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Viral-infecting agent is DNA: An experiment is performed by Hershey and Chase on bacteriophage showing phenomenon of transduction.
Bacteriophages are bacterial viruses. T2 is a bacteriophage which infects Escherichia coli, the bacterium occurs as commensal in human intestine. Escherichia coli can also be grown over culture medium. A.D. Hershey and Martha Chase (1952) grew two cultures of Escherichia coli.
One culture was supplied with radio-active sulphur, 35S. The other culture was provided with radioactive phosphorus, 32P. Radioactive sulphur gets incorporated into sulphur containing amino acids (cystine, cysteine and methionine), and therefore, becomes part of bacterial proteins. Radioactive phosphorus gets incorporated into nucleotides which form nucleic acids, mostly DNA. Therefore, bacteria of both the cultures became labeled (= hot).
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Hershey and Chase then introduced bacteriophage T2 in both the bacterial cultures. The virus entered the bacteria where it multiplied. The viral progeny was tested in both the cases. It was labeled, one type with radio-active protein and other type with radioactive DNA (Fig. 6.2).
Each type of bacteriophages were now introduced in separate cultures having normal or unlabeled bacteria. After some time both the cultures were gently shaken in a blender to remove remains of phage particles sticking to the surface of bacteria.
The culture was then centrifuged. The heavier (infected as well) bacteria settled down in the form of pellet. The supernatant contains viral coats which do not enter the bacterial cells. Both the pellet and supernatant were analyzed. It was found that phage with labeled protein did not make the bacteria labeled. Instead, radio-activity was restricted to the supernatant which was found to contain protein coat.
In the second culture where bacteriophage labeled with radioactive DNA was introduced, it was found that shaking did not produce any radio-activity in the supernatant having protein coats. Instead, the bacteria became labeled proving that only DNA of the phage entered the bacteria.
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The progeny of the two types of bacteriophages was again tested for radio-activity. Radioactivity was absent in the viruses derived from parents having labeled protein. The viruses derived from parents having labeled DNA possessed radio-activity. This shows that the genetic chemical is DNA and not the protein (Fig. 6.2).
Indirect Evidences that DNA is Hereditary Material:
DNA is genetic material is further confirmed by the following facts:
1. The total DNA content in all diploid cell of an organism and in cell of all the organisms of a species is the same.
2. Fertilization between male and female gametes double the amount of DNA in zygote which is the same as found in the somatic cells of the parent.
3 Polyploidy brings about a corresponding increase in the amount of DNA in the cell.
4. Composition of DNA is the same in all organisms of a species, but different in the organisms of different species.
5. Physical and chemical agents, which bring about mutations in genes also change the chemical structure of DNA.
