explain the procedure of the hershey and chase experiment

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.

In the early twentieth century, scientists debated whether genes were made of DNA or protein. Genes control how organisms grow and develop and are the material basis for organisms’ ability to inherit traits like eye color or fur color from their parents. By 1900, scientists had identified the complete chemical composition, or building blocks, of DNA. They had also verified that all cells contained DNA, though DNA’s function remained ambiguous. Up until the 1940s, some scientists accepted the idea that genes were not made of DNA. Instead, those scientists supported the idea that DNA was a molecule that maintained cell structure. Scientists supported that idea in part because of a hypothesis called the tetranucleotide hypothesis. Phoebus Levene, a researcher at the Rockefeller Institute for Medical Research in New York City, New York, proposed the tetranucleotide hypothesis for DNA in 1933. According to Levene and other proponents of the hypothesis, DNA consisted of repeating sets of four different building blocks, called nucleotides. Some scientists concluded that a repeating sequence of nucleotides in DNA limited potential for variability. Those scientists considered variability necessary for DNA to function as genetic material. In other words, genes needed to have the capacity for enough variation to account for the different traits scientists observe in organisms. Conversely, scientists found that proteins had many more building blocks and therefore more possible arrangements than DNA. From that, some scientists claimed that genes must have been made of protein, not DNA.

The Hershey-Chase experiments were not the first studies to oppose the prevailing theory in the early 1900s that genetic material was composed of proteins. In 1944, nearly a decade before Hershey and Chase’s work, scientists published sound evidence that genes were made of DNA rather than protein. Starting in 1935, Oswald Avery, another researcher at the Rockefeller Institute, with his research associates Colin MacLeod and Maclyn McCarty, performed experiments that showed that DNA facilitated a genetic phenomenon in bacteria called bacterial transformation. Bacterial transformation is the process by which a bacterium can get and use new genetic material from its surroundings. During bacterial transformation, a non-disease-causing bacterium can transform into disease-causing bacteria if the non-disease-causing bacteria is exposed to a disease-causing bacteria. Transformation can occur even if the disease-causing-strain is dead, implying that bacterial transformation happens when the non-disease-causing bacteria inherits genetic material from the disease-causing bacteria. Avery and his colleagues found that the inherited factor that caused bacterial transformation contained DNA. However, Avery’s group did not discount the possibility that some non-DNA component in their sample caused bacterial transformation, rather than the DNA itself. Because of that, many scientists maintained the idea that proteins must govern the genetic phenomenon of bacterial transformation.

Starting in 1951, Alfred Hershey and Martha Chase conducted a series of experiments, later called the Hershey-Chase experiments, that verified the findings of Avery and his colleagues. Hershey was a researcher who studied viruses at the Carnegie Institution of Washington in Cold Spring Harbor, New York. He studied viruses that infect bacteria, also called bacteriophages, or phages. Chase became Hershey’s research technician in 1950.

In their experiments, Hershey and Chase analyzed what happened when phages infect bacteria. By the 1950s, scientists had evidence for how phages infected bacteria. They found that when phages infect a host bacterium, the phages first attach themselves to the outside of the bacterium. Then, a piece of the phage enters the bacterium and subsequently replicates itself inside the cell. After many replications, the phage causes the bacterium to lyse, or burst, thereby killing the host bacteria. Scientists classified the replicating piece as genetic material. Scientists also found that phages contained two classes of biological molecules: DNA and protein. Hershey and Chase sought to determine if the replicating piece of phages that entered bacteria during infection, the genetic parts, were solely DNA.

To perform their experiments, Hershey and Chase utilized a technique called radioactive isotope labeling. Chemical elements can exist in different structural forms called isotopes. Isotopes of the same element are nearly identical, but scientists can distinguish between them by experimental means. One way to differentiate between chemical elements with different isotopes is by analyzing their radiation. Some isotopes are less stable than others and give off radioactive signals that scientists can detect. Hershey and Chase marked phages by incorporating radioactive isotopes of phosphorus and sulfur in those phages. They allowed some phages to replicate by infecting bacteria, specifically Escherichia coli , or E. Coli , that scientists had grown in radioactive sulfur. The researchers let other phages infect and replicate in E. Coli that scientists had grown in radioactive phosphorus. DNA contains phosphorus, but not sulfur, whereas protein contains sulfur, but not phosphorus. Therefore, when Hershey and Chase marked phages with radioactive isotopes of those elements, they placed separate, distinguishable tags on the protein and DNA parts of the phages.

The first Hershey-Chase experiment aimed to confirm previous experimental findings that the DNA and protein components of phages were separable. In 1950, Thomas Anderson at the University of Pennsylvania in Philadelphia, Pennsylvania, showed that phages consisted of a protein shell, or coat, with DNA inside the shell. Anderson found that the phages could release their DNA and leave behind what he called a protein ghost. Hershey and Chase replicated Anderson’s experimental results using their radioactive isotope labeling method. Hershey and Chase were able to separate the phages into radioactive sulfur-containing protein ghosts and radioactive phosphorus-containing DNA. They found that the radioactive sulfur protein ghosts could attach to bacterial membranes while the radioactive phosphorus DNA could not. Hershey and Chase also tested if enzymes, molecules that facilitate chemical reactions in cells, could degrade DNA. They found that enzymes did not degrade the DNA of intact phages, but did degrade the DNA of separated phages. Those results indicated that in the intact phages, the protein coat surrounded the DNA and protected the DNA from degradation.

In another Hershey-Chase experiment, Hershey and Chase showed that when certain phages infected E. Coli , the phages injected their DNA into the host bacterium. In 1951, Roger Herriot at Johns Hopkins University in Baltimore, Maryland, demonstrated that after phages infected bacteria, their protein ghosts remained attached to the outside of the bacterial cells while their DNA was released elsewhere. Hershey and Chase aimed to show where the phage DNA went when it exited the protein coat and entered the bacteria. The researchers allowed radioactive phosphorus-labeled phages to attach to bacterial cell membranes in a liquid solution and infect the bacteria. Using a centrifuge, Hershey and Chase rapidly spun the samples to separate the bacterial cells from the surrounding solution. After centrifugation, they found that most of the radioactive phosphorus was detected in the cells rather than in the surrounding solution, meaning that the phage DNA must have entered the cells when the phages infected the bacteria.

The most well-known Hershey-Chase experiment was the final experiment, also called the Waring Blender experiment, through which Hershey and Chase showed that phages only injected their DNA into host bacteria, and that the DNA served as the replicating genetic element of phages. In the previous experiment, Hershey and Chase found evidence that phages injected their DNA into host bacteria. In the Waring Blender experiment, the scientists found that the phages did not inject any part of their protein coats in the host bacteria and the protein coats remained outside the bacteria, adhered to the bacterial membranes. For their experiment, Hershey and Chase prepared two samples of infected E. Coli . They infected one sample with radioactive phosphorus-labeled phages, and the other sample with radioactive sulfur-labeled phages. Then, they stirred each sample in a Waring Blender, which was a conventional kitchen blender. They used a blender because centrifuges spun too fast and would destroy the bacterial cells. The shearing forces of the blender removed the phage particles that adhered to the bacterial membranes, but preserved the integrity of the cells and most of the phage material that entered the cell. In the phosphorus-labeled sample that marked DNA but not protein, the blender removed forty percent of the labeled particles. In the sulfur-labeled sample that marked protein but not DNA, the blender removed eighty percent of the labeled particles. Those results indicated that the blender removed much more of the protein parts of the phage than the DNA parts, suggesting that the protein likely remained adhered to the outside of the cell during infection. Since the protein remained outside the cell, it could not be the replicating genetic material.

The Waring Blender only removed eighty percent of the radioactive sulfur-labeled phage, so Hershey and Chase could not account for twenty percent of the phage protein material. To show that the missing twenty percent of the phage protein did not enter the bacterial cells and replicate, the researchers infected E. Coli with radioactive sulfur-labeled phage again so that only the protein parts of the phage were labeled. They prepared two samples. For one sample, Hershey and Chase stirred the cells in the blender to remove the phage particles adhered to the outer bacterial membrane. After stirring, they allowed the phages to cause the cells to lyse, releasing newly replicated phages. For the second sample, Hershey and Chase did not stir the cells in the blender and measured the resulting replicated phages after the bacterial cells lysed. In the blender-stirred sample, less than one percent of the replicated phages contained the radioactive sulfur label. However, in the sample that Hershey and Chase did not stir in the blender, almost ten percent of the phages contained the radioactive sulfur label. The blender maintains any phage material that entered the bacterial cell. If protein was genetic material that entered the cell and replicated, then Hershey and Chase would have found more sulfur-labeled protein in the sample they stirred with the blender. The sample that they did not stir had more of the sulfur-labeled protein because the protein coats remained on the outside of the cell. Hershey and Chase concluded that protein was not genetic material, and that DNA was genetic material.

Unlike Avery’s experiments on bacterial transformations, the Hershey-Chase experiments were more widely and immediately accepted among scientists. The Hershey-Chase experiments mostly ended scientists’ suspicions that genes were made of protein rather than DNA. However, historians have questioned the conclusiveness of the Hershey-Chase experiments. In all the Waring Blender experiments, some protein and DNA material remained unaccounted for. Even in the final experiment, when Hershey and Chase allowed the bacterial cells to lyse after stirring in the blender, the scientists still recovered a small amount of protein, implying that some protein entered the cells during infection. Furthermore, the amount of contaminating protein in the Hershey-Chase Experiments exceeded the amount of contaminating protein that Avery’s group found in their experiments.

Historians of science have studied why scientists more readily accepted the Hershey-Chase experiments than Avery’s experiments. Science historian Frederic Lawrence Holmes writes that scientists more readily accepted the results of the Hershey-Chase experiments because Hershey communicated directly with skeptical scientists. Hershey sent letters to his colleagues in which he detailed the experimental findings of the Hershey-Chase experiments. Another historian of science, Michel Morange, writes that the Hershey-Chase experiments were performed at a time when scientists were ready to accept that genetic material could be DNA. Avery’s group conducted their experiments when the tetranucleotide hypothesis was popular and few scientists held that genes contained DNA. According to Morange, because Hershey and Chase conducted their experiments years later, scientists had gathered more experimental evidence and were willing to seriously consider that genes contained DNA.

In 1953, James Watson and Francis Crick, two scientists at the University of Cambridge in Cambridge, England, modeled the three-dimensional structure of DNA and demonstrated how DNA might function as genetic material. In 1969, Hershey shared the Nobel Prize in Physiology or Medicine with two other scientists, Max Delbrück and Salvador Luria, partly for his work on the Hershey-Chase experiments.

• Avery, Oswald, Colin MacLeod, and Maclyn McCarty. "Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types." The Journal of Experimental Medicine 79 (1944): 137–58.
• Fry, Michael. “Chapter 4 – Hershey and Chase Clinched the role of DNA as Genetic Material: Phage Studies Propelled the Birth of Molecular Biology.” In Landmark Experiments in Molecular Biology , 111–42. Cambridge: Academic Press, 2016.
• Hershey, Alfred D., and Martha Chase. “Independent Functions of Viral Protein and Nucleic Acid in Growth of Bacteriophage” The Journal of General Physiology 36 (1952): 39–56.
• Holmes, Frederic L. Meselson, Stahl, and the Replication of DNA: A History of “The Most Beautiful Experiment in Biology.” New Haven and London: Yale University Press, 2001.
• Hopson, Janet L., and Norman K. Wessells. Essentials of Biology . New York: McGraw-Hill, 1990.
• Judson, Horace Freeland. The Eighth Day of Creation . Cold Spring Harbor: Cold Spring Harbor Laboratory Press, 1996.
• Morange, Michel. A History of Molecular Biology . Cambridge and London: Harvard University Press, 1998.
• Olby, Robert Cecil. The Path to the Double Helix: The Discovery of DNA . Seattle: University of Washington Press, 1974.
• Stahl, Franklin W., and Alfred D. Hershey. We Can Sleep Later: Alfred D. Hershey and the Origins of Molecular Biology . Woodbury: Cold Spring Harbor Laboratory Press, 2000.

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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
• https://byjus.com/biology/dna-genetic-material/
• https://bio.libretexts.org/Bookshelves/Genetics/Book%3A_Online_Open_Genetics_(Nickle_and_Barrette-Ng)/01%3A_Overview_DNA_and_Genes/1.02%3A_DNA_is_the_Genetic_Material
• https://www.toppr.com/guides/biology/the-molecular-basis-of-inheritance/the-genetic-material/
• https://www.nature.com/scitable/topicpage/discovery-of-dna-as-the-hereditary-material-340/
• https://www.biologydiscussion.com/genetics/dna-as-a-genetic-material-biology/56216
• https://www.nature.com/scitable/topicpage/discovery-of-the-function-of-dna-resulted-6494318/
• https://www.ndsu.edu/pubweb/~mcclean/plsc411/DNA%20replication%20sequencing%20revision%202017.pdf
• https://www.britannica.com/biography/Frederick-Griffith
• https://ib.bioninja.com.au/higher-level/topic-7-nucleic-acids/71-dna-structure-and-replic/dna-experiments.html
• 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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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.

• 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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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.

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.

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 17.

• 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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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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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.

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The first step was taken by Frederick Griffith in the year 1928. He was a bacteriologist who focused on epidemiology.  Griffith was studying how Streptococcus pneumoniae caused an infection. He was working with 2 strains of the bacteria called the S and R strains. S strain organisms, when cultured in the lab, gave rise to bacterial colonies with a smooth appearance. This was due to a shiny, polysaccharide coat, which is supposed to be their virulence factor. A virulence factor is any quality or factor of a pathogen that helps it in achieving its goal – causing a disease! The other strain was the R strain. This strain gave rise to colonies that didn’t possess the polysaccharide coat, and therefore had a ‘rough’ appearance. Therefore, the S strain was virulent and the R strain was avirulent.

Griffith took 4 mice and injected them with different solutions. The first one was injected with the S strain organisms; the second one was injected with the R strain organisms; the third mouse was injected with heat-killed S strain organisms; and the last one was injected with a mixture of heat-killed S strain and live R strain organisms. The result? The first and fourth mice died due to the infection, while the second and third mice survived. When he extracted the infectious agent from the dead mice, in both cases, he found S strain organisms.

Let’s break it down. The first 2 mice showed that S strain is the virulent strain, while the R strain is avirulent. The third mouse proved that heat-killed S strain organisms cannot cause an infection. Now here is where it gets interesting. The death of the 4 th mouse, and the retrieval of live S strain organisms showed that, somehow, the heat-killed S strain organisms had caused the transformation of live R strain organisms to live S strain organisms.

This was called the transformation experiment… not particularly creative in the naming department.

Also Read: Does Human DNA Change With Time?

While Griffith’s experiment had provided a surprising result, it wasn’t clear as to what component of the dead S strain bacteria were responsible for the transformation. 16 years later, in 1944, Oswald Avery, Colin Macleod and MacLynn McCarty solved this puzzle.

They worked with a batch of heat-killed S strain bacteria. They divided it into 5 batches. In the first batch, they destroyed the polysaccharide coat of the bacteria; in the second batch they destroyed its lipid content; they destroyed the RNA of the bacteria in the third batch; with the fourth batch, they destroyed the proteins; and in the last batch, they destroyed the DNA. Each of these batches was individually mixed with live R strain bacteria and injected into individual mice.

From all 5 mice, all of them died except the last mouse. From all the dead mice, live S strain bacteria was retrieved. This experiment clearly proved that when the DNA of the S strain bacteria were destroyed, they lost the ability to transform the R strain bacteria into live S strain ones. When other components, such as the polysaccharide coat, lipid, RNA or protein were destroyed, transformation still took place. Although the polysaccharide coat was a virulent factor, it wasn’t responsible for the transfer of the genetic matter.

Even after the compelling evidence provided by the Avery, Macleod and McCarty experiment, there were still a few skeptics out there who weren’t convinced. The debate still raged between proteins and DNA. However, the Hershey – Chase experiment permanently put an end to this long-standing debate.

Alfred Hershey and Martha Chase in 1952, performed an experiment that proved, without a doubt, that DNA was the carrier of information. For their experiment, they employed the use of the bacteriophage T2. A bacteriophage is a virus that only infects bacteria. This particular virus infects Escherichia coli . T2 had a simple structure that consisted of just 2 components – an outer protein casing and the inner DNA. Hershey and Chase took 2 different samples of T2. They grew one sample with 32 P, which is the radioactive isotope of phosphorus, and the other sample was grown with 35 S, the radioactive isotope of sulphur!

The protein coat has sulphur and no phosphorus, while the DNA material has phosphorus but no sulphur. Thus, the 2 samples were labelled with 2 different radioactive isotopes.

The viruses were then allowed to infect the E. coli . Once the infection was done, the experimental solution was subjected to blending and centrifugation. The former removed the ghost shells, or empty shells of the virus from the body of the bacteria. The latter separated the bacteria from everything else. The bacterial solution and the supernatant were then checked for their radioactivity .

In the first sample, where 32 P was used, the bacterial solution showed radioactivity, whereas the supernatant barely had any radioactivity. In the sample where 35 S was used, the bacterial solution didn’t show any radioactivity, but the supernatant did.

This experiment clearly showed that DNA was transferred from the phage to the bacteria, thus establishing its place as the fundamental carrier of genetic information.

Until the final experiment performed by Hershey and Chase, DNA was thought to be a rather simple and boring molecule. It wasn’t considered structured enough to perform such a complicated and extremely important function. However, after this experiment, scientists started paying much more attention to DNA, leading us to where we are in research today!

Also Read: A History Of DNA: Who Discovered DNA?

• How was DNA shown to be the genetic material?. The University of Texas at Austin
• The Genetic Material - DNA - CSUN. California State University, Northridge
• Home - Books - NCBI. National Center for Biotechnology Information

Mahak Jalan has a BSc degree in Zoology from Mumbai University in India. She loves animals, books and biology. She has a general assumption that everyone shares her enthusiasm about the human body! An introvert by nature, she finds solace in music and writing.

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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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What are GMOs (Genetically Modified Organisms)?

What is Evolution: A REALLY SIMPLE and Brief Explanation

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Skills: The Hershey & Chase Experiment ( DP IB Biology: HL )

Revision note.

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Skills: The Hershey & Chase Experiment

Which biomolecule is the heritable material.

• owing to the fact that there are 20 amino acids and only 4 nucleotide bases
• In the 1950s, Alfred Hershey and Martha Chase showed that DNA, not protein, is a factor of heredity responsible for carrying genetic information from one generation to another
• Viruses that infect bacteria were used in their experiment as they only consist of DNA encapsulated by a protein coat
• This would allow the biomolecule of heredity (ie. the one that caused bacterial cells to be used to produce viral progeny) to be easily determined

Analysis of results of the Hershey and Chase experiment provided evidence that DNA is the genetic material.

• DNA contains phosphorus but no sulfur
• Amino acids (that make up proteins) contain sulfur but no phosphorus
• Bacteria grown in separate media containing either radioactive sulfur ( 35 S) or radioactive phosphorus ( 32 P) were infected with viruses
• The progeny viruses contained either 35 S labelled proteins or 32 P labelled DNA
• Bacteria would be expected to contain the heritable material following infection
• Viruses are small so remained in the supernatant in the centrifuge tube
• Bacteria are larger so formed a pellet
• Only the bacteria infected by 32 P labelled viruses (DNA) were shown to be radioactive
• This suggested that DNA (and not protein) was transferred to bacteria and is the hereditary (genetic) material

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The Hershey & Chase Blendor Experiment (Part 2)

    During the process of infection of bacteria by phage , a quick spin in a Waring blendor shears phage away from the outside of the bacteria. Analysis of the bacterial pellet and supernatant fractions demonstrates that 32 P phosphorus-labelled DNA inside the phage shows up inside bacteria (left), whereas the 35 S sulfur-labelled protein in the external coat remains in the supernatant (right). Further, phage labelled with 32 P go on to produce progeny phage that are also labelled. This biological experiment confirms Avery 's conclusion that DNA is the transforming substance , and shows that transformation is inherited .

HOMEWORK : The Hershey & Chase blendor experiment is often described as " quick & dirty ": Why? Explain, in terms of  the key figure in Hershey & Chase (1952) (below).

Describe the experiment of Hershey and Chase to prove that DNA is the genetic material.

(1) hershey and chase performed their experiments with bacterium e. coli and bacteriophage t 2 . (2) the bacteriophage which has a dna and a protein coat attaches to the bacteria and transfers its genetic material into the bacterial cell. (3) some t 2 phages were grown in medium with radioactive, sulphur ( 35 s ) and the other t 2 phages were grown in a medium with radioactive phosphorous ( 32 p ). (4) the t 2 in ( 35 s ) medium contained radioactive protein because the dna does not contain sulphur whereas the t 2 phages in ( 32 p ) medium contained radioactive dna because the protein coat does not contain phosphorus. (5) the radioactive phages then attached to the e.coli . after the infection progressed, the viruses were separated by centrifugation. (6) the e.coli infected by t 2 phage which had radioactive dna were also radioactive, indicating that dna was the material that passed from the virus to the bacteria. (7) the bacteria that had been infected by the virus with a radioactive protein coat was not radioactive i.e., the protein had not entered the bacteria thus proving that dna was the genetic material that is passed from a virus to a bacteria..