The Hershey-Chase Experiment
 Alfred Hershey (1908-1997) was born
 in Owosso, Michigan. He studied at the
Michigan State College, where he
obtained B. S. in 1930, and Ph. D. in
1934. In 1967 he got an honorary D. Sc.
at the University of Chicago.
 Martha Chase (1930- ) recieved her bachelor's
degree in biology from the University of Dayton
and applied for a summer position, as an
undergrad, in Hershey's lab. She later recieved her
Ph. D in microbial physiology.
Escherichia coli
Escherichia coli is a bacterium that is a
common - but certainly not the most
abundant - inhabitant of the human intestine.
It also lives in the intestine of many other
animals, wild as well as domestic.
T2 Bacteriophage
 infects a bacterium & use the bacterium to
produce more viruses.
 contain 50% DNA(head) and 50% protein
 tail fibers of the phage attach to the
bacterial cell to initiate infection
 new viruses are produced within the
bacterial cell
T2 Bacteriophage
Discover of Bacteriophage
 Felix d'Herelle, was studying the feces of
patients who had recovered from a bacterial
dysentery.
 discovery of an organism capable of killing
bacteria - so small it could pass through a
filter.
 a bacterial virus----is an extremely simple
organism, composed only of protein and
DNA
 This discovery is useful in fighting disease
Which substance directed this takeover
DNA or protein?
 In 1952, American biologists Alfred
Hershey and Martha Chase set out to
determine what composed the genetic
material of a bacteriophage.
-
Bacteriophage attach a
bacteriuminject genetic material
(DNA) into bacteriumsynthesis
of DNA replicate & protein coat
and tailform a new
bacteriophage
Protein contains sulfur, but DNA
doesn't. Protein contains a small
amount of phosphorus; DNA
contains a lot of phosphorus.
DNA is the genetic
Alfred Hershey and Martha
Chase discovered that DNA is
the genetic material of a phage
known as T2
They knew that T2
A. Is one of many phages to infect bacterium
Escherichia coli (E. coli).
 Like other viruses, is little more than DNA
enclosed by a protein coat.
 Can quickly reprogram an E. coli cell to
produce T2 phages and release the viruses
when the cell lyses.
 They did not know is which viral
component - DNA or protein - was
responsible for reprogramming the host
bacterial cell
Experiment
A. 1: Viral protein and DNA were tagged
with different radioactive isotopes.
Protein Tagging: T2 and E. coli were
grown in media with radioactive
sulfur (35S) which incorporated only
into the phage protein. DNA Tagging:
T2 and E. coli were grown in media
containing radioactive phosphorus
(32P) which was incorporated only
into the phage DNA.
 2: Protein-labeled and DNA-
labeled T2 phages were
allowed to infect separate
samples of nonradioactive E.
coli cells.
 3: Cultures were agitated to
shake loose phages that
remained outside the bacterial
cells.
A.4: Mixtures were centrifuged
forcing the heavier bacterial
cells into a pellet on the bottom
of the tubes. The lighter viruses
remained in the supernatant.
5 : Radioactivity in the pellet
and supernatant was measured
and compared
Results:
 In tubes with E. coli infected with
protein labeled T2, most of the
radioactivity was in the supernatant
with viruses.
 In tubes with E. coli infected with
DNA-labeled T2, most of the
radioactivity was in the pellet with the
bacterial cells. When the bacteria
containing DNA-labeled phages were
returned to culture medium, the
bacteria released phage progeny which
contained 32P in their DNA.
Hershey and Chase found that
 When bacteriophages containing
35P
(radioactive), were allowed to
infect nonradioactive bacteria, all
the infected cells became
radioactive and, in fact, much of
the radioactivity was passed on to
the next generation of
bacteriophages.
 when the bacteria were infected
with bacteriophages labeled
with 35S and then the virus coats
removed (by whirling them in
an electric blender), practically
no radioactivity could be
detected in the infected cells.
Conclusions
A. Viral proteins remain outside
the host cell. Viral DNA is
injected into the host cell.
Injected DNA molecules cause
cells to produce additional
viruses with more viral DNA
and proteins. These data
provided evidence that nucleic
acids rather than proteins are
the hereditary material.
From these experiments, it was clear that:
 the DNA component of the
bacteriophages is injected into the
bacterial cell while the protein
component remains outside.
 However, it is the injected
component - DNA - that is able to
direct the formation of new virus
particles complete with protein
coats.
Additional evidence
 These experiment provided evidence that
DNA is the hereditary material in viruses.
Additional evidence pointed to DNA as the
genetic material in eukaryotes as well.
Some circumstantial evidence was:
 A eukaryotic cell doubles its DNA
content prior to mitosis.
 During mitosis, the doubled DNA is
equally divided between two daughter
cells.
 An organism's diploid cells have twice
the DNA as its haploid gametes.