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Isolating Hereditary Material: Frederick Griffith, Oswald Avery, Alfred Hershey, and Martha Chase

Frederick Griffith Discovers Bacterial Transformation

In the aftermath of the deadly 1918 flu epidemic, governments across the globe rushed to develop vaccines that could stop the spread of infectious diseases. In England, microbiologist Frederick Griffith was studying two strains of Streptococcus pneumoniae that varied dramatically in both their appearance and their virulence , or their ability to cause disease . Specifically, the highly virulent S strain had a smooth capsule, or outer coat composed of polysaccharides, while the nonvirulent R strain had a rough appearance and lacked a capsule (Figure 1). Mice injected with the S strain died within a few days after injection, while mice injected with the R strain did not die.

Through a series of experiments, Griffith established that the virulence of the S strain was destroyed by heating the bacteria. Thus, he was surprised to find that mice died when they were injected with a mixture of heat-killed S bacteria and living R bacteria (Figure 2), neither of which caused mice to die when they were injected alone. Griffith was able to isolate live bacteria from the hearts of the dead animals that had been injected with the mixed strains, and he observed that these bacteria had the smooth capsules characteristic of the S strain. Based on these observations, Griffith hypothesized that a chemical component from the virulent S cells had somehow transformed the R cells into the more virulent S form (Griffith, 1928). Unfortunately, Griffith was not able to identify the chemical nature of this " transforming principle " beyond the fact that it was able to survive heat treatment.

DNA Is Identified as the “Transforming Principle”

The actual identification of DNA as the "transforming principle" was an unexpected outcome of a series of clinical investigations of pneumococcal infections performed over many years (Steinman & Moberg, 1994). At the same time that Griffith was conducting his experiments, researcher Oswald Avery and his colleagues at the Rockefeller University in New York were performing detailed analyses of the pneumococcal cell capsule and the role of this capsule in infections. Modern antibiotics had not yet been discovered, and Avery was convinced that a detailed understanding of the pneumococcal cell was essential to the effective treatment of bacterial pneumonia. Over the years, Avery's group had accumulated considerable biochemical expertise as they established that strains of pneumococci could be distinguished by the polysaccharides in their capsules and that the integrity of the capsule was essential for virulence. Thus, when Griffith's results were published, Avery and his colleagues recognized the importance of these findings, and they decided to use their expertise to identify the specific molecules that could transform a nonencapsulated bacterium into an encapsulated form. In a significant departure from Griffith's procedure, however, Avery's team employed a method for transforming bacteria in cultures rather than in living mice, which gave them better control of their experiments.

Avery and his colleagues, including researchers Colin MacLeod and Maclyn McCarty, used a process of elimination to identify the transforming principle (Avery et al. , 1944). In their experiments (Figure 3), identical extracts from heat-treated S cells were first treated with hydrolytic enzymes that specifically destroyed protein , RNA , or DNA. After the enzyme treatments, the treated extracts were then mixed with live R cells. Encapsulated S cells appeared in all of the cultures, except those in which the S strain extract had been treated with DNAse, an enzyme that destroys DNA. These results suggested that DNA was the molecule responsible for transformation.

Avery and his colleagues provided further confirmation for this hypothesis by chemically isolating DNA from the cell extract and showing that it possessed the same transforming ability as the heat-treated extract. We now consider these experiments, which were published in 1944, as providing definitive proof that DNA is the hereditary material. However, the team's results were not well received at the time, most likely because popular opinion still favored protein as the hereditary material.

Hershey and Chase Prove Protein Is Not the Hereditary Material

From these experiments, Hershey and Chase determined that protein formed a protective coat around the bacteriophage that functioned in both phage attachment to the bacterium and in the injection of phage DNA into the cell. Interestingly, they did not conclude that DNA was the hereditary material, pointing out that further experiments were required to establish the role that DNA played in phage replication . In fact, Hershey and Chase circumspectly ended their paper with the following statement: "This protein probably has no function in the growth of intracellular phage. The DNA has some function. Further chemical inferences should not be drawn from the experiments presented" (Hershey & Chase, 1952). However, a mere one year later, the structure of DNA was determined , and this allowed investigators to put together the pieces in the question of DNA structure and function.

References and Recommended Reading

Avery, O. T., et al . Studies on the chemical nature of the substance inducing transformation of pneumococcal types. Journal of Experimental Medicine 79 , 137–157 (1944)

Griffith, F. The significance of pneumococcal types . Journal of Hygiene 27 , 113–159 (1928)

Hershey, A. D., & Chase, M. Independent functions of viral protein and nucleic acid in growth of bacteriophage. Journal of General Physiology 36 , 39–56 (1952)

Steinman, R. M., & Moberg, C. L. A triple tribute to the experiment that transformed biology . Journal of Experimental Medicine 179 , 379–384 (1994)

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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.

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The Hershey-Chase Blender Experiment

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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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A.D. Hershey

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A.D. Hershey (born Dec. 4, 1908, Owosso, Mich., U.S.—died May 22, 1997, Syosset, N.Y.) was an American biologist who, along with Max Delbrück and Salvador Luria , won the Nobel Prize for Physiology or Medicine in 1969. The prize was given for research done on bacteriophages (viruses that infect bacteria).

Hershey earned a doctorate in chemistry from Michigan State College (now Michigan State University) in 1934 and then took a position at Washington University School of Medicine in St. Louis, Mo. He joined the staff of the Genetics Research Unit of the Carnegie Institution of Washington in 1950 after giving up his position as professor at Washington University. In 1963 he became director of the Genetics Research Unit.

Hershey, Delbrück, and Luria began exchanging information on phage research in the early 1940s. In 1945 Hershey and Luria, working independently, demonstrated the occurrence of spontaneous mutation in both the bacteriophages and the host. The next year, Hershey and Delbrück independently discovered the occurrence of genetic recombination in phages— i.e., that different strains of phages inhabiting the same bacterial cell can exchange or combine genetic material. Delbrück incorrectly interpreted his results as specifically induced mutations, but Hershey and one of his students proved that the results they had obtained were recombinations by showing that the genetic processes in question correspond with the crossing-over of parts of similar chromosomes observed in cells of higher organisms.

Hershey is most noted for the so-called blender experiment that he performed with Martha Chase in 1952. By showing that phage DNA is the principal component entering the host cell during infection, Hershey proved that DNA, rather than protein, is the genetic material of the phage.

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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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Hershey, Chase and the blender experiment

The hershey-chase experiment, more popularly known as the blender experiment, came at a critical period in the history of modern genetics and marked the beginning of molecular biology as a branch of science. for, this experiment, the results of which were published on september 20, 1952, demonstrated that it was dna, not protein, that transmitted the genetic material of life. a.s.ganesh takes a look at this famous experiment and what came off it....

Updated - November 10, 2021 12:16 pm IST

Published - September 20, 2019 11:54 pm IST

Overview of the experiment performed by Hershey and Chase, showing DNA to be the genetic material for phage.

Often, during conversations pertaining to heredity, be it with respect to certain mannerisms or behaviour, you might have heard people allude to their DNA. This is because we now know that deoxyribonucleic acid, or DNA, holds the key to heredity to all forms of life and carries genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.

First isolated by Swiss physician Friedrich Miescher in 1869, DNA’s role as the carrier of life’s hereditary data wasn’t known for nearly a century. For, it was only in 1952 that it was firmly established that DNA was the substance that transmits genetic information. That was done through the Hershey-Chase experiment, also often referred to as the blender experiment.

Born in Michigan, the U.S. in 1908, Alfred Day Hershey attended public schools before going on to study B.S. in Bacteriology and doing a Ph.D. in Chemistry. He was drawn towards bacteriology and the biochemistry of life as a graduate student and even his doctoral thesis was on the chemistry of a bacteria. After receiving his Ph.D., Hershey moved into a career of research and teaching.

DNA or protein?

The foundation for the field of molecular biology was laid in the 1940s and the 1950s through research on bacteriophages. Bacteriophages, or simply phages, were known to be viruses – consisting only of DNA surrounded by a protein shell – that infect bacteria.

One of the key questions that was haunting the field was to find out which was the genetic material. The prevalent notion at the time was that it must be a protein, as its structure was complex enough to hold such data. Even though there was some research that pointed at DNA as the possible genetic material, most chemists, physicists and geneticists still held on to the then popular assumption.

Hershey, whose research on phages had provided him with a number of discoveries, set out to conclusively prove that the genetic material in phages was DNA. Along with his assistant Martha Chase, who had recently graduated, Hershey found a way to figure out the role played in replication by each of the phage components.

In experiments conducted in 1951-52, Hershey and Chase used radioactive phosphorus to tag the phage DNA and radioactive sulphur to tag the protein. These tagged phages were then allowed to infect a bacterial culture and begin the process of replication.

Role of blender

This process was interrupted at a crucial moment when the scientists whirled the culture in a blender. This was because Hershey and Chase had been able to determine that a blender produced the right shearing force to tear the phage particles from the bacterial walls, without damaging the bacteria.

Upon examination, it was clear that while the phage DNA had entered the bacterium and forced it to replicate phage particles, the phage protein was still outside, attached to the cell wall. In short, they were able to show that it was DNA, and not protein, that was responsible for communicating genetic information necessary for producing the next generation of phages.

Stimulates research

Hershey and Chase published their results on September 20, 1952. The Hershey-Chase experiment came to be popularly referred to as the blender experiment because of the fact that a simple blender had been used to achieve their test results.

These results stimulated research into DNA, and within months, molecular biologists James Watson and Francis Crick published their work establishing the double helix structure of the DNA molecule. In fact, Watson wrote in a 1997 memoriam that the Hershey-Chase experiment “made me ever more certain that finding the three-dimensional structure of DNA was biology’s next important objective”. It certainly turned out to be right.

Small in size, big prize

Alfred Hershey shared the Nobel Prize in Physiology or Medicine in 1969 with Max Delbruck, a physicist who did research in the U.S. after fleeing Nazi Germany in 1937, and Salvador Edward Luria, a biologist and physician from Italy who fled to France in 1938 and immigrated to the U.S. in 1940. They received the Nobel Prize for their contributions to molecular biology and their work on bacteriophages, which are viruses that infect bacteria.

Working independently, Hershey and Luria showed the occurrence of spontaneous mutation in bacteriophages and the host in 1945.

In the next year, Hershey and Delbruck separately discovered the occurrence of genetic recombination in phages. This showed that when different strains of phages infect the same bacterial cell, they can exchange or combine genetic material.

The three men turned out to be collaborators, despite the fact that they never worked together in the same laboratory.

They encouraged each other in their phage research by sharing results through correspondence and conversations.

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