Hershey and Chase Experiment | Discovery of DNA as Genetic Material

Phages have been widely used as tools by researchers in
molecular genetics. In 1952, Alfred Hershey and Martha
Chase performed experiments showing that DNA is the
genetic material of a phage known as T2. This is one of
many phages that infect Escherichia coli (E. coli), a bacterium
that normally lives in the intestines of mammals and
is a model organism for molecular biologists. At that time,biologists already knew that T2, like many other phages,
was composed almost entirely of DNA and protein. They
also knew that the T2 phage could quickly turn an E. coli
cell into a T2-producing factory that released many copies
of new phages when the cell ruptured. Somehow, T2 could
reprogram its host cell to produce viruses. But which viral
component—protein or DNA—was responsible?
Hershey and Chase answered this question by devising
an experiment showing that only one of the two components
of T2 actually enters the E. coli cell during infection. In their experiment, they used a radioactive isotope of sulfur to tag protein in one batch of T2 and a
radioactive isotope of phosphorus to tag DNA in a second
batch. Because protein, but not DNA, contains sulfur, radioactive
sulfur atoms were incorporated only into the protein
of the phage. In a similar way, the atoms of radioactive
phosphorus labeled only the DNA, not the protein, because
nearly all the phage’s phosphorus is in its DNA. In the experiment,
separate samples of nonradioactive E. coli cells
were infected with the protein-labeled and DNA-labeled
batches of T2. The researchers then tested the two samples
shortly after the onset of infection to see which type of
molecule—protein or DNA—had entered the bacterial
cells and would therefore be capable of reprogramming
them.
Hershey and Chase found that the phage DNA entered
the host cells but the phage protein did not. Moreover,
when these bacteria were returned to a culture medium, and
the infection ran its course, the E. coli released phages that
contained some radioactive phosphorus. This result further
showed that the DNA inside the cell played an ongoing role
during the infection process.
Hershey and Chase concluded that the DNA injected by
the phage must be the molecule carrying the genetic information
that makes the cells produce new viral DNA and
proteins. The Hershey-Chase experiment was a landmark
study because it provided powerful evidence that nucleic
acids, rather than proteins, are the hereditary material, at
least for certain viruses.

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