and Stahl solidly disproved two of the possible models of DNA replication, while strongly supporting another.

"The most beautiful experiment in biology"

The scientific community agreed that this was powerful evidence in support of the semi-conservative model . John Cairns, one of the leading molecular biologists of the era, called it, "the most beautiful experiment in biology." To this day, the Meselson and Stahl experiment is taught around the world as a classic example of the modern scientific method of experimentation. With one simple design, three scientific hypotheses were tested by the observation/verification of their predictions.

Scientists now have a detailed understanding of the molecular events of DNA replication (see our DNA III module). These molecular events occur just as predicted by the Watson and Crick model of DNA structure, and verify that the semi-conservative model of DNA replication is indeed correct. The Meselson and Stahl technique of labeling DNA strands with nitrogen isotopes is still employed by scientists around the world as they continue to explore the mysteries and complexities of DNA, the genetic material of life.

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00:00:19:21 John Cairns said on the telephone in a very excited voice, 00:00:22:24 he had just read Mendel's papers 00:00:27:04 [Imitating Cairns] "and you know they are the most beautiful 00:00:29:22 experiments in Biology." 00:00:32:04 And I gasped and I said, "John, John, you can't say that." 00:00:37:10 You said the Meselson-Stahl experiment 00:00:40:18 was the most beautiful experiment in Biology. 00:00:44:02 [Imitating Cairns] "Oh, did I? 00:00:46:12 Well, I was wrong." 00:00:53:22 Watson and Crick didn't make a discovery. 00:00:56:15 They proposed a model. 00:00:59:05 There are those who believed this model must be true 00:01:01:21 because it was so beautiful. 00:01:04:02 And there were those who believed it must be wrong 00:01:06:18 because biology is complicated. 00:01:09:02 And this model is too simple to be right. 00:01:12:01 Would you say? 00:01:13:05 Exactly, yes, 00:01:14:14 But there was no experimental proof of it. 00:01:17:01 They had a model which made a distinct prediction 00:01:20:18 about how DNA replicates and it needed to be tested. 00:01:24:11 And it's fun to test the hypothesis. 00:01:29:03 We agreed that we were going to work together 00:01:32:08 to figure out whether or not it was right. 00:01:38:23 When Frank and I showed semi-conservative replication, 00:01:43:03 It wasn't just a model, it was something real like that. 00:01:47:08 After our experiment, 00:01:48:18 it was now widely accepted that their model, 00:01:52:06 Watson-Crick model is right. 00:01:54:19 So it became the building block. 00:01:58:00 You might say for all of biology. 00:02:00:00 Yeah. 00:02:04:05 From very early childhood, I mean, practically infancy. 00:02:07:21 I loved science. 00:02:08:22 I loved to put wires together to make little radios, 00:02:13:24 which I could put under my pillow. 00:02:15:11 So my parents wouldn't know that I was listening 00:02:17:24 to them all night long. 00:02:19:21 And then I was very interested to know what makes life work. 00:02:24:08 And there had been, I think, in the house, 00:02:29:22 maybe even in my bedroom, that painting by Michelangelo, 00:02:36:02 God up high and Adam below and they're touching fingers. 00:02:42:11 I don't know if there is a spark. 00:02:45:08 I don't think there's a spark in the picture, 00:02:47:04 but I'm not sure. 00:02:48:15 But to me that meant that life is somehow electrical. 00:02:53:10 God is providing life through a spark. 00:02:57:14 And for some reason that made me interested in 00:02:59:12 electrochemistry. 00:03:01:19 Unlike Matthew, 00:03:02:20 I had no particularly strong interest in science as a youth. 00:03:09:11 What was understood that when I graduated from high school, 00:03:14:02 I would apply to the Naval Academy. 00:03:17:01 That's what my mother had in mind because she thought 00:03:19:09 I would look good in dress whites. 00:03:22:08 But then of course, World War II broke out. 00:03:25:01 So that plan changed fast and she decided 00:03:28:13 I should just go to college. 00:03:30:19 I think I was just too young to understand 00:03:33:13 the courses in humanities. 00:03:35:03 I hadn't had enough life experience to get a grip 00:03:38:23 on the questions they were even thinking about. 00:03:41:23 Science on the other hand was concrete. 00:03:44:17 Children can grasp science and among the sciences, 00:03:50:05 biology was the most appealing. 00:03:52:09 There, the fun was that you could figure out puzzles. 00:03:56:24 That is, there was a rational, concrete, 00:03:59:09 quantitative explanation for what you saw. 00:04:03:03 You could reason backwards as to what must be going on. 00:04:07:10 And that intrigued me enough to know that genetics 00:04:10:14 was something perhaps I could do. 00:04:20:19 I had the great, good luck to become Linus Pauling's 00:04:23:21 last graduate student. 00:04:25:09 His daughter was having a party at their swimming pool 00:04:28:12 and I'm in the water. 00:04:29:09 And Pauling comes out, the world's greatest chemist. 00:04:32:16 I'm all naked, practically, in a bathing suit. 00:04:36:13 And he's all dressed up with a jacket and a vest 00:04:40:04 and a neck tie. 00:04:41:11 And he looked down at me, 00:04:42:09 "Well, Matt, what are you gonna do next year?" 00:04:45:00 And I had already signed up 00:04:46:02 to go to the committee on mathematical biophysics 00:04:52:02 and Linus just looked down at me and he said, 00:04:54:00 "But Matt, that's a lot of baloney. 00:04:56:04 Come be my graduate student." 00:04:58:13 And so if I hadn't taken his course 00:05:00:19 on the nature of the chemical bond, 00:05:03:01 I would have had a very different life. 00:05:04:14 I wouldn't have met Frank, 00:05:06:19 I wouldn't be sitting here, that's for sure. 00:05:10:02 At the end of my PhD exam, 00:05:12:12 as we were walking out of the little exam room, 00:05:14:10 Linus Pauling turned to me and he said, 00:05:16:03 "Matt, you're very lucky you're entering this field 00:05:18:13 just at the right moment." 00:05:19:19 Yeah. 00:05:20:15 At the very beginning. 00:05:22:20 (upbeat music) 00:05:28:13 The first year of my being a graduate student at Caltech, 00:05:31:19 I wanted to get into biology. 00:05:33:17 I was a chemist and I thought the way to do that 00:05:36:12 would be to study molecular structure. 00:05:39:16 The only person who was looking at biology 00:05:42:01 from that point of view, other than Linus Pauling himself 00:05:45:10 was Max Delbruck. 00:05:47:03 He had a fearsome reputation. 00:05:49:13 Nevertheless, I got up my courage and went to see him. 00:05:52:10 He's not a fearsome creature at all really. 00:05:54:11 And the first thing he said was, 00:05:56:15 what do you think about these two papers 00:05:58:12 from Watson and Crick? 00:06:00:11 I said, I'd never heard of them. 00:06:03:01 I was still in the dark ages, 00:06:05:22 and he yelled at me. 00:06:07:08 He said, "Get out 00:06:08:04 and don't come back till you've read them." 00:06:13:12 There were two separate ideas that came together. 00:06:16:07 Crick's idea about how the base pairs linked onto the chains 00:06:21:22 and Jim's idea about how the base pairs were structured. 00:06:26:16 So there are four different building blocks in DNA, 00:06:29:07 adenine, thymine, guanine, and cytosine. 00:06:31:19 The surfaces of the G and the C are complementary 00:06:35:13 to each other and of the A and T are complementary 00:06:39:08 to each other so that they can fit together. 00:06:43:05 The way fingers would fit into a glove. 00:06:46:04 And importantly, when they put G opposite C, 00:06:50:21 the distance of the outside was exactly the same 00:06:55:07 as if they'd put A opposite T. 00:06:58:09 No other combination would give such a regular structure. 00:07:02:03 It was a gorgeous insight. 00:07:05:21 And then from that, 00:07:07:01 they made a hypothesis about how DNA is replicated. 00:07:11:02 It involved the two chains coming apart 00:07:15:12 and each one acting as a template for the synthesis 00:07:20:07 of a new chain on its surface. 00:07:24:12 When it's all done, here we have the two old chains, 00:07:27:24 each one now associated with a brand new chain. 00:07:32:08 What Watson and Crick proposed 00:07:34:07 was enormous stimulus to experimentation. 00:07:37:24 It was irresistibly beautiful. 00:07:39:21 Irresistibly beautiful. 00:07:42:22 Jim Watson was at Caltech the year after 00:07:46:19 he and Francis published their papers. 00:07:49:24 And so I got a chance to talk a lot with Jim then, 00:07:53:12 and that coming summer he was going to go 00:07:56:00 and teach the physiology course at Woods Hole. 00:08:01:16 I was a graduate student at Rochester at the time. 00:08:05:06 My chairman of the department who was also on my committee, 00:08:09:05 said I had to take a course in physiology. 00:08:12:18 And I said, the physiologist teacher here is a jerk. 00:08:15:13 I'll be damned if I'll take his course. 00:08:17:19 Well, send him to Woods Hole 00:08:19:23 to take the physiology course there. 00:08:22:16 And by serendipity, Jim Watson happened to be there 00:08:25:19 with some kid named Meselson hanging along with him. 00:08:29:19 We found that we had in fact deep, common interests. 00:08:33:22 I realized this is a guy who's really very smart 00:08:36:18 and I can learn a lot from him. 00:08:38:15 I remember a haze of beach parties, 00:08:42:03 lectures that I slept through 00:08:45:08 Well it was a kind of paradise. 00:08:48:04 The most interesting people in molecular biology. 00:08:51:08 Most of them were there. 00:08:52:22 So that's how we met. 00:08:54:10 And then it turns out Frank is coming that very September 00:08:57:07 to Caltech. 00:08:58:14 It would be a year from then I would come. 00:09:00:06 Are you sure? 00:09:01:03 Yep. 00:09:02:07 I still hadn't finished my thesis- 00:09:03:03 So I had to wait for a whole year before I saw you again? 00:09:05:18 That's right. 00:09:06:14 He said, when you get to Caltech we'll test Jim's idea. 00:09:11:13 What do you think about testing Jim's idea 00:09:13:22 of how DNA replicates? 00:09:15:19 And then he explained that to me, 00:09:17:21 I'd already heard about it and he explained it to me 00:09:20:23 and I absolutely - I committed, totally. 00:09:24:11 And then when Frank finally got there 00:09:26:22 and I wanted to start right away, he forbade it. 00:09:30:17 Why? 00:09:31:14 He said it would be bad for my character 00:09:34:06 to not complete my x-ray crystallography 00:09:37:09 before starting something new. 00:09:40:17 This tells you a lot about Frank's character. 00:09:46:18 With the Watson and Crick model, 00:09:48:16 the underlying question of course was, 00:09:50:23 was that really the right mechanism? 00:09:53:00 The famous Max Delbruck said no, no, no, no, 00:09:56:19 that model can't be right. 00:09:57:19 And he proposed a different model. 00:10:00:03 As Delbruck put it forth, 00:10:02:01 breaks are introduced in the parental molecule 00:10:05:14 as it's being replicated 00:10:07:13 and then carefully sealed up in certain ways. 00:10:10:16 Others proposed one in which 00:10:12:12 the original DNA molecule stays intact. 00:10:16:01 And the new DNA molecule is made of all new DNA. 00:10:21:14 So there were three targets out there 00:10:24:18 that in principle could be distinguished, 00:10:27:00 if you could trace the fate of the old chains, 00:10:30:11 what becomes of the two old chains. 00:10:32:16 And one step led to the next, really. 00:10:35:09 I mean, the first idea was using density somehow, 00:10:39:10 which is not a very good idea yet, 00:10:41:07 except it leads you to the next one. 00:10:43:03 Matt's idea from the very beginning 00:10:45:10 was that somehow stable isotopes could be used. 00:10:50:01 That would be incorporated into the DNA 00:10:53:02 and impart upon the DNA, a different density. 00:10:56:10 You grow bacteria in a medium, 00:10:59:03 which instead of having this ordinary isotope of nitrogen 00:11:03:11 N14, you can buy nitrogen 15 ammonium chloride, 00:11:09:24 the heavy kind. 00:11:12:11 And if you grow the bacteria for a number of generations, 00:11:15:13 you can be sure that essentially all of the DNA 00:11:19:12 is labeled with heavy nitrogen, good. 00:11:23:08 Now, we resuspend those cells in a medium that just has 00:11:26:13 ordinary, nitrogen 14, the light one. 00:11:30:12 And now the question is as the DNA molecules replicate, 00:11:34:23 how will the heavy nitrogen from those parent molecules 00:11:39:04 be distributed amongst the daughter molecules 00:11:42:19 that are produced in successive duplications? 00:11:46:13 Then some sensitive method for separating DNA, 00:11:50:24 according to its density would be devised. 00:11:54:18 I ran across an article about the centrifugation 00:11:58:06 of cesium chloride solution 00:11:59:24 to measure the molecular weight. 00:12:02:01 If the DNA was in there with the cesium, 00:12:05:17 it would find its position in the density gradient. 00:12:09:07 If it was heavy DNA, 00:12:11:15 it would tend to be down near the bottom of the tube 00:12:14:02 where the cesium was concentrated and the density was high. 00:12:18:14 If the DNA was light DNA, made of light isotopes, 00:12:22:18 it would be higher up in the tube. 00:12:29:00 You could think about it this way. 00:12:30:21 If you jumped into the Great Salt Lake, 00:12:32:21 as we all know you float, 00:12:34:15 you go right to the top because you are less dense 00:12:38:00 than the water. 00:12:38:21 But if you have a bathing suit with pockets in it, 00:12:41:10 and you stuffed some lead weights in your pockets, 00:12:44:22 you'll sink down. 00:12:46:05 Cause you're more dense than the water. 00:12:48:22 Now imagine that the salt in the Great Salt Lake 00:12:51:22 is not uniformly distributed, 00:12:54:09 but is concentrated near the bottom 00:12:57:12 and rather less concentrated near the top. 00:13:01:12 Now, if you put just the right number of heavy weights 00:13:04:13 in your pocket, you won't float because you'll be too dense. 00:13:09:05 You won't float at the top and you won't go all the way 00:13:11:13 to the bottom because you're not dense enough. 00:13:14:03 You'll instead come to rest somewhere, 00:13:16:13 halfway between the top and the bottom, 00:13:19:01 you will have found your place in that gradient. 00:13:23:17 And that's the very basis by which the experiment 00:13:26:12 finally worked and worked so beautifully. 00:13:29:01 And then it was just a question of looking 00:13:30:22 in the centrifuge while it's running. 00:13:33:24 And when it reaches equilibrium to see where 00:13:37:07 the heavy and light DNA are. 00:13:39:02 All the makings were there, 00:13:40:13 then to do the experiment itself, 00:13:42:21 it was obvious that the experiment was going 00:13:45:08 to give an answer. 00:13:46:23 Driving it all was the fact that Frank 00:13:49:14 wanted to know how life works. 00:13:57:12 Yeah, yeah. 00:14:01:14 [Mumbles] 00:14:04:09 I don't know that drove it all but- 00:14:07:14 Each person is trying to come up with something 00:14:09:21 as a gift to the other guy. 00:14:12:02 That's true. 00:14:12:23 I think 00:14:13:19 That's true 00:14:14:15 So it becomes a very connected 00:14:17:02 relationship because the next day you want 00:14:20:12 to have something to offer. 00:14:23:09 Matt was ready to step out into an area, 00:14:26:22 pretty heavily uncharted, 00:14:29:23 to answer an important question. 00:14:32:08 And the pieces had to be built as he went along. 00:14:36:15 (upbeat music) 00:14:40:18 The prediction of the Watson and Crick model, 00:14:43:04 was the two parent chains come apart. 00:14:45:00 Each one makes a new daughter molecule 00:14:47:01 and that's replication. 00:14:48:16 So that would predict that after exactly one generation, 00:14:52:16 when everything has doubled in the bacterial culture, 00:14:56:02 that you'd find the DNA molecules all have one old strand, 00:15:01:12 which is labeled heavy. 00:15:03:01 And one new strand, 00:15:05:01 which is labeled light and therefore their density 00:15:08:01 should be halfway between fully heavy and fully light, 00:15:12:06 that would be the prediction for what you see 00:15:15:07 at exactly one generation. 00:15:17:08 What do you predict to see for the next generation? 00:15:20:17 Well, each molecule would, again, separate its chains. 00:15:24:08 One of which is heavy. 00:15:26:00 The other of which is light and the only 00:15:28:14 growth medium available is light growth medium. 00:15:32:08 Then the light chain would make another light chain 00:15:35:04 to go with it, a complement. 00:15:37:05 The heavy chain would make another, 00:15:39:15 a light chain to go with it. 00:15:42:00 So after two generations you have DNA, 00:15:45:11 half of which is half heavy. 00:15:47:13 And the other half of which is all light 00:15:53:13 And fantastically, 00:15:55:05 that's exactly the result that one could see. 00:16:02:24 In order to say that the Watson-Crick model 00:16:06:08 fits the data very well, but the other two models do not, 00:16:10:14 we have to see what they'd predict. 00:16:12:23 Start with the Dispersive Model. 00:16:15:10 After one generation, 00:16:17:02 the two molecules resulting would indeed be half heavy, 00:16:21:10 but in the next generation, 00:16:23:10 there would be a subsequent dispersion of the label. 00:16:26:18 So you'd be getting molecules that were 00:16:29:20 three quarters light, and one quarter heavy. 00:16:34:22 And in each generation, 00:16:36:10 the molecules would get lighter and lighter. 00:16:39:16 The fully Conservative Model simply imagined that duplex DNA 00:16:44:16 fully heavy now, somehow created the appearance of a fully 00:16:51:09 light duplex molecule in which both chains 00:16:54:07 are made of light DNA. 00:16:59:15 Most of the times when you get an experimental result, 00:17:03:19 it doesn't speak to you with such clarity. 00:17:07:19 These pictures of the DNA bands interpreted themselves. 00:17:18:14 It felt like a...supernatural. 00:17:21:22 It felt like you were in touch with the gods 00:17:24:10 or something like that. 00:17:25:16 I remember I presented this result that summer 00:17:29:16 early in the summer in France at a phage meeting, 00:17:33:23 complete with the photographs 00:17:36:02 of the density gradient bandings. 00:17:39:05 And at the end of it, 00:17:41:17 I stopped and there was total silence and somebody said, 00:17:45:16 "Well, that's it." 00:17:53:21 The intellectual freedom at Caltech. 00:17:56:02 We could do whatever we wanted. 00:17:58:00 It was very unusual for such young guys 00:18:00:19 to do such an important experiment. 00:18:02:23 So suddenly, whereas before that, 00:18:05:12 like Max would be talking with Sinsheimer 00:18:08:00 about the genetic code. 00:18:09:23 And before we did our experiment, 00:18:11:10 I was definitely not - at least 00:18:13:11 I felt I wasn't - supposed to be at those discussions. 00:18:16:15 But afterwards, I could be a full member. 00:18:20:06 We had this wonderful house, 00:18:21:15 big house across the street from the lab. And our roommates, 00:18:26:19 we all, 00:18:27:15 we talked about these experiments at almost every dinner. 00:18:31:01 So we had this wonderful intellectual atmosphere, 00:18:35:11 John Drake, Howard Temin. 00:18:38:00 Why are you frowning? 00:18:39:06 He told the dirtiest jokes I've ever heard. 00:18:41:04 No that was Roger Milkman. 00:18:42:20 [Crosstalk] 00:18:45:11 Positions one and two. 00:18:46:18 That's true, that's true, that's true. 00:18:49:05 So it was a very lively, intense, friendly atmosphere. 00:18:56:12 It was lively enough and conveniently located enough 00:19:00:12 that over time we had visits from William O. Douglas, 00:19:05:17 Judge Douglas. 00:19:06:13 Judge Douglas of the Supreme Court 00:19:08:11 And here Dick Feynman probably one of the world's greatest 00:19:13:13 physicists at that time, 00:19:15:04 or maybe ever, palled around with us. 00:19:17:17 He came over to our big house and played his drums, 00:19:22:00 sat down on the floor, played the drums. 00:19:24:21 I'm just a graduate student 00:19:26:03 and he's the world's greatest physicist, 00:19:29:17 but that's what it was like. 00:19:30:22 It was a very friendly wide open place. 00:19:33:10 Frank and I are very lucky. 00:19:36:20 The way I think of it is that there's a river, 00:19:39:17 which is a period of time when the fundamental things, 00:19:42:21 the structure of DNA, how replication happens, 00:19:45:10 the genetic code. 00:19:47:06 And then, when these problems are solved. 00:19:50:22 There are lots of little rivulets. 00:19:51:18 The river divides into thousands of branches 00:19:55:16 using these fundamental insights into how life works 00:20:00:16 and applying them to specific questions, 00:20:03:14 questions of disease etc. 00:20:07:09 So to me, with some exceptions, 00:20:11:21 this was a really interesting time 00:20:14:00 when it was still a big river. 00:20:16:04 Also, now you can cut this out, 00:20:19:04 but also the Meselsons, Matt's parents, were kind enough to 00:20:22:14 keep the liquor cabinet fully stocked at all times. 00:20:29:22 (upbeat music) 00:20:49:15 My throat is a little bit? 00:20:51:22 I have a cough drop 00:20:54:06 (whispering) I don't want a cough drop. I want a non-alcoholic beer. 00:20:58:21 I require a margarita. 00:21:00:22 I've worked for the CIA. 00:21:04:08 I vaporized many people, including many of your friends, 00:21:08:16 Big black beard, 00:21:09:19 and blew out some of his pipe smoke and still 00:21:12:20 holding his pipe stem in his teeth said, 00:21:15:01 "Oh Matt history is just what people think it was."

General Public
Educators of H. School / Intro Undergrad

Cohesin and Genome Organization: Jan Michael Peters

Talk Overview

Matt Meselson and Frank Stahl were in their mid-20s when they performed what is now recognized as one of the most beautiful experiments in modern biology. In this short film, Matt and Frank share how they devised the groundbreaking experiment that proved semiconservative DNA replication, what it was like to see the results for the first time, and how it felt to be at the forefront of molecular biology research in the 1950s. This film celebrates a lifelong friendship, a shared love of science, and the serendipity that can lead to foundational discoveries about the living world.

Please head to the Science Communication Lab’s website for more films like this along with educator resources, full video transcript, and most up to date content.

Speaker Bio

Frank stahl.

Frank Stahl received his PhD at the University of Rochester, where he studied genetic recombination in phage. He performed postdoctoral studies at Caltech, during which he completed the famous Meselson-Stahl experiment, and joined the faculty at the University of Oregon in Eugene in 1959. He is now an emeritus faculty member who enjoys teaching and… Continue Reading

Matthew Meselson

Dr. Meselson has made important contributions to the areas of DNA replication, repair and recombination as well as isolating the first restriction enzyme. Currently, he is Professor of Molecular and Cellular Biology at Harvard University, where his lab studies aging in the model organism bdelloid rotifers. Meselson is also a long-time advocate for the abolition… Continue Reading

More Talks in Genetics and Gene Regulation

Related Resources

Meselson M. and Stahl F. The replication of DNA in Escherichia coli . PNAS July 15, 1958 44 (7) 671-682.

Teaching resources from XBio: How DNA Replicates

Sarah Goodwin (Wonder Collaborative): Executive Producer Elliot Kirschner (Wonder Collaborative): Executive Producer Shannon Behrman (iBiology): Executive Producer Brittany Anderton (iBiology): Producer Derek Reich (ZooPrax Productions): Videographer Eric Kornblum (iBiology): Videographer Rebecca Ellsworth (The Edit Center): Editor Adam Bolt (The Edit Center): Editor Gb Kim (Explorer’s Guide to Biology): Illustrations Chris George: Design and Graphics Maggie Hubbard: Design and Graphics Marcus Bagala: Original music Samuel Bagala: Original music

Reader Interactions

ANGELA DIXON says

February 8, 2021 at 9:39 pm

Thank you – I cannot tell you how much I enjoyed this video. It was as if I was sitting in Dr. Stahl’s living room, having a conversation with these two great scientists. What an elegant experiment! You have really captured the essence of two incredible scientists in this video.

Marieke Mackintosh says

March 23, 2021 at 12:08 pm

Thank you for sharing this incredible footage of these brilliant human beings. What a joy it is to watch them reminisce and teach. I cannot wait to show this to my students.

Neeraja Sankaran says

August 30, 2022 at 7:00 am

Hi.. this is not a comment except to say that this is a beautiful video. Could you give me the full citation please, I’d like to include it in a bibliography

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Replication

We know that existing cells transfer genetic information to the new cells during cell division. What do you think will happen if each new cell receives only half the set of genetic information? Yes, the cell will not be able to function properly. This is why the DNA in an existing cells needs to be doubled (or copied) so that each new cell gets the full set of instructions. This process is DNA replication. Let’s learn more about it.

The Machinery Of DNA Replication

Dna polymerase.

It is the main enzyme needed for DNA replication. It uses DNA as a template to catalyze the polymerization of deoxynucleotides. DNA Polymerases should have the following properties:

Highly efficient to carry out polymerization of a large number of nucleotides in a very short time. The process of replication in E. coli , that has only 4.6 x 10 6 bp (base pairs) is completed within 18 minutes. This indicates that the average rate of polymerization is approximately 2000 bp per second.
High degree of accuracy since any mistake will lead to mutations.

Deoxyribonucleoside triphosphates

They act as substrates for polymerization. Replication is also a very energy-expensive process. Therefore, the triphosphates also provide energy for polymerization.

Origin of replication

Replication cannot start randomly at any place in DNA. It starts at a specific place in E.coli DNA called the ‘origin of replication’.

Replication fork

Since it will require a lot of energy to separate the two DNA strands for their entire length, replication usually starts within a small opening in the DNA helix. This is the ‘replication fork’. DNA polymerase can function only in one direction 5′ → 3′. Therefore, replication of the DNA strand with polarity 3′ → 5′ is continuous .

This is also called the ‘leading strand’. While on the DNA strand with polarity 5′ → 3′, it is discontinuous and results in the formation of fragments of new DNA. This strand is the ‘lagging strand’.

This is the enzyme that joins the discontinuously synthesized fragments of DNA. In eukaryotes, DNA replication occurs in the S phase of the cell cycle. Moreover, the replication of DNA and cell division should be highly coordinated. If a cell fails to divide after DNA replication, it results in polyploidy – a condition in which a cell has more than the normal number of chromosomes.

DNA Replication [Wikimedia Commons]

Solved Example For You

Q1: In the Meselson and Stahl experiment, if DNA was extracted after 60 minutes in the 14 NH 4 Cl medium, what would be the DNA densities?

75% light and 25% intermediate
25% light and 75% intermediate
50% light and 50% intermediate
15% light and 85% intermediate

Sol: The answer is option ‘a’. After 60 minutes, i.e. three generations in the 14 NH 4 Cl medium, the extracted DNA will show 75% light density and 25% intermediate density.

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Molecular Basis of Inheritance

Gene Expression
Human Genome Project and DNA Fingerprinting
Translation
The Genetic Material
Transcription

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Department News

Matthew meselson’s “beautiful experiment” turns fifty.

September 22, 2008

Matthew Meselson

Fifty years ago, Matthew Meselson and Franklin Stahl – a Caltech grad student and post-doc, respectively – published an experiment in which they proved that DNA replication occurs when each strand copies itself to produce two identical daughter molecules, each a hybrid of old and new.

The now-famous experiment was a validation of the double-helix model of DNA, which had been proposed five years before by James Watson and Francis Crick, but was still being hotly debated. Brilliantly conceived and expertly executed, the Meselson-Stahl work has been called “the most beautiful experiment in biology.” Its details are a textbook example of scientific creativity and rigor marking the beginning of the era of molecular biology. The technique Meselson invented to separate DNA and other macromolecules—density-gradient centrifugation—is a mainstay in molecular biology labs today.

Meselson, now Thomas Dudley Cabot Professor of the Natural Sciences at Harvard’s Department of Molecular and Cellular Biology, was just 28 when he and Stahl conducted their experiment. “No matter what the age, the feeling at the time that you make a discovery, when you realize it and it crystallizes in your mind, is euphoric, of course,” Meselson says. “But then it’s like a good meal. It’s wonderful to remember, but you do get hungry again. And so it’s something you’d like to keep doing.”

Meselson’s encores came as he worked with the other greats of the early days of molecular biology, making one discovery after another about the fundamental processes of life. With Francois Jacob and Sydney Brenner, he used density-gradient centrifugation to demonstrate the existence of messenger RNA. Later, he showed that genetic recombination results from the breaking and rejoining of DNA molecules, discovered methyl-directed DNA mismatch repair, and isolated the first restriction enzyme. These discoveries, as well as his work as an advisor to the government and an activist against chemical and biological weapons, have brought him many honors, including the Albert Lasker Award for Special Achievement in Medical Science in 2004.

To commemorate the 50th anniversary of Meselson-Stahl, MCB conferred the first Meselson Award at its annual retreat in September. The annual prize will go to the graduate student who has performed the most beautiful experiment of the past year. At the event, Meselson will offer recollections of his early life in science, and Stahl will join the proceedings teleconference.

“I still remember when I first heard about the Meselson-Stahl experiment and I was amazed,” MCB Chair Catherine Dulac said. “It’s a lesson in the process of coming up with a meaningful approach to an important scientific problem, and it will be good for our students to hear about that.”

How They Did It

The idea for the Meselson-Stahl experiment had its roots in a slightly different problem that Meselson encountered as a graduate student. Jacques Monod gave a lecture on the induction of the beta-galactosidase enzyme activity by lactose in E.coli. It was unknown at the time whether exposure to the inducer activated a pre-existing enzyme, or if it triggered synthesis of new protein. Meselson came up with the idea to grow the E.coli in heavy water, letting the deuterium isotope of hydrogen label all the proteins. Then, by inducing the cells in regular water and measuring the density of the resulting enzyme activity, he could determine if the protein was newly-made.

That experiment was never done, but it provided the blueprint for the later DNA work, which really began when Meselson met Stahl at the Marine Biological Laboratory at Woods Hole in the summer of 1954. Meselson was a teaching assistant in James Watson’s physiology course; Stahl was a student in the course. A year later, Stahl landed at Caltech as a postdoc, and the two of them began to work together on methods for density separation of macromolecules. Meselson invented equilibrium density-gradient centrifugation using cesium chloride, a technique that enabled the DNA experiment by allowing the determination of very small density differences among large biomolecules.

For the pivotal experiment, the two men grew bacteria in a medium containing the heavy isotope of nitrogen (15N) for many generations until all the DNA was heavy, and then switched the organisms into media containing the lighter isotope, 14N. After the switch, Meselson and Stahl found that the DNA of the daughter cells had an intermediate density, suggesting that heavy parental DNA strands went one each to the daughters, paired with a newly synthesized light strand.

The inspiration for the experiment, and others that followed, Meselson says, was DNA itself. The double helix, he says, told researchers what to do. “The two complementary chains said ‘this is how I replicate.’ The sequence of nucleotides said ‘this is how I carry information.’ The tautomeric base forms said ‘this is how I mutate.’ The double helix was like a director, setting the agenda for research. We just had to go out and do it.”

An Interest in Sex

Meselson joined the Harvard faculty in 1960, where he has continued his long and successful search for answers, but also for compelling questions. In recent years, he has been considering the problem of sex, or more specifically, the reasons for sex. The fact that most asexual organisms are evolutionary dead-ends implies that sexual reproduction has an essential benefit that outweighs its costs. The dogma holds that sexual reproduction, and the recombination that occurs between homologous chromosomes during meiosis, is essential in shuffling the genetic deck and providing the variation upon which natural selection acts.

In 1989, Meselson heard about the bdelloid rotifers, a large and highly successful group of microscopic aquatic animals who reproduce without sex, and have been doing so for millions of years. All females, the bdelloids have been called an “evolutionary scandal.” To Meselson, they offered a unique chance to understand the advantages of sex.

What his work on the bdelloids revealed, however, is stranger even than a life with no sex. Meselson and his Harvard colleagues Irina Arkhipova and Eugene Gladyshev recently found that the rotifer genome is packed with foreign DNA, taken from plants, bacteria and even fungi. The work, published last May in Science, is important because it shows that the rotifers are not exactly genetic virgins—they incorporate external DNA into their own genomes to an extent that has not been seen before in any other animal.

How? The answer could lie in the rotifers’ unusual lifestyle. The bdelloid rotifer can survive a complete drying-out at any stage of life. This resistance to desiccation may be related to another unusual feature of the rotifers that Meselson and Gladyshev recently discovered: The bdelloids are also highly immune to the killing effects of radiation. One reason is that the animals are experts at DNA repair. Meselson believes, but has not proven, that desiccation causes the same kind of DNA damage as radiation. If so, it’s possible that, during dry periods, rotifer DNA is fragmented; foreign genes could slip in and eventually be incorporated by the DNA repair enzymes.

However, sitting in a puddle picking up random DNA is not sex. Sex involves the swapping of different versions of the same gene by homologous recombination. To see if that can happen in rotifers, the researchers are now attempting to demonstrate desiccation-induced homologous recombination of rotifer genes in the laboratory. They are also comparing the DNA sequences of closely related bdelloids collected from around the world. If otherwise quite different strains show up with regions of identical sequence, a situation called allele-sharing, that will be a sure sign of homologous gene transfer, and therefore, sexual reproduction.

If it turns out that the bdelloids have substituted desiccation for sexual reproduction, Meselson says, that would give strong support to the view that sexual reproduction, or at least some form of homologous recombination between individuals is essential for reproductive success.

A Question of Aging

The rotifer’s resistance to desiccation and radiation may also provide clues to human aging and disease. Protection from radiation in other organisms comes from a robust defense against reactive oxygen species, which damage DNA and proteins and have been blamed for aging and aging-related diseases including cancer and Alzheimer’s disease.

Meselson wonders if bdelloids are resistant to radiation and desiccation, too, because they have developed a super-scavenger that mops up reactive oxygen species before they can damage proteins. The next thing to do, he says, is to isolate this hypothetical scavenger, and see, “Is it some common old thing like glutathione or is it some super elixir of youth? This scavenger question for me is like a gift from the muse, because it’s yet another very interesting thing to work on.”

If there were a fountain of youth, one might be forgiven for thinking the 78-year-old Meselson is keeping it to himself. He has just returned from a stint working in the lab of his friend Miroslav Radman in Paris, is running his own lab at Harvard, conducts experiments at Woods Hole, and is in the middle of writing up a grant proposal on his bdelloid work.

Just before the MCB retreat, Meselson travelled to Bath, England, to accept yet another honor, the Mendel Medal from the Genetics Society of the United Kingdom. The award makes a fitting bridge from old to new: Past winners include his mentor Max Delbruck and his colleagues Jacob, Brenner, Watson and Crick. His lecture, at a special symposium on the evolution of sexual and asexual reproduction featured his current work. With a schedule like this, who’s got time for retirement?

View Matthew Meselson’s Faculty Profile

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Meselson–Stahl Experiment

Discovery and history, the background of replication of dna, semi-conservative replication , the dispersive hypothesis, the conservative hypothesis, the semi conservative, procedure and protocol followed by meselson and stahl, results and observation , ruling out conservative hypothesis, ruling out dispersive hypothesis, importance and application of meselson and stahl experiment, what did meselson-stahl experiment show, why were 14 n and 15n used in meselson and stahl’s experiment, how did meselson and stahl turn e.coli dna into heavy dna, what is the semiconservative model of dna replication.

The Meselson-Stahl experiment was a groundbreaking scientific study conducted in 1958 by Matthew Meselson and Franklin Stahl. This experiment provided strong evidence supporting the theory of semi-conservative DNA replication, which was proposed by James Watson and Francis Crick.

Read more about an Introduction to DNA Replication

After the brilliant work of describing the DNA structure, Watson and Crick also proposed a hypothesis that the DNA replication process is semi-conservative. This hypothesis was strengthened by the experiment of Meselson and Stahl in which they elucidated the nature of replication of DNA.

The cell was first discovered to be dividing by Hugo Von Mohl in 1835. Later, cell division was filmed and captured by Kurt Michel in 1943, who was renowned for micro-cinematography. It became evident that when the cell divides the nucleus also divides, and whatever material is present in the nucleus also undergoes division. So, the DNA present in the nucleus must undergo division. This division of DNA was named DNA replication. There were a lot of other hypotheses explaining the nature of DNA replication, but the Meselson-Stahl Experiment stood out the most as it supported the already existing hypothesis of Watson and Crick.

DNA is the basic code of life. When the cell divides it also divides and is transferred to the daughter cells. Just like the production of daughter cells, there are production daughter DNA strands. This production is simply the copying of the hereditary material and is called DNA replication. The process begins at specific sites where there is a characteristic nucleotide sequence.

Several enzymes have been found to assist in this process most important of which is DNA polymerase. This enzyme is of a single type in the case of eukaryotes while there are three types (DNA polymerase I, II, and III) in the case of prokaryotes.

The DNA polymerase I is relatively a short sized enzyme that plays only a supportive role, but DNA polymerase III is the main enzyme that causes replication in E. coli. DNA polymerase III has a large size, almost 10 times larger than DNA polymerase I. It moves along the DNA strand, adding nucleotides at a very quick rate of 1000 per second. However, DNA polymerase cannot itself initiate DNA replication. It requires the enzyme primase to create RNA primer, a complementary sequence of 10 nucleotides. DNA polymerase III identifies this and starts further adding nucleotides to complete the new daughter strand.

In this replication, it is cleared that when the DNA helix replicates it unwinds itself and turns out to create two strands. This unwinding is done by Helicase enzyme. One of the parent strands act as a template strand and on this, the complementary base pair attach to form a new stand. After the formation of this new strand or daughter strand, they wind together to form a supercoiled DNA double Helix with the help of topoisomerase enzymes. The same process happens with the other parent strand of DNA.

Thus, it was concluded that the new copies or replicas of the parent DNA helix consist of half (one of the strands) of the parent DNA. That is why it is called semi-conservative as it contains fifty percent of older or parent DNA.

Hypotheses Regarding DNA replication

Three hypotheses were tried to elucidate the DNA replication process up to their own extent.

The Dispersive hypothesis
The Semi-conservative Hypothesis

This hypothesis was based on the model, which was put forward by Max Delbruck, a German – American biophysicist. He introduced the concepts of physics at the molecular level in biology. According to the dispersive hypothesis, the parent DNA is digested into around ten or more segments. These segments are used as templates to make new segments. The old and new segments then mix and coil to form the daughter DNA strands.

Simply, this model suggested that the parent DNA strand would disintegrate, and the new strands formed will be the blend of the newly formed nucleotides (made by polymerase enzymes) and older nucleotides (from the parent DNA).

This hypothesis considers the DNA a template as a whole. The histone proteins bind to the DNA and revolve around it, producing a new DNA copy. In simple terms, the DNA duplex will remain the same. It will not be unwound and will create completely new copies.

As described earlier, it supports the hypothesis of Watson and Crick. Each of the parent DNA strands acts as a template and the new bases or nucleotides will form. This means it will be half new and half old i.e. semi-conservative replication.

In all of the above hypotheses, there is a prognosis about how the parent DNA would distribute itself. In essence, the conservative hypothesis tells us that the parent DNA will not mix up with the daughter DNA, but it will retain its identity. However, the daughter DNA might be new, but it will have the same sequence as the parent DNA.

The semi conservative hypothesis tells us that after the replication process the new DNA duplex formed will have fifty percent of the parent material and fifty percent new formed or daughter material (nucleotides), as complete single strands.

Whereas, the dispersive theory explains that the whole DNA duplex will divide itself in about 10 nucleotides in size and the old fragments will join randomly with the newly formed fragment or more precisely the nucleotides.

Experimental Process and Protocol

All three hypotheses were assessed by Mathew Meselson and Franklin Stahl. They performed experiments in California. As their experiment was highly based on the studies and hypotheses of Watson and Crick. They realized that DNA is made up of nucleotides. These nucleotides are further comprised of a phosphate group, deoxyribose sugar and most importantly nitrogenous base.

The nitrogenous bases are present in each nucleotide of DNA. All the bases present in those nucleotides have nitrogen. They realized it would be greatly helpful to tag the parent DNA.

The best way to tag it was to change the nitrogen atoms present in the parent DNA. They used a nitrogen isotope to make a difference between the daughter and parent strand. Due to this remarkable use of isotopes and biophysics, this experiment is regarded as the most beautiful experiment.

The experiment began with the preparation of a culture media for the bacteria. The microorganism chosen was E. coli . The culture media consisted of NH4CL and 15 N isotope of nitrogen. This isotope of nitrogen has a higher molecular weight than the normal existing nitrogen isotope 14 N.

When the E. coli was allowed to grow in the culture containing high molecular weight isotope of nitrogen, the bacteria started to incorporate 15 N atoms of nitrogen into its DNA. So, after several generations, the DNA of the bacteria grown in 15 N culture becomes denser than the normal bacteria grown in normal culture.

The other E. coli ware grown in a normal culture media containing the common isotope of nitrogen, the 14 N nitrogen. Thus, the DNA of this bacterial culture was less dense than 15 N culture media.

After completing enough generations, they transferred the bacteria from the 15 N media to 14 N media. They allowed the bacteria containing 15 N in their DNA to grow in a culture containing 14 N nitrogen.

They obtained DNA from the mixture and dissolved it in solution containing cesium chloride. Later, they centrifuged this mixture at high speed i.e. using the ultracentrifugation technique. The high centrifugal forces cause the ions of CsCl to produce a density gradient. The gradient is produced as the ions migrate to the bottom of the tube due to ultra-centrifugation. A constant density is established throughout the solution. The DNA samples start to float in the solution. After keeping the tube at rest for a while, the different DNAs of different densities maintain a specific position according to their densities.

As the DNA with 15 N isotope is heavier, it finds its place at the bottom of the tube. On the other hand, the DNA with 14 N isotopes of nitrogen finds its place on an elevated level in the tube. However, a type of DNA is also found in between the elevated level and bottom level.

Both the scientists concluded that if conservative hypothesis were to be true, then there would be DNAs of only two densities. As the parent DNA would have maintained its integrity and gave birth to a new daughter strand. However, they found three types of densities regarding DNA in their experiment.

According to the semi-conservative replication hypothesis, there would be a hybrid density DNA. As one strand will be of 14 N and one strand will be of 15 N isotope. However, concerning dispersive hypothesis the new DNA formed will be of intermediate density just like the semi-conservative. But, each of the strands from the double helix will not solely contain either 14 N or 15 N nitrogen, it will be a mixture of both isotopes.

To rectify these confusions, Meselson and Stahl sampled the second replication of 15 N bacteria in the 14 N medium. They followed the same procedure and protocol and put the DNA in the test tube with Cesium Chloride and established a density gradient.

It was found that there were DNAs of two different densities, unlike the first time when there were three. One of the two had intermediate density just like in the previous experiment and the second one had the density of pure 14 N isotope.

This result did not support the dispersive hypothesis of replication as it would have given one DNA having a density lower than the intermediate density found in the experiment.

This experiment provided modern biology with the knowledge of DNA replication. This knowledge granted the vision to peek into hereditary diseases and disorders. It is a simple yet provoking experiment that has been accepted by a lot of scientists.

Despite the affirmative result provided by the experiment of the Meselson-Stahl, it took a few years for acceptance by the scientific community. The Meselson and Stahl experiment did not only provide evidence for the semi-conservative theory which was put forward by Watson and Crick, this experiment also confirmed the Watson and Crick model of DNA structure. Thus, it strengthened the standpoint of Watson and Crick which was taking years to get accepted.

In 1952, a scientist studied the experiment by implementing it on the cancer researches. He did not negate the semi-conservative replication hypothesis. This further supported the experiment.

However, the only thing that fell short in this experiment was the proper explanation about the DNA subunits. This discovery of Meselson and Stahl allowed the scientists to discover the mode of transmission of DNA and follow up on the genetic disorders.

This experiment is simply based on tagging the DNA and separating them based on their densities relative to the solution created by using Cesium Chloride.

It was a known fact by then that DNA is made up of nucleotides and these nucleotides contained nitrogen.

Meselson and Stahl utilized the common nitrogen by replacing it with a heavier isotope so that they can identify the parent and daughter DNA in solution by mixing the DNAs of different densities.

They experimented using simple techniques as ultra-centrifugation and density grading.

This experiment is a great milestone in modern biology as those were early times when atomic physics was applied in biological studies. Meselson and Stahl made three major outcomes.

DNA is made up of two strands, which was based on the Watson and Crick Model
If the parent DNA has two strands, then each of the strand acts as a template and retains its integrity and forms a daughter strand. Thus, the semi-conservation is evident
Each of the parent DNA strand is shared by two daughter DNA.

In any case, the investigation helped researchers to clarify legacy by indicating how DNA saves hereditary data through all the progressive DNA replication cycles as a cell grows, divides and repeats the cycle.

Frequently Asked Questions

Meselson-Stahl’s experiment confirmed that DNA replicates semi-conservatively and ruled out the other proposed mechanisms of DNA replication.

Nitrogen is one of the abundant elements in DNA structure. Meselson and Stahl used the isotopes of N14 and N15 in the experiment to incorporate them into the DNA of newly growing organisms and separate DNAs of different densities to observe the mode of DNA replication.

Meselson and Stahl cultured E.coli in 15NH4Cl containing medium over many generations. As a result, 15N was incorporated into the bacterial DNA, converting it into heavy DNA.

According to this model, both strands of DNA segregate in the replication process, and each strand acts as a template to synthesize a new daughter strand. Resultingly, daughter DNA contains one parent strand and one new strand.

John Cairns to Horace F Judson, in The Eighth Day of Creation: Makers of the Revolution in Biology (1979). Touchstone Books, ISBN 0-671-22540-5 . 2nd edition: Cold Spring Harbor Laboratory Press, 1996 paperback: ISBN 0-87969-478-5 .
Watson JD, Crick FH (1953). “The structure of DNA”. Cold Spring Harb. Symp. Quant. Biol. 18 : 123–31. doi : 10.1101/SQB.1953.018.01.020 . PMID 13168976 .
Bloch DP (December 1955). “A Possible Mechanism for the Replication of the Helical Structure of Desoxyribonucleic Acid” . Proc. Natl. Acad. Sci. U.S.A. 41 (12): 1058–64. doi : 10.1073/pnas.41.12.1058 . PMC 528197 . PMID 16589796 .
Delbrück M (September 1954). “On the Replication of Desoxyribonucleic Acid (DNA)” (PDF). Proc. Natl. Acad. Sci. U.S.A. 40 (9): 783–8. doi : 10.1073/pnas.40.9.783 . PMC 534166 . PMID 16589559 .

DNA Replication ( Edexcel A (SNAB) A Level Biology )

Revision note.

DNA Replication

Before a (parent) cell divides, it needs to copy the DNA contained within it
Doubling the DNA ensures that the two new (daughter) cells produced will both receive full copies of the parental DNA
The DNA is copied via a process known as semi-conservative replication (semi = half)
The process is called this because in each new DNA molecule produced, one of the polynucleotide DNA strands (half of the new DNA molecule) is from the original DNA molecule being copied
The other polynucleotide DNA strand (the other half of the new DNA molecule) has to be newly created by the cell
Therefore, the new DNA molecule has conserved half of the original DNA and then used this to create a new strand

The importance of retaining one original DNA strand

It ensures there is genetic continuity between generations of cells
In other words, it ensures that the new cells produced during cell division inherit all their genes from their parent cells
Replication of DNA and cell division also occurs during growth

Semi conservative replication of DNA

Semi-conservative replication

DNA replication occurs in preparation for mitosis , the number of DNA molecules in the parent cell must be doubled before mitosis takes place
DNA replication occurs during the S phase of the cell cycle (which occurs during interphase , when a cell is not dividing )
The enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands to form two single polynucleotide DNA strands
Each of these single polynucleotide DNA strands acts as a template for the formation of a new strand made from free nucleotides that are attracted to the exposed DNA bases by base pairing
The new nucleotides are then joined together by the enzyme DNA polymerase
The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule
This method of replicating DNA is known as semi-conservative replication because half of the original DNA molecule is kept ( conserved ) in each of the two new DNA molecules

DNA Polymerase

These nucleotides are known as nucleoside triphosphates or ‘activated nucleotides’
The extra phosphates activate the nucleotides, enabling them to take part in DNA replication
The bases of the free nucleoside triphosphates align with their complementary bases on each of the template DNA strands
The enzyme DNA polymerase synthesises new DNA strands from the two template strands
It does this by catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands
DNA polymerase cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides)
Hydrogen bonds then form between the complementary base pairs of the template and new DNA strands

Process of Semi-Conservation Replication 1

Nucleotides are bonded together by DNA polymerase to create the new complementary DNA strands

Leading & lagging strands

DNA polymerase can only build the new strand in one direction (5’ to 3’ direction)
As DNA is ‘unzipped’ from the 3’ towards the 5’ end, DNA polymerase will attach to the 3’ end of the original strand and move towards the replication fork (the point at which the DNA molecule is splitting into two template strands)
This means the DNA polymerase enzyme can synthesise the leading strand continuously
This template strand that the DNA polymerase attaches to is known as the leading strand
The other template strand created during DNA replication is known as the lagging strand
On this strand, DNA polymerase moves away from the replication fork (from the 5’ end to the 3’ end)
This means the DNA polymerase enzyme can only synthesise the lagging DNA strand in short segments (called Okazaki fragments)
A second enzyme known as DNA ligase is needed to join these lagging strand segments together to form a continuous complementary DNA strand
DNA ligase does this by catalysing the formation of phosphodiester bonds between the segments to create a continuous sugar-phosphate backbone

The synthesis of the complimentary strand occurs differently on the leading and lagging strands of DNA

Meselson and Stahl’s Experiment

Scientists were unsure if DNA replication was conservative or semi-conservative
Two scientists called Matthew Meselson and Franklin Stahl , showed that DNA replication was semi-conservative by experimenting with isotopes of nitrogen

Meselson and Stahl's Experiment

DNA contains nitrogen in its bases
As the bacteria replicated, they used nitrogen from the broth to make new DNA nucleotides
After some time, the culture of bacteria had DNA containing only heavy ( 15 N) nitrogen
This showed that the DNA containing the heavy nitrogen settled near the bottom of the centrifuge tube
If conservative DNA replication had occurred, the original template DNA molecules would only contain the heavier nitrogen and would settle at the bottom of the tube, whilst the new DNA molecules would only contain the lighter nitrogen and would settle at the top of the tube
If semi-conservative replication had occurred, all the DNA molecules would now contain both the heavy 15 N and light 14 N nitrogen and would therefore settle in the middle of the tube (one strand of each DNA molecule would be from the original DNA containing the heavier nitrogen and the other (new) strand would be made using only the lighter nitrogen)
The DNA from this second round of centrifugation settled in the middle of the tube, showing that each DNA molecule contained a mixture of the heavier and lighter nitrogen isotopes
If more rounds of replication were allowed to take place, the ratio of 15 N: 14 N would go from 1:1 after the first round of replication, to 3:1 after the second and 7:1 after the third

Meselson and Stahl's experiment that showed bacterial DNA replicated via semi-conservative DNA replication

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The Experimental Proof Of DNA Replication

The process by which cells duplicate their genetic material during cell division—the replication of DNA—was still largely a mystery. This sparked a race to understand how DNA replication happens among several well-known experts. The experimental evidence of DNA replication, which showed that DNA replication is a semi-conservative process, was one of the most important advances in this science.

Meselson and Stahl Experiment

Before we dive into the details of the Meselson and Stahl experiment, let’s first understand why it was such an important experiment in the field of molecular biology. At the time of the experiment, there was a lot of debate about how DNA replication occurred. There were three main hypotheses: conservative, semi-conservative, and dispersive . The conservative model suggested that the original DNA molecule remained intact during replication, with the newly synthesized DNA molecules consisting entirely of new nucleotides. The dispersive model suggested that the original DNA molecule was broken down and its nucleotides were randomly distributed between the newly synthesized DNA molecules. The semi-conservative model, on the other hand, proposed that each newly synthesized DNA molecule consisted of one original strand and one newly synthesized strand. The Meselson and Stahl experiment provided evidence in support of the semi-conservative model and helped to establish the basic mechanism of DNA replication

In 1958, Matthew Meselson and Franklin Stahl conducted a groundbreaking experiment that provided evidence for the semi-conservative nature of DNA replication. Their experiment involved growing E. coli bacteria in a medium containing a heavy isotope of nitrogen, N-15. This heavy isotope is incorporated into the DNA nucleotides as the bacteria grow and divide, resulting in a DNA molecule with a higher density than normal DNA.

Here’s a step-by-step breakdown of Meselson’s experiment:

E. coli bacteria were grown in a medium containing N-15 as the sole nitrogen source, allowing the bacteria to incorporate the heavy isotope into their DNA molecules.
The bacteria were then transferred to a medium containing a lighter isotope of nitrogen, N-14. This allowed the bacteria to begin replicating their DNA using the lighter nitrogen isotope.
After one round of replication, the DNA was extracted from the bacteria and subjected to a process called density gradient centrifugation . This technique separates molecules based on their density by subjecting them to a centrifugal force.
The DNA was loaded onto a tube containing a gradient of a heavy substance called cesium chloride (CsCl). The CsCl gradient allowed the DNA molecules to settle at the point in the tube where their density matched that of the surrounding CsCl.
The DNA was then spun at a high speed in the centrifuge, causing the DNA molecules to move through the CsCl gradient until they settled at their equilibrium density.
Meselson and Stahl observed that the DNA formed a single band in the tube, indicating that all the DNA molecules had the same density.
This result was unexpected, as it was predicted that if DNA replication was conservative, then all the DNA molecules would have either the heavy isotope or the light isotope, resulting in two distinct bands in the CsCl gradient.

The result of Meselson’s experiment led him to conclude that DNA replication is not conservative, but rather semi-conservative. This means that each strand of the original DNA molecule serves as a template for the synthesis of a new complementary strand, resulting in two DNA molecules, each with one original and one newly synthesized strand.

DNA Replication

DNA replication is the process by which cells make an exact copy of their genetic material before cell division. This process ensures that each daughter cell receives a complete set of genetic information. The replication of DNA is a complex process that involves several enzymes and other molecules.

Semi-Conservative Model

The mechanism of DNA replication was first proposed by Watson and Crick in 1953. They proposed that DNA replication is a semi-conservative process, meaning that each daughter DNA molecule contains one original strand and one newly synthesized strand. This model was supported by experiments conducted by Matthew Meselson and Franklin Stahl in 1958.

DNA Replication Fork

The site of DNA replication is called the replication fork. The replication fork is a Y-shaped structure that forms when the two strands of DNA are separated. At the replication fork, DNA synthesis occurs in both directions, creating two replication bubbles.

FAQs on Meselson and Stahl Experiment

Q1: how was the experimental proof of dna replication obtained.

The experimental proof of DNA replication was obtained through a series of experiments by researchers such as Meselson and Stahl, in which they used isotopes of nitrogen to trace the replication process.

Q2: What is the significance of DNA replication?

DNA replication is essential for the transmission of genetic information from one generation to the next. It ensures that each daughter cell receives an identical copy of the genetic material and maintains the genetic stability of the organism.

Q3: What is the role of enzymes in DNA replication?

Enzymes play a crucial role in DNA replication. They are responsible for unwinding the DNA double helix, separating the parent strands, synthesizing new strands of DNA, and proofreading the newly synthesized strands to ensure that they are accurate copies of the parent strands.

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Mode of DNA replication: Meselson-Stahl experiment
The experiment done by Meselson and Stahl demonstrated that DNA replicated semi-conservatively, meaning that each strand in a DNA molecule serves as a template for synthesis of a new, complementary strand. Although Meselson and Stahl did their experiments in the bacterium E. coli, we know today that semi-conservative DNA replication is a ...
Semi-Conservative DNA Replication: Meselson and Stahl
Replication is the process by which a cell copies its DNA prior to ... Meselson and Stahl's Elegant Experiment. ... Meselson and Stahl opted for nitrogen because it is an essential chemical ...
Meselson-Stahl experiment
The Meselson-Stahl experiment is an experiment by Matthew Meselson and Franklin Stahl in 1958 which supported Watson and Crick's hypothesis that DNA replication was semiconservative.In semiconservative replication, when the double-stranded DNA helix is replicated, each of the two new double-stranded DNA helices consisted of one strand from the original helix and one newly synthesized.
The Meselson And Stahl Experiment on DNA Replication
Meselson and Stahl Experiment. Meselson and Stahl Experiment was an experimental proof for semiconservative DNA replication. In 1958, Matthew Meselson and Franklin Stahl conducted an experiment on E.coli which divides in 20 minutes, to study the replication of DNA. Semi conservative DNA Replication through Meselson and Stahl's Experiment.
How DNA Replicates
You can also hear Matt Meselson describing the Meselson-Stahl experiment in Video 1. Video 1 Matt Meselson describes his experiment with Frank Stahl on DNA replication. The genetic material of eukaryotic cells is organized in the form of chromosomes , each a single linear, double-stranded DNA molecule ( Figure 1 ).
The Meselson-Stahl Experiment (1957-1958), by Matthew Meselson and
In an experiment later named for them, Matthew Stanley Meselson and Franklin William Stahl in the US demonstrated during the 1950s the semi-conservative replication of DNA, such that each daughter DNA molecule contains one new daughter subunit and one subunit conserved from the parental DNA molecule. The researchers conducted the experiment at California Institute of Technology (Caltech) in ...
Meselson-Stahl Experiment
DNA containing a mixture of 15 N and 14 N ends up in an intermediate position between the two extremes. By spinning DNA extracted at different times during the experiment, Meselson and Stahl were able to see how new and old DNA interacted during each round of replication. The beauty of this experiment was that it allowed them to distinguish ...
Classics: Meselson and Stahl: The art of DNA replication
Matthew Meselson and Franklin Stahl's experiments on the replication of DNA, published in PNAS in 1958 ( 2 ), helped cement the concept of the double helix. Meselson, a graduate student, and Stahl, a postdoctoral researcher, both at the California Institute of Technology (Pasadena), gave validity to a model that many scientists saw as ...
Density matters: The semiconservative replication of DNA
A comprehensive historical description of the collaboration between Meselson and Stahl, the milieu in which they worked, and their remarkable path to success was prepared by the late Frederic Lawrence Holmes and titled Meselson, Stahl, and the Replication of DNA: A History of "the Most Beautiful Experiment in Biology" ( 2).This account highlights the personalized side of the story and ...
Density matters: The semiconservative replication of DNA
I first learned of the Meselson-Stahl experiment while I was a graduate student at Yale, when I attended the second annual meeting of the Biophysical Society, in Cambridge, MA, in early 1958. ... We were evidently observing the patching step in this putative process of excision repair, and the lack of a density shift was because of the fact ...
PDF DNA Replication
Meselson-Stahl experiment has been called "the most beautiful experiment in biology." The key to the Meselson-Stahl experiment was growing cells ... bond to form and for the process to proceed on to the addition of the next nucleotide. Because nucleotides are added at the 3' end of the growing strand, DNA synthesis proceeds in a 5'-to-3 ...
Semi-conservative replication (video)
DNA replication has three possible methods: conservative, dispersive, and semi-conservative. Here, we focus on the Meselson-Stahl experiment, which proved DNA replication is semi-conservative. In this process, each new DNA pair consists of one old strand and one new strand, ensuring accurate genetic information transfer. Created by Efrat Bruck.
Semiconservative replication
This process is known as semi-conservative replication because two copies of the original DNA molecule are produced, each copy conserving (replicating) the information from one half of the original DNA molecule. ... Meselson-Stahl experiment using isotopes to discover semiconservative replication. Multiple experiments were conducted to ...
The Meselson-Stahl Experiment: "the Most Beautiful Experiment in
We begin with the theoretical and experimental background that provided the motivation and context for the Meselson-Stahl experiment. Footnote 1 In 1953 Francis Crick and James Watson proposed a three-dimensional structure for deoxyribonucleic acid (DNA): two polynucleotide chains helically wound about a common axis (Watson and Crick 1958).This was the famous "Double Helix" where the ...
Meselson and Stahl Experiment
Meselson and Stahl experiment gave the experimental evidence of DNA replication to be semi-conservative type.It was introduced by the Matthew Meselson and Franklin Stahl in the year 1958.Matthew Meselson and Franklin Stahl have used E.coli as the "Model organism" to explain the semiconservative mode of replication. There are three modes of replication introduced during the 1950s like ...
An Elegant Experiment to Test the Process of DNA Replication
The 1958 experiment by Matthew Meselson and Franklin Stahl is an example of such an experiment, and is one of the most famous in all of molecular biology. With one cleverly designed experiment, they tested the predictions of three different scientific hypotheses simultaneously, and the field of DNA biology was changed forever. DNA replication
The Most Beautiful Experiment: Meselson and Stahl • iBiology
Talk Overview. Matt Meselson and Frank Stahl were in their mid-20s when they performed what is now recognized as one of the most beautiful experiments in modern biology. In this short film, Matt and Frank share how they devised the groundbreaking experiment that proved semiconservative DNA replication, what it was like to see the results for ...
Replication: Concepts, Meselson and Stahl Experiment, Videos ...
In 1958, Matthew Meselson and Franklin Stahl performed the following experiment to prove this mode of replication: Experiment. Meselson and Stahl grew E.coli on a medium that contains 15 NH 4 Cl as the only nitrogen source for many generations [Note: 15 N is the heavy isotope of nitrogen].
Matthew Meselson'S "Beautiful Experiment" Turns Fifty
Matthew Meselson. Fifty years ago, Matthew Meselson and Franklin Stahl - a Caltech grad student and post-doc, respectively - published an experiment in which they proved that DNA replication occurs when each strand copies itself to produce two identical daughter molecules, each a hybrid of old and new. The now-famous experiment was a validation of the double-helix model of DNA, which had ...
The Most Beautiful Experiment: Meselson and Stahl
Matt Meselson and Frank Stahl share the story of their groundbreaking experiment from 1958 that definitively showed semiconservative DNA replication.Matt Mes...
Meselson-Stahl Experiment
The Meselson-Stahl experiment was a groundbreaking scientific study conducted in 1958 by Matthew Meselson and Franklin Stahl. This experiment provided strong evidence supporting the theory of semi-conservative DNA replication, which was proposed by James Watson and Francis Crick. Read more about an Introduction to DNA Replication.
DNA Replication
Meselson and Stahl's Experiment. Bacteria were grown in a broth containing the heavy (15 N) nitrogen isotope. DNA contains nitrogen in its bases; As the bacteria replicated, they used nitrogen from the broth to make new DNA nucleotides; After some time, the culture of bacteria had DNA containing only heavy (15 N) nitrogen; A sample of DNA from the 15 N culture of bacteria was extracted and ...
Meselson and Stahl Experiment-DNA Replication
The mechanism of DNA replication was first proposed by Watson and Crick in 1953. They proposed that DNA replication is a semi-conservative process, meaning that each daughter DNA molecule contains one original strand and one newly synthesized strand. This model was supported by experiments conducted by Matthew Meselson and Franklin Stahl in 1958.

Three Proposed Models for DNA Replication

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DNA Replication and Meselson And Stahl's Experiment

Meselson and Stahl Experiment

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How DNA Replicates

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What Events Preceded the Experiment?

Setting Up the Experiment

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Dig Deeper 1: Alternatives to Semi-Conservative replication

Dig Deeper 2: The idea for using density for separation

Dig Deeper 3: The role of the centrifuge in the Meselson–Stahl experiment

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The Meselson-Stahl Experiment (1957–1958), by Matthew Meselson and Franklin Stahl

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The Meselson-Stahl Experiment: “the Most Beautiful Experiment in Biology”

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An Elegant Experiment to Test the Process of DNA Replication: The work of Meselsohn and Stahl

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The Most Beautiful Experiment: Meselson and Stahl

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