Life: The Science of Biology, Ninth Edition
Sadava • Hillis • Heller • Berenbaum
Working with Data
The Hershey–Chase Experiment
(Textbook Figure 13.4)
Introduction
Less than a decade after Avery, MacLeod, and
McCarty’s work, Alfred Hershey and Martha
Chase did their famous blender experiment which
showed that DNA is the genetic material. In
parallel experiments, bacteriophages containing
either 32P-labeled DNA or 35S-labeled proteins
were allowed to infect bacteria. Following
infection, the cultures were agitated in a blender
to detach viruses from the bacterial cells and then
centrifuged. Bacterial cells formed a pellet at the
bottom, while the viral particles remained in the
supernatant. In the first experiment, most of the
radioactivity was found in the infected bacterial,
while in the second experiment, most of the
radioactivity was found in the phage coat.
Together, these experiments demonstrated that
DNA, not protein, carried the genetic instructions
for infecting bacteria. Although Avery, McLeod,
and McCarty had provided earlier evidence that
DNA was the transforming principle, some doubt
had remained among the scientific community.
This classic experiment by Hershey and Chase
settled the long-standing debate, and was
eventually accepted as definitive proof that DNA
is the genetic material. The discovery that RNA
could also function as a genetic material came several years later in 1956. Two
independent groups, one led by Heinz Fraenkel-Conrat and the other by Alfred Gierer
and Gerhard Schramm, identified RNA as the source of hereditary information in tobacco
mosaic virus (TMV), a virus that infects plants.
Original Paper
Hershey, A. D. and M. Chase. 1952. Independent functions of viral protein and nucleic
acid in growth of bacteriophage. The Journal of General Physiology 36: 39–56.
http://www.jgp.org/cgi/content/abstract/36/1/39
© 2011 Sinauer Associates, Inc.
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Links
(For additional links on this topic, refer to the Chapter 13 Experiment Links.)
Memorial University of Newfoundland: Hershey and Chase 1952
http://www.mun.ca/biology/scarr/4241_mkm_hersheychase.htm
PubMed: Tobacco mosaic virus RNA as genetic determinant: genesis of a discovery
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1
1256209&dopt=Abstract
Descriptions of Plant Viruses Web: tobacco mosaic virus
http://www.dpvweb.net/dpv/showdpv.php?dpvno=370
Analyze the Data
Hershey and Chase prepared two types of phage T2 particles: one with their protein coat
labeled with radioactive sulfur (35S; in the original paper written S35), and one with their
DNA labeled with radioactive phosphorus (32P; in the original paper written P32). Two
previous observations were especially important: First, just after an infection cycle began,
electron micrographs showed phage particles still attached to the outside of the cell wall
of the host bacteria, only the particles looked like empty “ghosts.” Second, these ghosts
could be prepared in the test tube by incubating T2 phage in water (plasmolysis). To
determine what was released from the plasmolyzed phage, the scientists incubated both
types of labeled phage in water. They tested for the presence of labeled DNA or protein,
whether or not the molecules were acid-soluble (had been hydrolyzed to monomers—
nucleotides or amino acids), whether or not they could attach to bacteria, and whether or
not they reacted to antibodies for intact T2 phage. The results are shown in Table 1.
© 2011 Sinauer Associates, Inc.
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Question 1
Compare the DNase treatment of intact phage and plasmolyzed phage. What can you
conclude about the presence of DNA in intact phage and ghosts?
Question 2
What happened to the DNA after the phage was plasmolyzed? Why did this not happen
to the protein?
Question 3
Explain the antibody data in terms of what remains of phage particles when they become
ghosts.
The fragile attachment of phage to the outside of host bacteria led the scientists to allow
attachment and then agitate the bacteria-phage to try to remove the latter. Then, the
distributions of radioactive labels were determined. Also, the infection of the host
bacteria was tested. Data are shown in Figure 1.
Figure 1
Question 4
What are the differences between the distribution of the labeled protein and DNA? What
conclusion can you draw about the infection process?
Question 5
Relate the data and conclusions seen in Figure 1 to those in Table 1.
Question 6
Why was the infection rate of bacteria an important measurement?
© 2011 Sinauer Associates, Inc.
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Question 7
Can you explain why over 30% of the 32P label was in the extracellular fraction?
© 2011 Sinauer Associates, Inc.
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