How did Hershey and Chase proved that DNA is the hereditary material? Explain their experiment with suitable diagram.

Explanation

A.D. Hershey and Martha Chase (1952) grew two cultures of Escherichia call One culture was supplied with radioactive sulphur S. The other culture was provided with radioactive phosphorus, P. Radioactive sulphur gets incorporated into sulphur containing amino acids (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 labelled (=hot)

Hershey and Chase then introduced bacteriophage T, in both the bacterial cultures. The virus entered the bacteria where it multiplied. The viral progeny was tested in both the cases. It was labelled. one type with radio active protein and other type with radioactive DNA (Fig. 6.3). Each type of bacteriophages were now introduced in separate cultures having normal or unlabelled bacteria. After some time both the cultures were gently shaken in a blender at 10000 rpm to remove the empty phage capsids (or ghosts) 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 lighter viral coats which do not enter the bacterial cells. Both the pellet and supernatant were analysed. It was found that phage with labelled protein did not make the bacteria labelled. Instead. radio-activity was restricted to the supernatant which was found to contain only empty phage capsids or ghosts. In the second culture where bacteriophage labelled with radioactive DNA was introduced, it was found that shaking did not produce any radio-activity in the supernatant having empty capsids coats. Instead, the bacteria became labelled proving that only DNA of the phage entered the bacteria. The progeny of the two types of bacteriophages was again tested for radio activity. Radioactivity was absent in the viruses derived from parents having labelled protein. The viruses derived from parents having labelled DNA possessed radio-activity. This shows that the genetic chemical is DNA and not the protein