DNA
is the genetic material
Many biologists despite
the earlier experiments of Griffith, Avery and others, still believed that
protein, not DNA, was the hereditary material in a cell. As eukaryotic chromosomes
consist of roughly equal amounts of protein and DNA, it was said that only a
protein had sufficient chemical diversity and complexity to encode the
information required for genetic material. In 1952, however, the results of the
Hershey-Chase experiment finally provided convincing evidence that DNA is the
genetic material.
Alfred Hershey and Martha Chase (1952)
conducted experiments on bacteriophages that infect bacteria. Phage T2
is a virus that infects the bacterium Escherichia coli. When phages
(virus) are added to bacteria, they adsorb to the outer surface, some material
enters the bacterium, and then later each bacterium lyses to release a large
number of progeny phage. Hershey and Chase wanted to observe whether it was DNA
or protein that entered the bacteria. All nucleic acids contain phosphorus, and
contain sulphur (in the amino acid cysteine and methionine). Hershey and Chase
designed an experiment using radioactive isotopes of Sulphur (35 S) and
phosphorus (32P) to keep separate track of the viral protein and nucleic acids
during the infection process. The phages were allowed to infect bacteria in
culture medium which containing the radioactive isotopes 35S or 32P. The
bacteriophage that grew in the presence of 35S had labelled proteins and
bacteriophages grown in the presence of 32P had labelled DNA.
The differential
labelling thus enabled them to identify DNA and proteins of the phage.
Hershey and Chalse mixed
the labelled phages with unlabeled E. coli and allowed bacteriophages to
attack and inject their genetic material. Soon after infection (before lysis of
bacteria), the bacterial cells were gently agitated in a blender to loosen the
adhering phase particles. It was observed that only 32P was found associated
with bacterial cells and 35S was in the surrounding medium and not in the
bacterial cells. When phage progeny was studied for radioactivity, it was found
that it carried only 32 P and not 35S (Fig. 5.2). These results clearly
indicate that only DNA and not protein coat entered the bacterial cells.
Hershey and Chase thus conclusively proved that it was DNA, not protein, which
carries the hereditary information from virus to bacteria.
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