The Hershey–Chase experiments were a series of experiments conducted
in 1952 by
Alfred Hershey and
Martha Chase that helped to confirm
DNA is genetic material. While
DNA had been known to biologists
since 1869, many scientists still assumed at the time that proteins
carried the information for inheritance because
DNA appeared simpler
than proteins. In their experiments, Hershey and Chase showed that
when bacteriophages, which are composed of
DNA and protein, infect
DNA enters the host bacterial cell, but most of their
protein does not. Although the results were not conclusive, and
Hershey and Chase were cautious in their interpretation, previous,
contemporaneous, and subsequent discoveries all served to prove that
DNA is the hereditary material.
Hershey shared the 1969
Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine with Max
Salvador Luria for their “discoveries concerning the
genetic structure of viruses.”
1 Historical background
2 Methods and results
Experiment and conclusions
3.2 Other experiments
6 External links
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
a repeating set of identical nucleotides, 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,
DNA was the biomolecule that carried genetic
information. Before that, Oswald Avery, Colin MacLeod, and Maclyn
McCarty had shown that
DNA led to the transformation of one strain of
Streptococcus pneumoniae to another. The results of these experiments
provided evidence that
DNA was the biomolecule that carried genetic
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 isolate 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. This allowed each to be observed and analyzed
separately. Since phosphorus is contained in
DNA but not amino acids,
radioactive 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 amino acids but not
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
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 originally labeled ,remained labeled, while the
progeny of the phages originally labeled with 35S were unlabeled.
Hershey–Chase experiment helped confirm that DNA, not
protein, is the genetic material.
Hershey and Chase showed that the introduction of deoxyribonuclease
(referred to as DNase), an enzyme 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 to bacteria that were susceptible to
T2, although they contained no
DNA and were simply the remains of the
original bacterial capsule. They concluded that the protein protected
DNA from DNAse, but that once the two were separated and the phage
was inactivated, the DNAse could hydrolyze the phage DNA. However,
it subsequently became clear that in some viruses, RNA is the genetic
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. The lack of
DNA remaining in the solution after the bacteriophages had
been allowed to adsorb to the bacteria showed that the phage
transferred into the bacterial cell. The presence of almost all the
radioactive 35S in the solution showed that the protein coat that
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.
DNA § History of
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.
Francis Crick correctly hypothesized, in their journal
article "Molecular Structure of Nucleic Acids: A Structure for
Deoxyribose Nucleic Acid", the double helix 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, that
is, both protein and
DNA are linear sequences of amino acids and
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, the structure of
DNA was discovered by James Watson
Francis Crick with the help of previously documented experimental
Maurice Wilkins and Rosalind Franklin. Knowledge of the
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 was composed of sequences
DNA base pairs known as triplets or codons which represent
one of the twenty amino acids. Genetic coding helped researchers to
understand the mechanism of gene expression, the process by which
information from a gene is used in protein synthesis. Since then, much
research has been conducted to modulate steps in the gene expression
process. These steps include transcription, RNA splicing, translation,
and post-translational modification 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 techniques. The
DNA molecule was created by Paul Berg in 1972 when
DNA from the monkey virus SV40 with that of the lambda
Experiments on hereditary material during the time of the
Experiment often used bacteriophages as a model
organism. Bacteriophages lend themselves to experiments on hereditary
material because they incorporate their genetic material into their
host cell's genetic material (making them useful tools), they multiply
quickly, and they are easily collected by researchers.
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, crime
investigation and genealogy. It provided the background knowledge for
further applications in
DNA forensics, where
DNA fingerprinting uses
data originating from DNA, not protein sources, to deduce genetic
^ a b c d Hershey A, Chase M; Chase (1952). "Independent functions of
viral protein and nucleic acid in growth of bacteriophage" (PDF). J
Gen Physiol. 36 (1): 39–56. doi:10.1085/jgp.36.1.39.
PMC 2147348 . PMID 12981234.
^ Dahm R (January 2008). "Discovering DNA: Friedrich Miescher and the
early years of nucleic acid research". Hum. Genet. 122 (6): 565–81.
doi:10.1007/s00439-007-0433-0. PMID 17901982.
Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine 1969". Nobel Foundation.
^ a b O'Connor, Clare (2008). "Isolating hereditary material:
Frederick Griffith, Oswald Avery, Alfred Hershey, and Martha Chase".
Scitable by Nature Education. Retrieved 20 March 2011.
^ Pauling L, Corey RB; Corey (February 1953). "A Proposed Structure
For The Nucleic Acids". Proc. Natl. Acad. Sci. U.S.A. 39 (2): 84–97.
PMC 1063734 . PMID 16578429.
Rosalind Franklin and the Double Helix. Physics Today, March
2003". Physics Today. Retrieved 2011-04-06.
^ Crick, Francis (1988). "Chapter 8: The genetic code". What mad
pursuit: a personal view of scientific discovery. New York: Basic
Books. pp. 89–101. ISBN 0-465-09138-5.
^ Berk V, Cate JH; Cate (June 2007). "Insights into protein
biosynthesis from structures of bacterial ribosomes". Curr. Opin.
Struct. Biol. 17 (3): 302–9. doi:10.1016/j.sbi.2007.05.009.
^ Jackson DA, Symons RH, Berg P; Symons; Berg (October 1972).
"Biochemical method for inserting new genetic information into
Simian Virus 40: circular SV40
DNA molecules containing lambda phage
genes and the galactose operon of Escherichia coli". Proc. Natl. Acad.
Sci. U.S.A. 69 (10): 2904–9. Bibcode:1972PNAS...69.2904J.
doi:10.1073/pnas.69.10.2904. PMC 389671 .
PMID 4342968. CS1 maint: Multiple names: authors list (link)
^ Jobling MA, Gill P; Gill (October 2004). "Encoded evidence:
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Hershey–Chase experiment animation
Clear depiction and simple summary
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