The Hershey–Chase experiments were a series of
experiment
An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs whe ...
s conducted in 1952
by
Alfred Hershey and
Martha Chase that helped to confirm that
DNA
Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
is
genetic material
Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nucleic aci ...
.

While DNA had been known to
biologist
A biologist is a scientist who conducts research in biology. Biologists are interested in studying life on Earth, whether it is an individual Cell (biology), cell, a multicellular organism, or a Community (ecology), community of Biological inter ...
s since 1869, many scientists still assumed at the time that
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s carried the information for inheritance because DNA appeared to be an inert molecule, and, since it is located in the nucleus, its role was considered to be phosphorus storage. In their experiments, Hershey and Chase showed that when
bacteriophage
A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...
s, which are composed of DNA and protein, infect bacteria, their DNA enters the host bacterial cell, but most of their protein does not. Hershey and Chase and subsequent discoveries all served to prove that DNA is the hereditary material.
Hershey shared the 1969
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine () is awarded yearly by the Nobel Assembly at the Karolinska Institute for outstanding discoveries in physiology or medicine. The Nobel Prize is not a single prize, but five separate prizes that, acco ...
with
Max Delbrück and
Salvador Luria
Salvador Edward Luria (; ; born Salvatore Luria; August 13, 1912 – February 6, 1991) was an Italian microbiologist, later a Naturalized citizen of the United States#Naturalization, naturalized U.S. citizen. He won the Nobel Prize in Physiology ...
for their "discoveries concerning the genetic structure of viruses".
[
]
Historical background
In the early twentieth century, biologists thought that proteins carried genetic information. This was based on the belief that proteins were more complex than DNA.
Phoebus Levene's influential "tetranucleotide hypothesis", which incorrectly proposed that DNA was a repeating set of identical
nucleotide
Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...
s, supported this conclusion. The results of the
Avery–MacLeod–McCarty experiment, published in 1944, suggested that DNA was the genetic material, but there was still some hesitation within the general scientific community to accept this, which set the stage for the Hershey–Chase experiment.
Hershey and Chase, along with others who had done related experiments, confirmed that DNA was the biomolecule that carried genetic information. Before that,
Oswald Avery
Oswald Theodore Avery Jr. (October 21, 1877 – February 20, 1955) was a Canadian-American physician and medical researcher. The major part of his career was spent at the Rockefeller Hospital in New York City. Avery was one of the first molecu ...
,
Colin MacLeod, and
Maclyn McCarty
Maclyn McCarty (June 9, 1911 – January 2, 2005) was an American geneticist, a research scientist described in 2005 as "the last surviving member of a Manhattan scientific team that overturned medical dogma in the 1940s and became the first to ...
had shown that DNA led to the transformation of one strain of ''
Streptococcus pneumoniae
''Streptococcus pneumoniae'', or pneumococcus, is a Gram-positive, spherical bacteria, hemolysis (microbiology), alpha-hemolytic member of the genus ''Streptococcus''. ''S. pneumoniae'' cells are usually found in pairs (diplococci) and do not f ...
'' to another. The results of these experiments provided evidence that DNA was the biomolecule that carried genetic information.
Methods and results

Hershey and Chase needed to be able to examine different parts of the phages they were studying separately, so they needed to distinguish the phage subsections. Viruses were known to be composed of a protein shell and DNA, so they chose to uniquely label each with a different elemental
isotope
Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
. This allowed each to be observed and analyzed separately. Since phosphorus is contained in DNA but not amino acids,
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
phosphorus-32 was used to label the DNA contained in the
T2 phage. Radioactive sulfur-35 was used to label the protein sections of the T2 phage, because sulfur is contained in protein but not DNA.
Hershey and Chase inserted the radioactive elements in the bacteriophages by adding the isotopes to separate media within which bacteria were allowed to grow for 4 hours before bacteriophage introduction. When the bacteriophages infected the bacteria, the
progeny contained the radioactive isotopes in their structures. This procedure was performed once for the sulfur-labeled phages and once for phosphorus-labeled phages.
The labeled progeny were then allowed to infect unlabeled bacteria. The phage coats remained on the outside of the bacteria, while genetic material entered. Disruption of phage from the bacteria by
agitation in a blender followed by
centrifugation
Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...
allowed for the separation of the phage coats from the bacteria. These bacteria were
lysed to release phage progeny. The progeny of the phages that were labeled with radioactive phosphorus remained labeled, whereas the progeny of the phages labeled with radioactive sulfur were unlabeled. Thus, the Hershey–Chase experiment helped to confirm that DNA, not protein, is the genetic material.
Hershey and Chase showed that the introduction of
deoxyribonuclease (referred to as
DNase
Deoxyribonuclease (DNase, for short) refers to a group of glycoprotein endonucleases which are enzymes that catalyze the hydrolytic cleavage of phosphodiester linkages in the DNA backbone, thus degrading DNA. The role of the DNase enzyme in cells ...
), an
enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
that breaks down DNA, into a solution containing the labeled bacteriophages did not introduce any
32P into the solution. This demonstrated that the phage is resistant to the enzyme while intact. Additionally, they were able to
plasmolyze the bacteriophages so that they went into osmotic shock, which effectively created a solution containing most of the
32P and a heavier solution containing structures called "ghosts" that contained the
35S and the protein coat of the virus. It was found that these "ghosts" could
adsorb
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...
to bacteria that were susceptible to T2, although they contained no DNA and were simply the remains of the original viral capsule. They concluded that the protein protected the DNA from DNase, but that once the two were separated and the phage was inactivated, the DNase could hydrolyze the phage DNA.
Experiment and conclusions
Hershey and Chase were also able to prove that the DNA from the phage is inserted into the bacteria shortly after the virus attaches to its host. Using a high-speed blender they were able to force the bacteriophages from the bacterial cells after
adsorption
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
. The lack of
32P-labeled DNA remaining in the solution after the bacteriophages had been allowed to adsorb to the bacteria showed that the phage DNA was transferred into the bacterial cell. The presence of almost all the radioactive
35S in the solution showed that the protein coat that protects the DNA before adsorption stayed outside the cell.
Hershey and Chase concluded that DNA, not protein, was the genetic material. They determined that a protective protein coat was formed around the bacteriophage, but that the internal DNA is what conferred its ability to produce progeny inside a bacterium. They showed that, in growth, protein has no function, while DNA has some function. They determined this from the amount of radioactive material remaining outside of the cell. Only 20% of the
32P remained outside the cell, demonstrating that it was incorporated with DNA in the cell's genetic material. All of the
35S in the protein coats remained outside the cell, showing it was not incorporated into the cell, and that protein is not the genetic material.
Hershey and Chase's experiment concluded that little sulfur-containing material entered the bacterial cell. However no specific conclusions can be made regarding whether material that is sulfur-free enters the bacterial cell after phage adsorption. Further research was necessary to conclude that it was solely bacteriophages' DNA that entered the cell and not a combination of protein and DNA where the protein did not contain any sulfur.
Discussion
Confirmation
Hershey and Chase concluded that protein was not likely to be the hereditary genetic material. However, they did not make any conclusions regarding the specific function of DNA as hereditary material, and only said that it must have some undefined role.
Confirmation and clarity came a year later in 1953, when
James D. Watson and
Francis Crick
Francis Harry Compton Crick (8 June 1916 – 28 July 2004) was an English molecular biologist, biophysicist, and neuroscientist. He, James Watson, Rosalind Franklin, and Maurice Wilkins played crucial roles in deciphering the Nucleic acid doub ...
correctly hypothesized, in their journal article "
Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid", the
double helix
In molecular biology, the term double helix refers to the structure formed by base pair, double-stranded molecules of nucleic acids such as DNA. The double Helix, helical structure of a nucleic acid complex arises as a consequence of its Nuclei ...
structure of DNA, and suggested the
copying mechanism by which DNA functions as hereditary material. Furthermore, Watson and Crick suggested that DNA, the genetic material, is responsible for the synthesis of the thousands of proteins found in cells. They had made this proposal based on the structural similarity that exists between the two macromolecules: both protein and DNA are linear sequences of monomers (amino acids and nucleotides, respectively).
Other experiments
Once the Hershey–Chase experiment was published, the scientific community generally acknowledged that DNA was the genetic code material. This discovery led to a more detailed investigation of DNA to determine its composition as well as its 3D structure. Using
X-ray crystallography
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
, the structure of DNA was discovered by James Watson and Francis Crick with the help of previously documented experimental evidence by
Maurice Wilkins
Maurice Hugh Frederick Wilkins (15 December 1916 – 5 October 2004) was a New Zealand-born British biophysicist and Nobel laureate whose research spanned multiple areas of physics and biophysics, contributing to the scientific understanding ...
and
Rosalind Franklin
Rosalind Elsie Franklin (25 July 192016 April 1958) was a British chemist and X-ray crystallographer. Her work was central to the understanding of the molecular structures of DNA (deoxyribonucleic acid), RNA (ribonucleic acid), viruses, coal ...
.
[
]
Knowledge of the structure of DNA led scientists to examine the nature of genetic coding and, in turn, understand the process of protein synthesis.
George Gamow proposed that the
genetic code
Genetic code is a set of rules used by living cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or RNA sequences of nucleotide triplets or codons) into proteins. Translation is accomplished ...
was composed of sequences of three DNA base pairs known as triplets or
codons
Genetic code is a set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets or codons) into proteins. Translation is accomplished by the ribosome, which links pro ...
which represent one of the twenty amino acids.
Genetic coding helped researchers to understand the mechanism of
gene expression
Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...
, the process by which information from a gene is used in
protein synthesis
Protein biosynthesis, or protein synthesis, is a core biological process, occurring inside cells, balancing the loss of cellular proteins (via degradation or export) through the production of new proteins. Proteins perform a number of critica ...
. Since then, much research has been conducted to modulate steps in the gene expression process. These steps include
transcription,
RNA splicing
RNA splicing is a process in molecular biology where a newly-made precursor messenger RNA (pre-mRNA) transcription (biology), transcript is transformed into a mature messenger RNA (Messenger RNA, mRNA). It works by removing all the introns (non-cod ...
,
translation
Translation is the communication of the semantics, meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The English la ...
, and
post-translational modification
In molecular biology, post-translational modification (PTM) is the covalent process of changing proteins following protein biosynthesis. PTMs may involve enzymes or occur spontaneously. Proteins are created by ribosomes, which translation (biolog ...
which are used to control the chemical and structural nature of proteins.
Moreover, genetic engineering gives engineers the ability to directly manipulate the genetic materials of organisms using
recombinant DNA
Recombinant DNA (rDNA) molecules are DNA molecules formed by laboratory methods of genetic recombination (such as molecular cloning) that bring together genetic material from multiple sources, creating sequences that would not otherwise be fo ...
techniques. The first recombinant DNA molecule was created by
Paul Berg in 1972 when he combined DNA from the monkey virus
SV40 with that of the
lambda phage
Lambda phage (coliphage λ, scientific name ''Lambdavirus lambda'') is a bacterial virus, or bacteriophage, that infects the bacterial species ''Escherichia coli'' (''E. coli''). It was discovered by Esther Lederberg in 1950. The wild type of ...
.
Experiments on hereditary material during the time of the Hershey–Chase experiment often used bacteriophages as a
model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...
. Bacteriophages lend themselves to experiments on hereditary material because they incorporate their
genetic material
Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nucleic aci ...
into their
host cell's genetic material (making them useful tools), they multiply quickly, and they are easily collected by researchers.
Legacy
The Hershey–Chase experiment, its predecessors, such as the
Avery–MacLeod–McCarty experiment, and successors served to unequivocally establish that hereditary information was carried by DNA. This finding has numerous applications in
forensic science
Forensic science combines principles of law and science to investigate criminal activity. Through crime scene investigations and laboratory analysis, forensic scientists are able to link suspects to evidence. An example is determining the time and ...
,
crime investigation and
genealogy
Genealogy () is the study of families, family history, and the tracing of their lineages. Genealogists use oral interviews, historical records, genetic analysis, and other records to obtain information about a family and to demonstrate kin ...
. It provided the background knowledge for further applications in DNA forensics, where
DNA fingerprinting
DNA profiling (also called DNA fingerprinting and genetic fingerprinting) is the process of determining an individual's deoxyribonucleic acid (DNA) characteristics. DNA analysis intended to identify a species, rather than an individual, is cal ...
uses data originating from DNA, not protein sources, to deduce
genetic variation
Genetic variation is the difference in DNA among individuals or the differences between populations among the same species. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources ...
.
References
External links
Hershey–Chase experiment animationClear depiction and simple summary
{{DEFAULTSORT:Hershey-Chase Experiment
Genetics experiments
Bacteriophages
1952 in biology