Ans.

(2)

The experiments of Hershey and Chase on bacteriophages proved that DNA is a genetic material. They incorporated radioactive isotope of phosphorus (${}^{32}\mathrm{P}$) into phage DNA and that of sulphur (${}^{35}\mathrm{S}$) into proteins of a separate phage culture. Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not radioactive protein because DNA contains phosphorus but proteins do not. Similarly, viruses grown on radioactive sulphur contain radioactive protein but not radioactive DNA because DNA does not contain sulphur. After blending and centrifugation, they found that bacteria which was infected with viruses that had radioactive DNA were radioactive while bacteria that were infected with viruses that had radioactive proteins were not radioactive which indicated that proteins did not enter the bacteria from the viruses rather, DNA passed from virus to bacteria.