Experiments that Inspire
The Hershey-Chase Experiments (1952)
One of the greatest threats to human health is viral infection.
Diseases caused by viruses range from the merely inconvenient (warts,
the common cold), to the worrisome (flu, mumps, and measles), to the
potentially fatal (hepatitis, polio, and AIDS). Humans are not alone
in this suffering - all cellular organisms, even bacteria, can be
attacked by viruses.
The first bacterial viruses were discovered in 1917 by scientists
working independently in London and Paris. The French scientist,
Felix d'Herelle, was studying the feces of patients who had recovered
from a bacterial dysentery. This somewhat unpleasant work led him to
the discovery of an organism capable of killing bacteria - so small it
could pass through a filter. He coined the term "bacteriophage,"
meaning eater of bacteria, to describe his discovery. d'Herelle was
hopeful that this discovery would be useful in fighting disease. It
was - in ways he never anticipated: The study of a bacterial virus
turned out to be crucial in establishing the identity of DNA as the
genetic material of all living things.
In 1952, American biologists Alfred Hershey and Martha Chase set out
to determine what composed the genetic material of a bacteriophage.
They knew that a bacterial virus was an extremely simple organism,
composed only of protein and DNA. The protein makes up the exterior
of the virus, and the DNA is contained within it. When a bacterium is
infected by a bacteriophage, the bacterium's internal machinery falls
under the control of the virus, which uses the bacterium to produce
more viruses. What Hershey and Chase wanted to know was: Which
substance directed this takeover - DNA or protein?
The experiment that Hershey and Chase devised to differentiate between
these possibilities was simple and took advantage of the differences
in the composition of protein and DNA. Protein contains sulfur, DNA
doesn't. Protein contains a small amount of phosphorus; DNA contains
a lot of phosphorus.
Hershey and Chase added bacteriophage to cultures containing either
radioactive sulfur or radioactive phosphorus. The bacteriophages
grown in the cultures with radioactive sulfur picked it up and
incorporated it into their protein. The bacterial viruses grown in
the culture with radioactive phosphorus picked that up, incorporating
a little of it into the protein, but most of it into their DNA.
Hershey and Chase now had two types of bacteriophages: one with a
radioactive external protein coat, the other with highly radioactive
DNA. They were ready to begin their experiment.
Each of the two types of radioactive bacteriophage was added to a
separate culture of bacteria. The bacteriophages were allowed to
infect the bacteria, then the cultures were whirled in a kitchen
blender, causing any part of the bacteriophages that hadn't got inside
the bacteria to fall off. Next the cultures were spun in a
centrifuge, which separates materials suspended in liquid according to
their weights. The heavier bacterial cells fell to the bottom and
formed a pellet, the lighter bacteriophages and loose phage parts
remained in the liquid.
Where was the radioactivity now? It depended on which radioactive
element you looked for. In the cultures infected by bacteriophages
with radioactive sulfur (with labeled protein), most of the
radioactivity was in the liquid with the phages. In the cultures
infected by bacteriophages with radioactive phosphorus (with most of
the label in their DNA), most of the radioactivity was in the pellet
of infected bacteria. Thus, the radioactive protein hadn't entered
the bacterial cells, but the DNA had.
The final proof that DNA, not protein, was the genetic material was
provided by the offspring of the phosphorus-labeled bacteriophages.
They had radioactive DNA, passed down from their parents, but no
radioactive protein. These experiments convinced the scientific
community that DNA alone was the material of heredity, and inspired
Watson and Crick
to begin their efforts to discover its structure.
Go to Graphics Gallery: The
Hershey-Chase Blender Experiment,
Examples of Viral
Go to next story: One Gene/One Enzyme Hypothesis
Return to About Biotech directory