Hershey (Alfred Hershey) and Chase (Martha Chase)
performed a series of experiments in 1953 to prove that the DNA is the genetic
material. The DNA was known to biologists since 1869, but still some scientists
assumed that protein has some role in the inheritance. The key concept of their
experiments was that Phage infection proved that DNA is the genetic material of
viruses. When the DNA and protein components of bacteriophages were labelled with
different radioactive isotopes, only the DNA is transmitted to the progeny
phages produced by infecting bacteria.
First they labelled the bacteriophages with 35S,
since Sulphur is a component of protein coat of bacteriophage, the 35S
is incorporated to the coat of the bacteriophage. This bacteriophage was
allowed to infect the bacteria. The bacteriophages infect the bacteria in a
manner that their protein coat is left behind out side the bacterial cell and
only the DNA of the bacteriophage enters the bacteria and incorporates with the
genome of the bacteria. After the infection of the bacteriophage centrifugation
is done to separate the capsids and the bacterial cells. It was found that 35S
was only found in the protein coat and not in the bacterial cells.
Secondly they labelled the bacteriophages with 32P.
Since Phosphorus is the essential component of the DNA. The 32P was
incorporated in the DNA of the bacteriophages. These labelled bacteriophages
were allowed to infect the bacteria. The bacteriophages left their protein coat
outside the bacterial cell and they injected their DNA into the bacterial cell
which incorporated with the genome of bacteria. Then centrifugation was done to
separate the protein coat and the bacteria. On testing 32P was found
in the bacterial cell. And not in the protein coat.
This experiment prominently proved that the DNA is the
only genetic material, and the protein has nothing to do with the inheritance and
genetic information.
