hershey and chase phage experiment

Hershey and Chase Experiment

Hershey and Chase experiment give practical evidence in the year 1952 of DNA as genetic material using radioactive bacteriophage . Griffith also explained the transformation in bacteria and concluded that the protein factor imparts virulence to the rough strain, but it was not proved to be genetic material.

Avery , Macleod and McCarthy further studied the Griffith experiment and concluded that the DNA was the genetic material responsible for transforming the avirulent rough strain to the virulent strain. To resolve the query of genetic material, many researchers were engaged to know whether the cause of inheritance is protein or DNA.

Many assessments then led to the discovery of “ DNA ” as genetic material or the cause of inheritance . One of the best experiments that provide DNA evidence as genetic material is the “ Hershey and Chase experiment ”. We will study the definition, steps (radioactive labelling, infection, blending and centrifugation) and observation of the Hershey and Chase experiment in this context.

Content: Hershey and Chase Experiment

Radioactive labelling of bacteriophage, centrifugation, observation, definition of hershey and chase experiment.

Hershey and Chase’s experiment has demonstrated the DNA is the genetic material where they have taken the radioactive T2-bacteriophage (Viruses that infect E.coli bacteria). T2-bacteriophage or Enterobacteria phage T2 belongs to the Group-I bacteriophage.

t2 bacteriophage used in hershey and chase experiment

Video: Hershey and Chase Experiment

Hershey and Chase Experiment Steps

Hershey and Chase gave full evidence of the DNA being a genetic material by their experiments. To perform the experiment, Hershey and Chase have taken T-2 bacteriophages (invaders of E.coli bacteria). The experiment includes the following steps:

Hershey and Chase have grown T-2 bacteriophages in the two batches. In batch-1, we need to grow the bacteriophages in the medium containing radioactive sulphur (S 35 ) and radioactive phosphorus (P 32 ) in batch-2. After incubation, we could see that the radioactive sulphur (S 35 ) will tag the phage protein. The radioactive phosphorus (P 32 ) will tag the phage DNA.

Hershey and Chase Experiment batch1 radioactive labelling

After radioactive labelling of the phage DNA and protein, Hershey and Chase infected the bacteria, i.e. E.coli by using the radioactively labelled T-2 phage. In batch-1, T-2 phage tagged with S 35 and in batch-2 T-2 phage labelled with P 32 were allowed to infect the bacterial cells of E.coli .

After the attachment of T-2 bacteriophage to the E.coli , the phage DNA will enter the cytoplasm of E.coli . The phage DNA will take up the host cell machinery. Degradation of the bacterial genome occurs by the T2-phages where they use the ribosomes to form structural proteins of the capsid, tail fibres, base plate etc.

At the time of blending or agitation, the bacterial cells are agitated to remove the viral coats . As a result of the agitation, we get a solution containing bacterial cells and viral particles like capsid, tail fibres, base plate, DNA etc.

After the centrifugation, we could observe the results to identify the heritable factor . The phage DNA labelled with P 32 will transfer the radioactivity in the host cell. Thus, the radioactive P 32 enters a bacterial cell and exists in the form of “Pellets”. The phage protein tagged with S 35 will not transfer its radioactivity in the host cell. As a result, radioactive S 35 will appear in the form of “Supernatant” in the solution.

The P 32 labelled phage DNA will transfer its radioactivity to the host cell DNA, while S 35 labelled phage protein will not do so. The P 32 labelled phage DNA will remain inside the E.coli cell even after blending and centrifugation. According to the Hershey and Chase experiment, we can conclude that the DNA is the genetic material because the P 32 tagged T2-phage DNA will transfer the radioactivity to the host cell ( E.coli ) not the S 35 labelled T2-phage protein.

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Biology archive

Course: biology archive > unit 15.

DNA as the "transforming principle"

Hershey and Chase: DNA is the genetic material

Classic experiments: DNA as the genetic material
The discovery of the double helix structure of DNA
Discovery of the structure of DNA
Discovery of DNA

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Video transcript

Microbe Notes

DNA Experiments (Griffith & Avery, McCarty, MacLeod & Hershey, Chase)

DNA, deoxyribonucleic acid, is the carrier of all genetic information. It codes genetic information passed on from one generation to another and determines individual attributes like eye color, facial features, etc. Although DNA was first isolated in 1869 by a Swiss scientist, Friedrich Miescher, from nuclei of pus-rich white blood cells (which he called nuclein ), its role in the inheritance of traits wasn’t realized until 1943. Miescher thought that the nuclein, which was slightly acidic and contained a high percentage of phosphorus, lacked the variability to account for its hereditary significance for diversity among organisms. Most of the scientists of his period were convinced by the idea that proteins could be promising candidates for heredity as they were abundant, diverse, and complex molecules, while DNA was supposed to be a boring, repetitive polymer. This notion was put forward as the scientists were aware that genetic information was contained within organic molecules.

Table of Contents

Interesting Science Videos

Griffith’s Transformation Experiment

In 1928, a young scientist Frederick Griffith discovered the transforming principle. In 1918, millions of people were killed by the terrible Spanish influenza epidemic, and pneumococcal infections were a common cause of death among influenza-infected patients. This triggered him to study the bacteria Streptococcus pneumoniae and work on designing a vaccine against it . It became evident that bacterial pneumonia was caused by multiple strains of S. pneumoniae, and patients developed antibodies against the particular strain with which they were infected. Hence, serum samples and bacterial isolates used in experiments helped to identify DNA as the hereditary material.

He used two related strains of S. pneumoniae and mice and conducted a series of experiments using them.

When type II R-strain bacteria were grown on a culture plate, they produced rough colonies. They were non-virulent as they lacked an outer polysaccharide coat. Thus, when RII strain bacteria were injected into a mouse, they did not cause any disease and survived.
When type I S-strain bacteria were grown on a culture plate, they produced smooth, glistening, and white colonies. The smooth appearance was apparent due to a polysaccharide coat around them that provided resistance to the host’s immune system. It was virulent and thus, when injected into a mouse, resulted in pneumonia and death.
In 1929, Griffith experimented by injecting mice with heat-killed SI strain (i.e., SI strain bacteria exposed to high temperature ensuing their death). But, this failed to harm the mice, and they survived.
Surprisingly, when he mixed heat-treated SI cells with live RII cells and injected the mixture into the mice, the mice died because of pneumonia. Additionally, when he collected a blood sample from the dead mouse, he found that sample to contain live S-strain bacteria.

Conclusion of Griffith’s Transformation Experiment

Based on the above results, he inferred that something must have been transferred from the heat-treated S strain into non-virulent R strain bacteria that transformed them into smooth coated and virulent bacteria. Thus, the material was referred to as the transforming principle.

Following this, he continued with his research through the 1930s, although he couldn’t make much progress. In 1941, he was hit by a German bomb, and he died.

Avery, McCarty, and MacLeod Experiment

During World War II, in 1943, Oswald Avery, Maclyn McCarty, and Colin MacLeod working at Rockefeller University in New York, dedicated themselves to continuing the work of Griffith in order to determine the biochemical nature of Griffith’s transforming principle in an in vitro system. They used the phenotype of S. pneumoniae cells expressed on blood agar in order to figure out whether transformation had taken place or not, rather than working with mice. The transforming principle was partially purified from the cell extract (i.e., cell-free extract of heat-killed type III S cells) to determine which macromolecule of S cell transformed type II R-strain into the type III S-strain. They demonstrated DNA to be that particular transforming principle.

Initially, type III S cells were heat-killed, and lipids and carbohydrates were removed from the solution.
Secondly, they treated heat-killed S cells with digestive enzymes such as RNases and proteases to degrade RNA and proteins. Subsequently, they also treated it with DNases to digest DNA, each added separately in different tubes.
Eventually, they introduced living type IIR cells mixed with heat-killed IIIS cells onto the culture medium containing antibodies for IIR cells. Antibodies for IIR cells were used to inactivate some IIR cells such that their number doesn’t exceed the count of IIIS cells. that help to provide the distinct phenotypic differences in culture media that contained transformed S strain bacteria.

Observation of Avery, McCarty, and MacLeod Experiment

The culture treated with DNase did not yield transformed type III S strain bacteria which indicated that DNA was the hereditary material responsible for transformation.

Conclusion of Avery, McCarty, and MacLeod Experiment

DNA was found to be the genetic material that was being transferred between cells, not proteins.

Hershey and Chase Experiment

Although Avery and his fellows found that DNA was the hereditary material, the scientists were reluctant to accept the finding. But, not that long afterward, eight years after in 1952, Alfred Hershey and Martha Chase concluded that DNA is the genetic material. Their experimental tool was bacteriophages-viruses that attack bacteria which specifically involved the infection of Escherichia coli with T2 bacteriophage.

T2 virus depends on the host body for its reproduction process. When they find bacteria as a host cell, they adhere to its surface and inject its genetic material into the bacteria. The injected hereditary material hijacks the host’s machinery such that a large number of viral particles are released from them. T2 phage consists of only proteins (on the outer protein coat) and DNA (core) that could be potential genetic material to instruct E. coli to develop its progeny. They experimented to determine whether protein or DNA from the virus entered into the bacteria.

Bacteriophage was allowed to grow on two of the medium: one containing a radioactive isotope of phosphorus( 32 P) and the other containing a radioactive isotope of sulfur ( 35 S).
Phages grown on radioactive phosphorus( 32 P) contained radioactive P labeled DNA (not radioactive protein) as DNA contains phosphorus but not sulfur.
Similarly, the viruses grown in the medium containing radioactive sulfur ( 35 S) contained radioactive 35 S labeled protein (but not radioactive DNA) because sulfur is found in many proteins but is absent from DNA.
E. coli were introduced to be infected by the radioactive phages.
After the progression of infection, the blender was used to remove the remains of phage and phage parts from the outside of the bacteria, followed by centrifugation in order to separate the bacteria from the phage debris.
Centrifugation results in the settling down of heavier particles like bacteria in the form of pellet while those light particles such as medium, phage, and phage parts, etc., float near the top of the tube, called supernatant.

Observation of Hershey and Chase Experiment

On measuring radioactivity in the pellet and supernatant in both media, 32 P was found in large amount in the pellet while 35 S in the supernatant that is pellet contained radioactively P labeled infected bacterial cells and supernatant was enriched with radioactively S labeled phage and phage parts.

Conclusion of Hershey and Chase Experiment

Hershey and Chase deduced that it was DNA, not protein which got injected into host cells, and thus, DNA is the hereditary material that is passed from virus to bacteria.

Fry, M. (2016). Landmark Experiments in Molecular Biology. Academic Press.
https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/04%3A_Molecular_Biology/4.02%3A_DNA_the_Genetic_Material
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https://biolearnspot.blogspot.com/2017/11/experiments-of-avery-macleod-and.html
https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-discovery-and-structure/a/classic-experiments-dna-as-the-genetic-material

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A century of phage lessons

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One hundred years after the first description of viruses that infect bacterial cells, the contribution of these bacteriophages to fundamental biology, biotechnology and human health continues unabated and deserves celebration.

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Hershey-Chase experiments

Alfred day hershey (1908–1997).

During the twentieth century in the United States, Alfred Day Hershey studied phages, or viruses that infect bacteria, and experimentally verified that genes were made of deoxyribonucleic acid, or DNA. Genes are molecular, heritable instructions for how an organism develops. When Hershey started to study phages, scientists did not know if phages contained genes, or whether genes were made of DNA or protein. In 1952, Hershey and his research assistant, Martha Chase, conducted phage experiments that convinced scientists that genes were made of DNA. For his work with phages, Hershey shared the 1969 Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria. Hershey conducted experiments with results that connected DNA to the function of genes, thereby changing the way scientists studied molecular biology and the development of organisms.

The Hershey-Chase Experiments (1952), by Alfred Hershey and Martha Chase

In 1951 and 1952, Alfred Hershey and Martha Chase conducted a series of experiments at the Carnegie Institute of Washington in Cold Spring Harbor, New York, that verified genes were made of deoxyribonucleic acid, or DNA. Hershey and Chase performed their experiments, later named the Hershey-Chase experiments, on viruses that infect bacteria, also called bacteriophages. The experiments followed decades of scientists’ skepticism about whether genetic material was composed of protein or DNA. The most well-known Hershey-Chase experiment, called the Waring Blender experiment, provided concrete evidence that genes were made of DNA. The Hershey-Chase experiments settled the long-standing debate about the composition of genes, thereby allowing scientists to investigate the molecular mechanisms by which genes function in organisms.

How Did Scientists Prove That DNA Is Our Genetic Material?

Griffith experiment, avery, macleod and mccarty experiment, hershey and chase experiment.

Three seminal experiments proved, without doubt, that DNA was the genetic material, and not proteins. These experiments were the Griffith experiment, Avery, MacLeod, and McCarthy Experiment, and finally the Hershey-Chase Experiment.

DNA is the fundamental component of our being. The human body is merely the carrier for this genetic material, passing it down from generation to generation. Our purpose is to ensure the survival of the species. Humans are to DNA like a fruit is to a seed. We are just an outer covering to ensure the safe passage and protection of the source code of our existence through time. Makes you feel pretty useless, doesn’t it?

However, that’s not what I want you to focus on. The main focus is, how did we discover that DNA is the carrier of information? How did we determine that it wasn’t something else, like proteins? After all, proteins are also present in every cell.

For a long time this debate had been going on. Even after Gregor Mendel formed the 3 laws of inheritance , it wasn’t accepted by the scientific community for 45 years. The reason? There was no concept of DNA or genes being the information carriers! The whole debate was finally put to rest by 3 main experiments carried out by independent researchers, which formed the basis of all our evolutionary and molecular biology studies.

What Is DNA And How Does It Work?

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What Does RNA Do In A Cell?

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Chromatin: Structure And Function Within A Cell

Is 98% Of Our DNA Really Junk?

Mendel’s Laws Of Inheritance: The Law Of Segregation

Why Do Some People Look More Like Their Grandparents Than Their Parents?

Photosynthesis: The Biochemistry Behind How Plants Make Their Food

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Can We Bring Back Dinosaurs (Just Like In Jurassic Park)?

Concept 18 Bacteria and viruses have DNA too.

Joshua Lederberg was only 20 when he proposed the experiment in bacterial conjugation. The experiment worked almost on the first try. Within six weeks, he had enough results to prove that bacteria mated.

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Hershey and Chase Experiment

Hershey and chase experiment: an introduction.

There were many scientists who knew that the element essential for inheritance is found within the body of an organism, but they failed to discover it. Many experiments were performed to extract the chromosomal components, but the question of inheritance remains unanswered. However, with the advent of Griffith’s experiments, the path was opened for the discovery of genetic material.

Working off on Griffith's experiment, Avery and his colleagues successfully isolated DNA and demonstrated that DNA is the genetic material. However, until Hershey and Chase published their experimental data, not everyone agreed with this theory.

The Hershey and Chase Experiment

Hershey and Chase Experiment Diagram

To establish that DNA serves as the genetic material, the Hershey-Chase experiment was carried out in 1952.

E. coli and the bacteriophage T 2 were used in the tests conducted by Hershey and Chase.

The bacteriophage binds to the bacteria and introduces its genetic material into the bacterial cell . It has DNA and a protein coat.

Some T 2 phages were cultivated in radioactive sulphur ( 35 S) media, while the other T 2 phages were cultured in a radioactive phosphorus ( 32 P) medium.

While the T 2 phages in ( 32 P) medium contained radioactive DNA because the protein coat does not contain phosphorus, the T 2 in ( 35 S) medium contained radioactive protein due to the absence of sulphur in the DNA.

After that, the radioactive phages joined the E. coli. As the illness grew worse, centrifugation was used to separate the viruses.

The fact that the radioactive DNA in the T 2 phage-infected E. coli was similarly radioactive suggests that DNA was the substance that was transferred from the virus to the bacteria.

Conclusion of Hershey and Chase Experiment: The bacteria that had been infected by the virus and coated with a radioactive protein coat were not radioactive, demonstrating that DNA is the genetic material transmitted from a virus to a bacteria.

Why is DNA Considered a Genetic Material?

It was discovered that DNA dominated the genetic makeup of the majority of species . There were notable exceptions, including certain viruses whose genetic makeup was RNA . But what distinguishes DNA from other molecules such as proteins , carbohydrates etc. as genetic material? Important requirements for being a genetic material are:

Able to replicate itself.

Structurally and chemically stable.

Give room for a mutation that could result in evolution .

Able to communicate itself with "Mendelian Characters".

The majority of other compounds, including proteins, carbohydrates and lipids , did not meet the aforementioned requirements. Although RNA could meet the requirements, DNA remained the favoured genetic material over RNA for the following reasons:

RNA is less stable structurally than DNA.

RNA is less stable chemically than DNA.

Due to its double-stranded structure, DNA can more easily correct replication faults.

RNA is required for protein synthesis because DNA can not code for it directly.

Pulse Chase Experiment

The Pulse-Chase Analysis is a technique used in Biochemistry and genetic experiments to look at the biological activity that is happening over time by exposing the cells to the same substance first in a labelled form (the pulse) and then in an unlabelled form (the second pulse) (chase).

This technique can be used to track a cell's activity over an extended period of time. Protein kinase C, ubiquitin and numerous other proteins have been studied using this technique. The technique was additionally employed to demonstrate the existence and utility of Okazaki fragments. To clarify the secretory process, George Palade used a pulse-chase of radioactive amino acids.

Alfred Hershey and Martha Chase carried out a series of tests in 1952 that helped to establish that DNA is the genetic material. These investigations are known as the Hershey-Chase experiments. Despite the fact that DNA has been known to biologists since 1869, many scientists at the time still believed that proteins contained genetic information because DNA seemed to be less complex than proteins.

In their tests, Hershey and Chase demonstrated that when bacteriophages , which are made up of DNA and protein, infect bacteria, only a small portion of their protein actually reaches the host bacterial cell. The prior, current and later discoveries all served to indicate that DNA is the hereditary material, even though the results were inconclusive and Hershey and Chase were circumspect in their interpretation. Max Delbruck, Salvador Luria and Hershey received the 1969 Nobel Prize in Physiology or Medicine for their discoveries relating to genetics.

FAQs on Hershey and Chase Experiment

1. What was Griffith's transforming principle?

Griffith was the one who initially conceived the idea of the transformative principle. The principle proved successful in converting a strain of non-pathogenic bacteria into a strain of pathogenic bacteria. Hereditary material is distinguished by a number of qualities, including the ability to undergo phenotypic change. Griffith referred to the component that was responsible for the altered phenotype as the transforming principle. It was determined through a series of studies carried out by Avery, McCartys and MacLeod that the hereditary material in question was DNA.

2. What is the semi-conservative DNA replication model?

The "semi-conservative DNA replication" model was proposed by Watson and Crick. The two DNA strands split apart in accordance with this theory. For the synthesis of a new strand, each strand serves as a template. Based on complementary base pairing with the template, the new strand is created. One parent strand and one freshly produced strand make up each new DNA molecule. This is how the single copy of the original DNA molecule is divided into two copies.

3. What is the biochemical nature of the transforming principle?

To find the transforming principle, bacteriologists did a number of experiments.

Alcohol precipitated the transforming principle. This demonstrated that it wasn't a carbohydrate.

Proteases were unable to eliminate the transforming principle. So, the protein was not the cause.

The lipases were unable to remove the transforming principle. This demonstrated that it wasn't a lipid.

Ribonuclease could not inactivate the transforming principle, hence RNA was not effective.

Deoxyribonuclease may be used to inactivate the transforming principle.

DNA was the transforming principle. As a result, DNA was the genetic material.

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A Single-Molecule Hershey-Chase Experiment

David van valen.

1 Division of Engineering and Applied Sciences, Mathematics and Astronomy California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA

2 Division of Physics, Mathematics and Astronomy California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA

Hannah Tuson

3 Department of Biochemistry, University of Wisconsin, Madison, 433 Babcock Drive, Madison, WI 53706, USA

Paul Wiggins

4 Department of Physics, University of Washington, Box 351560, Seattle, WA 98195, USA

Rob Phillips

Associated data.

Ever since Hershey and Chase used phages to establish DNA as the carrier of genetic information in 1952, the precise mechanisms of phage DNA translocation have been a mystery [ 1 ]. Although bulk measurements have set a time-scale for in vivo DNA translocation during bacteriophage infection, measurements of DNA ejection by single bacteriophages have only been made in vitro. Here, we present direct visualization of single bacteriophages infecting individual Escherichia coli cells. For bacteriophage λ, we establish a mean ejection time of roughly 5 min with significant cell-to-cell variability, including pausing events. In contrast, corresponding in vitro single-molecule ejections are more uniform and finish within 10 s. Our data reveal that when plotted against the amount of DNA ejected, the velocity of ejection for two different genome lengths collapses onto a single curve. This suggests that in vivo ejections are controlled by the amount of DNA ejected. In contrast, in vitro DNA ejections are governed by the amount of DNA left inside the capsid. This analysis provides evidence against a purely intrastrand repulsion-based mechanism and suggests that cell-internal processes dominate. This provides a picture of the early stages of phage infection and sheds light on the problem of polymer translocation.

A schematic of our experimental design is shown in Figure 1A . To visualize DNA translocation, we use a cyanine dye to stain the viral DNA while it still remains in the capsid [ 2 ]. Phages are first incubated in the appropriate DNA stain before dialyzing away excess dye. The stained phages are then briefly bound to bacterial cells, which are pipetted into a flow chamber. The sample is then washed with buffer and then imaged with time-lapse bright-field and fluorescence microscopy. The signature of an ejection event is a loss of fluorescence in the virus and a concomitant increase in the fluorescence within the bacterial cell ( Figure 1B ). We were encouraged that such an experiment was possible from previous studies that have shown that dye-stained phages can remain intact and can infect cells [ 2 , 3 ]. Cyanine dyes have also been used to study the kinetics of viruses in living eukaryotic cells, demonstrating that some dyes have limited cytotoxicity [ 4 ]. A screen was performed and identified SYTOX Orange as a DNA stain with appropriate specificity/sensitivity (see Figure S1 available online) and physiological parameters ( Figures S2 – S4 ).

An external file that holds a picture, illustration, etc.
Object name is nihms-388521-f0001.jpg

A Schematic for Monitoring DNA Translocation with Pre-Ejection Labeling

(A) The DNA is stained while still in the capsid. During ejection, the phage DNA carries its complement of cyanine dye with it, transferring fluorescence intensity from the virus to the cellular interior. Eventually, the dye falls off the phage DNA and rebinds to the bacterium’s genome.

(B) The timing of ejection is determined by measuring the loss of fluorescence intensity from the capsid; the concomitant increase in intensity in the cellular interior serves to verify that phage DNA has entered the cell. See also Figures S1 – S3 .

Single-Cell DNA Ejection Trajectories

A typical in vivo ejection event for phage λcI60 (48.5 kbp) is shown in Figure 2 . The attachment of the viruses to the host is revealed by diffraction-limited spots on the cell surface ( Figure 2A ). We identified pixels associated with either the virus or the cell ( Figure 2B ) and queried the fluorescence intensity as a function of time. As shown in Figure 2C , the ejection process is characterized by a loss of fluorescence intensity in the phage and a concomitant increase in fluorescence in the cellular interior ( Figure 2D ; representative Movies S1 and S2 ). The fluorescence inside the cell is diffuse: this reflects the dye molecules unbinding kinetics from the phage DNA (residence time ~1 s [ 5 ]) and redistributing themselves along the host genome, as verified in the Supplemental Experimental Procedures ( Figures S3B and S3C ). In the particular trajectory shown, the increase in cellular fluorescence is roughly equal to the decrease in phage fluorescence. The decrease in signal at the end of the trajectories in Figure 2D can be accounted for by photobleaching ( Figure S3C ).

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Dynamics of DNA Ejection

(A) Viruses attached to the cell surface in this fluorescence image merged with its bright-field counterpart.

(B) Segmentation masks of the cell (white), the phage that ejects its DNA (green), and the phages that do not eject their DNA (red).

(D) Fluorescence intensity as a function of time is presented. The intensity of the phage segmented region and the cell segmented region are each plotted separately. The fluorescence intensity inside the nonejecting phage mask is stable; this is shown in Figure S4A . Note that in this ejection there appear to be steps and pauses.

The scale bars in (A) and (C) are 2 mm.

Single-Phage In Vivo Ejections Are Two Orders of Magnitude Slower Than In Vitro and Display Pausing

The results of a number of ejection events for λcI60 are shown in Figure 3 . For the measurements shown here, the viralfluorescence decreases on a timescale of minutes, a factor of 10–100 times longer than the corresponding dynamics in vitro [ 6 , 7 ]. Here, the ejection time is the time required for 80% of the fluorescence intensity to leave the viral capsid. The mean and SD for the ejection time for λcI60 was 5.2 ± 4.2 min (n = 45). In addition to the variability in ejection time, a number of ejections demonstrated pausing events, which we define as a nondecreasing fluorescence level greater than 2 min (our time resolution was typically 1 min); the mean pause time for λcI60 was 5.4 ± 4.1 min (n = 14). Both singlestep and paused ejections are shown in Figures 3A and 3B , respectively. A sample paused ejection is shown in Movie S3 .

An external file that holds a picture, illustration, etc.
Object name is nihms-388521-f0003.jpg

Ejection Trajectories from Single-Cell Infections for λcI60

The red trajectories show the time history of the DNA intensity within the virus, and the blue trajectories show the concomitant increase in the fluorescence in the cellular interior. The solid-red color highlights a characteristic ejection, and the lighter-red color displays other ejection events for reference. The conversion between arbitrary units and kilobase pairs (kbp) was done by first subtracting each trace’s minimum observed fluorescence from itself. Each trace was then normalized by the maximum drop in the phage fluorescence and then multiplied by the genome length, which is 48.5 kbp for λcI60. Only two representative traces for the intensity within the cell are shown, with the remaining trajectories available online. Multiple lysate preparations from a single stock of CsCl-purified phages were used with similar results.

(A) Trajectories displaying a rapid and continuous ejection are illustrated.

(B) Trajectories that exhibit pausing events are demonstrated.

An Ensemble View of In Vivo Ejection Shows that the Amount of DNA in the Viral Capsid Is Not the Governing Control Parameter

Measurements on both the wild-type and shortened genomes provide an opportunity to quantitatively examine the DNA translocation kinetics. One quantity of interest is the first-passage time for ejection: for each trajectory, we extracted the first-passage time for 20%, 50%, and 80% of completion, as determined by the decrease in the starting phage fluorescence. The first-passage time distributions for λcI60 and λb221 are shown in Figure 4A . By taking the mean of this distribution, we obtain the mean first-passage time ( Figure 4B ). We note that the mean first-passage time is a quantity that is amenable to theoretical calculations [ 9 ]. Another way to view the mean first-passage time is as an “average” ejection trajectory. When viewed in this way, one interpretation of Figure 4B is that, within the error of the measurement, the “average” trajectories for λcI60 and λb221 have considerable overlap. For both λcI60 and λb221, the velocities plateau after ~50% of the genome length ( Figures 4C and 4D ).

An external file that holds a picture, illustration, etc.
Object name is nihms-388521-f0004.jpg

An Ensemble View of Ejection Times and Dynamics for Phage λ

(A) Distributions of first-passage times for different fractions of completion of ejection are shown. The histograms are determined by setting ejection thresholds of 20%, 50%, and 80% complete as measured using the fluorescence. The distributions for both λcI60 (48.5 kbp) and λb221 (37.7 kbp) are shown.

(B) Mean first-passage times for λcI60 and λb221 are presented. The means of the distributions shown in (A) were used to calculate the mean first-passage time. There is little difference in the mean first-passage times of absolute amounts of DNA ejected between λcI60 and λb221.

(C) Velocity of ejecting DNA plotted as a function of the amount of DNA remaining in the capsid is illustrated. The initial portion of the ejection process is faster for λcI60 than λb221. There is no significant overlap between the two curves.

(D) Velocity of ejecting DNA plotted as a function of the amount of DNA ejected is demonstrated. There is a small difference between the two curves for the first 20 kbp and significant overlap after that.

Only trajectories without pauses were used to generate (C) and (D). Error bars for (B)–(D) represent SE (n = 45 for λcI60; n = 18 for λb221).

We also plotted the mean velocity at different amounts of DNA remaining in the capsid during an ejection ( Figure 4C ). There is little overlap between the two curves, and for lower amounts of DNA remaining in the capsid, the velocity for λb221 is higher than λcI60. This is to be contrasted with in vitro measurements where there is significant overlap between the two curves, with the “data collapse” in that case signifying that the dynamics are equivalent after λcI60 has ejected its first 10.8 kbp [ 7 ]. An alternative way to visualize these data is to plot the mean velocity versus the amount of DNA ejected into the cell ( Figure 4D ). When plotted in this fashion, there is considerable overlap between the two curves, with a small difference observed for the first 20 kbp of ejection. This analysis is consistent with the mean first-passage time analysis, which showed considerable overlap when the first-passage time was also plotted against the amount of DNA ejected.

For bacterial viruses, genome delivery is at the heart of the viral life cycle, yet this critical process remains enigmatic as does the in vivo process of polymer translocation more generally. Beyond a purely intellectual understanding of this process, phage-mediated transfer of nucleic acids has medical and evolutionary implications [ 10 – 13 ]. Our objective was to design and perform an experiment with sufficient temporal resolution that would permit us to measure single-molecule DNA transfer in real time; we accomplished this for both WT (48.5 kbp) and mutant (37.7 kbp) λ phage using SYTOX Orange and fluorescence microscopy. These experiments reveal that the DNA translocation process is subject to strong cell-to-cell variability (1–20 min). A number of ejections also exhibited pauses and stalls. Our single-molecule measurements are consistent with earlier estimates of a minute timescale for in vivo genome delivery of phage λ from bulk experiments [ 14 , 15 ].

A number of different hypotheses have been formulated for the actual translocation mechanism for phage λ. In addition to the driving force due to the packaged DNA, these models propose that thermal fluctuations, hydrodynamic drag, and active molecular motors might each play a role in bringing the viral DNA into the bacterial cell [ 9 , 15 – 18 ]. Our results provide both surprises and useful insights that constrain the space of possible models and will guide future modeling efforts. One key result is that the length of DNA remaining inside the capsid is not the sole control parameter that governs the ejection dynamics, as it is in vitro. In the in vitro experiments, the approximate collapse of the data from the different genome lengths on a single curve revealed that the DNA within the capsid is driving the kinetics of ejection [ 7 , 8 ]. By way of contrast, in the in vivo ejection experiments reported here, an approximate data collapse is only revealed when the velocity is plotted with respect to how much DNA is out of the capsid and in the cell rather than how much DNA remains within the capsid. Data collapse has been previously used to identify control parameters for in vitro DNA ejection as well as the lysis-lysogeny decision [ 7 , 8 , 19 ].

No collapse is seen when the velocity is plotted against the amount of DNA remaining inside the capsid. This has significant implications for the role energy stored in the compacted DNA plays during the in vivo ejection. If some significant portion of the ejection process were governed solely by the energy in the compacted DNA, then during that portion we would expect the dynamics of λcI60 and λb221 to be identical when the amount of DNA remaining in the capsid is identical. This is the in vitro case as studied in [ 7 , 8 ]. Because the DNA-DNA repulsion inside the capsid is highest when the capsid contains more DNA, such a period would likely be at the beginning of the ejection process. As seen in Figure 4C , however, there exists no period of overlap between the velocity curves for the two phage strains and, hence, no period during the ejection process where the length of DNA in the capsid and, hence, intrastrand repulsion, is the sole control parameter. Two-step models in which the first half of the genome is delivered by the energy stored in the compacted DNA and the remainder is delivered by another mechanism are also not consistent with our data.

Another consequence of the data collapse ( Figure 4D ) is the possibility that the amount of DNA ejected (as opposed to the amount of DNA in the capsid) is a key control parameter for this system. This picture is consistent with models in which the mechanism is internal to the cell because the only information such a mechanism would utilize is the amount of DNA that has been brought inside the cell. One limitation to applying this argument is that only two genome lengths have been tested here. Such reasoning also does not exclude a mixed picture, as mentioned above.

The origin of the apparent pauses might provide information about the ejection mechanism, because DNA-based motors acting against a load have been observed to pause [ 20 , 21 ]. However, the pauses observed here are much longer than the pauses observed for motors, and it is possible that they could simply be a reflection of the cell-to-cell variability in turgor pressure. Postpause resuming of DNA entry could thus indicate a secondary mechanism in conjunction with pressure, for high turgor cells. Another possibility is that the pauses observed here might also be related to mechanisms proposed for pauses observed in vitro for phage T5 [ 22 , 23 ]. However, this is unlikely because pauses are not observed for phage λ in vitro [ 6 , 7 ].

Our results are contrary to what was shown in T7, in which a constant DNA ejection rate was seen with bulk measurements [ 15 ]. T7 has a capsid similar in size to λ (60 versus 58 nm, respectively) with a 40 kbp genome; however, its tail is considerably shorter (23 versus 150 nm, respectively) [ 24 , 25 ]. It has been suggested that a constant velocity is suggestive of a purely enzyme-driven model such as a molecular motor [ 15 ]. Such a feature is not seen in our data because Figure 4D shows that once ~20 kbp of DNA has been ejected, there is a marked decrease in the ejection velocity. However, the non-linear force-velocity relationship seen in vitro and the presence of pN level forces from the DNA-DNA repulsion inside the capsid make it unclear whether a constant ejection rate prediction would be true for λ.

The current data do not match previous calculations of ejection dynamics for mechanisms based on DNA binding proteins and thermal fluctuations [ 9 ]. Those calculations predict that after the first 10.8 kbp of DNA from λcI60 has been ejected, it should have the same dynamics as λb221, which is inconsistent with our data. Also perplexing is the timescale of ejection. The origin of the friction that sets the timescale for ejection is poorly understood, both in vitro and in vivo. A number of models assume a linear relationship between force and velocity, but it is now known that this assumption is not true in vitro [ 7 , 9 , 26 ]; we suspect that it is not true in vivo either.

In summary, we have examined the DNA ejection process for bacteriophage λ in vivo at the single-molecule level. We note that the techniques explored in this work may be generalizable to the study of other bacteriophages. It would be especially interesting to see a comparison between the bulk and single-molecule dynamics for bacteriophage T7 because bulk experiments have shown that the speed is constant throughout the ejection process in vivo [ 15 , 24 ], as opposed to the variable rate reported here. We also note that the experimental platform presented here can be used to explore the effects of various genetic, chemical, and mechanical perturbations on the ejection process.

Experimental Procedures

Real-time imaging of dna ejection in vivo.

Glass coverslips were cleaned by sonication for 30 min in 1 M KOH followed by sonication in 100% ethanol with copious rinsing with purified water in between, and then dried on a hot plate. The coverslips were then briefly (5 s) immersed in a fresh solution of 1% polyethyleneimine, transferred into purified water, and finally dried with a stream of air. A microscope slide, double-sided tape, and the treated coverslip were then assembled into a flow chamber.

E. coli strain LE392 was grown up overnight in LB media at 37°C (see Supplemental Experimental Procedures for all buffer formulations). The saturated culture was then diluted 1:100 in M9 maltose-sup and grown for 3 hr at 37°C until the culture reached an optical density 600 of ~0.3. Plate lysate of the desired phage strain (see Supplemental Experimental Procedures ) was centrifuged for 5 min at 13,000 × g to remove bacterial debris. The supernatant was recovered and then stained with SYTOX Orange at a final concentration of 500 nM for 3 hr at room temperature. Prior to binding stained phages to E. coli , free dye was removed by diluting 100 μl of the phage suspension and then centrifuging the sample across a 100 kDa (EMD Millipore,UFC910008) filter four times. Each round of centrifugation led to a 40-fold dilution of dye, reducing the final free concentration of dye to less than 200 pM. After the final round, the phages were brought up to the original volume of 100 μl with M9sup. Phages were then bound to cells by mixing ~50 μl of cells with phage at a multiplicity of infection (moi) of approximately seven for 1 min at room temperature. The cells were then flowed into the flow chamber and allowed to adhere to the surface for 2 min at room temperature. Occasionally, phages were bound to cells by mixing 10 μl of cells with phage at a moi of approximately one to five and incubating on ice for 30 min. Incubation of the cells in the flow chamber took place on ice as well. The initiation of phage ejection is slowed down considerably within this time period in either of these two conditions [ 19 , 27 , 28 ].

After the incubation, the flow chamber was washed with 200 μl of M9sup with 1% GODCAT mixture, 1% β-mercaptoethanol, and 0.5% glucose. The chamber was then sealed with valap and imaged on a Nikon Ti-E Perfect Focus microscope using a mercury lamp and TRITC filter (Semrock, LF561) set at 37°C. Snapshots of both the phase and fluorescence channels were taken either one or four times a minute, with a fluorescence excitation time of 500 or 300 ms, respectively. Images were collected using a Hamamatsu C8484 camera, a Photometrics CoolSNAP ES2 camera, or an Andor iXON EMCCD camera. We observed better conservation of fluorescence between the phage and the cell with the Hamamatsu and Photometrics cameras as opposed to the Andor camera. The electron multiplier gain of the Andor camera allowed for shorter exposure times and higher time resolution.

Supplementary Material

Acknowledgments.

We are grateful to a number of people for help with experiments, advice, and critical commentary on the manuscript, including Heun Jin Lee, Maja Bialecka, Phillips laboratory, Talia Weiss, Vilawain Fernandes, Kari Barlan, Paul Grayson, Ido Golding, Lanying Zeng, Bill Gelbart, Chuck Knobler, Francois St. Pierre, and Drew Endy. We are also grateful to Ron Vale, Tim Mitchison, Dyche Mullins, and Clare Waterman as well as several generations of students from the MBL Physiology Course where this work has been developed over several summers. We also gratefully acknowledge financial support from several sources, including a National Institutes of Health (NIH) Medical Scientist Training Program Fellowship, a Yaser AbuMostafa Hertz Fellowship, and a NIH Director’s Pioneer Award. We also acknowledge the support of National Science Foundation grant number 0758343.

Supplemental Information Supplemental Information includes four figures, two tables, Supplemental Experimental Procedures, and three movies and can be found with this article online at doi:10.1016/j.cub.2012.05.023.

Biology Article
Dna Genetic Material

DNA As Genetic Material - Hershey And Chase Experiment

Even though researchers discovered that the factor responsible for the inheritance of traits comes from within the organisms; they failed to identify the hereditary material. The chromosomal components were isolated but the material which is responsible for inheritance remained unanswered. Griffith’s experiment was a stepping stone for the discovery of genetic material. It took a long time for the acceptance of DNA as genetic material. Let’s go through the discovery of DNA as genetic material.

Experiments of Hershey and Chase

We know about Griffith’s experiment and experiments that followed to discover the hereditary material in organisms. Based on Griffith’s experiment, Avery and his team isolated DNA and proved DNA to be the genetic material. But it was not accepted by all until Hershey and Chase published their experimental results.

In 1952, Alfred Hershey and Martha Chase took an effort to find the genetic material in organisms. Their experiments led to an unequivocal proof to DNA as genetic material. Bacteriophages (viruses that affect bacteria) were the key element for Hershey and Chase experiment.

The virus doesn’t have their own mechanism of reproduction but they depend on a host for the same. Once they attach to the host cell, their genetic material is transferred to the host. Here in case of bacteriophages, bacteria are their host. The infected bacteria are manipulated by the bacteriophages such that bacterial cells start to replicate the viral genetic material. Hershey and Chase conducted an experiment to discover whether it was protein or DNA that acted as the genetic material that entered the bacteria.

DNA as Genetic Material

Experiment: The experiment began with the culturing of viruses in two types of medium. One set of viruses (A) was cultured in a medium of radioactive phosphorus whereas another set (B) was cultured in a medium of radioactive sulfur. They observed that the first set of viruses (A) consisted of radioactive DNA but not radioactive proteins . This is because DNA is a phosphorus-based compound while protein is not. The latter set of viruses (B) consisted of radioactive protein but not radioactive DNA.

The host for infection was E.coli bacteria. The viruses were allowed to infect bacteria by removing the viral coats through a number of blending and centrifugation.

Observation: E.coli bacteria which were infected by radioactive DNA viruses (A) were radioactive but the ones that were infected by radioactive protein viruses (B) were non-radioactive.

Conclusion: Resultant radioactive and non-radioactive bacteria infer that the viruses that had radioactive DNA transferred their DNA to the bacteria but viruses that had radioactive protein didn’t get transferred to the bacteria. Hence, DNA is the genetic material and not the protein.

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				DNA
				RNA
				protein coat
				the entire phage
				no part
		S was found in progeny phages rather than P, Hershey and Chase would have concluded that

				proteins contain phosphorus.
				DNA contains sulfur.
				phage DNA enters the host cell.
				phage protein enters the host cell.
				phage can kill the cell.


				P did not enter the cell.
				P remained inside the cells after vigorous shaking.
				P was removed from the cells by vigorous shaking.
				P and S remained inside the cells after vigorous shaking.
				P and S were removed from the cells after vigorous shaking.
		P.

				True
				False


				True
				False

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Hershey and Chase Experiment
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The Hershey-Chase experiments were a series of experiments conducted in 1952 [1] ... DNA molecule was created by Paul Berg in 1972 when he combined DNA from the monkey virus SV40 with that of the lambda phage. [11] Experiments on hereditary material during the time of the Hershey-Chase experiment often used bacteriophages as a model organism.
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Hershey and Chase's experiment has demonstrated the DNA is the genetic material where they have taken the radioactive T2-bacteriophage (Viruses that infect E.coli bacteria). T2-bacteriophage or Enterobacteria phage T2 belongs to the Group-I bacteriophage. The genome of the T2-bacteriophage comprises linear, ds-DNA and they are a part of the ...
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The DNA of the virus was tagged with radioactive phosphorus in one experiment, and this ended up in the pellet. The protein shell of the virus was tagged with radioactive sulphur, and this ended up in the supernatant. The heavier bacterial cells formed the pellet, so Hershey and Chase knew that DNA was the genetic material of the virus, as the ...
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If 35 S was found in progeny phages rather than 32 P, Hershey and Chase would have concluded that: A) proteins contain phosphorus. B) DNA contains sulfur. C) phage DNA enters the host cell. D) phage protein enters the host cell. E) phage can kill the E. coli cell.
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Protein was finally excluded as the hereditary material following a series of experiments published by Alfred Hershey and Martha Chase in 1952. These experiments involved the T2 bacteriophage, a ...
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Al Hershey had sent me a long letter summarizing the recently completed experiments by which he and Martha Chase established that a key feature of the infection of a bacterium by a phage was the injection of the viral DNA into the host bacterium. Their experiment was thus a powerful new proof that DNA is the primary genetic material.". The ...
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In 1952, Hershey and his research assistant, Martha Chase, conducted phage experiments that convinced scientists that genes were made of DNA. For his work with phages, Hershey shared the 1969 Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria. ... Hershey and Chase conducted a series of experiments, later called the ...
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In 1952, Alfred Hershey and Martha Chase 2 performed a famous experiment in which radiolabelled phages were sheared off bacterial cells using a high-speed blender, helping the researchers to ...
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In 1952, Hershey and his research assistant, Martha Chase, conducted phage experiments that convinced scientists that genes were made of DNA. For his work with phages, Hershey shared the 1969 Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria. Hershey conducted experiments with results that connected DNA to the function ...
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Moreover, new phage made by these infected bacteria contained radioactive 32P. Therefore, phage DNA was used inside the bacteria to make new phage particles. The phage coat is just the package that delivers the phage DNA into the bacteria. We concluded that the phage DNA alone carries the instructions needed to replicate phages inside the bacteria.
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Ever since Hershey and Chase used phages to establish DNA as the carrier of genetic information in 1952, the precise mechanisms of phage DNA translocation have been a mystery [1]. Although bulk measurements have set a timescale for in vivo DNA translocation during bacteriophage infection, measurements of DNA ejection by single bacteriophages have only been made in vitro.
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The Hershey and Chase Experiment. Hershey and Chase Experiment Diagram. To establish that DNA serves as the genetic material, the Hershey-Chase experiment was carried out in 1952. E. coli and the bacteriophage T2 were used in the tests conducted by Hershey and Chase. The bacteriophage binds to the bacteria and introduces its genetic material ...
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Ever since Hershey and Chase used phages to establish DNA as the carrier of genetic information in 1952, the precise mechanisms of phage DNA translocation have been a mystery [].Although bulk measurements have set a time-scale for in vivo DNA translocation during bacteriophage infection, measurements of DNA ejection by single bacteriophages have only been made in vitro.
The Hershey and Chase Experiments : DNA as the Genetic Material
In 1952, Alfred Hershey and Martha Chase took an effort to find the genetic material in organisms. Their experiments led to an unequivocal proof to DNA as genetic material. Bacteriophages (viruses that affect bacteria) were the key element for Hershey and Chase experiment. The virus doesn't have their own mechanism of reproduction but they ...
Hershey and Chase Experiment
In the Hershey and Chase experiment, radioactively-labeled: A) ... 32 P and 35 S were removed from the cells after vigorous shaking. 4. Hershey and Chase labeled the phage DNA with radioactive 32 P. A) True: B) False: 5. The phage used in the experiment consisted of a DNA molecule surrounded by a protein coat. A)

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Hershey and Chase: DNA is the genetic material

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DNA Experiments (Griffith & Avery, McCarty, MacLeod & Hershey, Chase)

Griffith’s Transformation Experiment

Conclusion of Griffith’s Transformation Experiment

Avery, McCarty, and MacLeod Experiment

Observation of Avery, McCarty, and MacLeod Experiment

Conclusion of Avery, McCarty, and MacLeod Experiment

Hershey and Chase Experiment

Observation of Hershey and Chase Experiment

Conclusion of Hershey and Chase Experiment

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Pulse Chase Experiment

FAQs on Hershey and Chase Experiment

A Single-Molecule Hershey-Chase Experiment

Hannah Tuson

Paul Wiggins

Rob Phillips

Single-Cell DNA Ejection Trajectories

Single-Phage In Vivo Ejections Are Two Orders of Magnitude Slower Than In Vitro and Display Pausing

An Ensemble View of In Vivo Ejection Shows that the Amount of DNA in the Viral Capsid Is Not the Governing Control Parameter

Experimental Procedures

Supplementary Material

DNA As Genetic Material - Hershey And Chase Experiment

Experiments of Hershey and Chase

DNA as Genetic Material

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