(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.

(2)

Hershey and Chase conducted an experiment with ${}^{32}\mathrm{P}$ and ${}^{35}\mathrm{S}$ labelled ${\mathrm{T}}_2$ type bacteriophage and concluded that DNA is the genetic material.

(3)

Both DNA and RNA are able to mutate. RNA being a catalyst is reactive and unstable and mutates at a faster rate. Consequently, viruses having RNA genome and having shorter life span mutate and evolve faster.

(4)

The unequivocal proof that DNA is the genetic material came from the experiments of Alfred Hershey and Martha Chase (1952). They worked with viruses that infect bacteria called bacteriophages.

(2)

Semi-conservative replication of DNA was proved by the work of Matthew Meselson and Franklin Stahl (1958) using bacterium Escherichia coli.

(1)

Ribozymes (ribonucleic acid enzymes) are RNA molecules that are capable of catalysing specific biochemical reactions similar to the action of protein enzymes.

(1)

(2)

Taylor et al. (1957) conducted experiment on Vicia faba (broad bean) to prove semi-conservative replication of DNA. He fed dividing cells of root tips of Vicia faba with radioactive ${}^3\mathrm{H}$ containing thymine instead of normal thymine and found that all the chromosomes became radioactive. Labelled thymine was then replaced with normal one. Next generation came to have radioactivity in one of the two chromatids of each chromosome while in subsequent generation radioactivity was present in 50% of the chromosomes. This is possible only if out of the two strands of a chromosome, one is formed afresh while the other is conserved at each replication.

(3)

Genetic material should be structurally and chemically stable otherwise its expression will change and lead to loss of several metabolic functions, etc.

(3)

Transformation was first studied by S.F. Griffith in 1928 while studying Streptococcus pneumoniae. He found that R-type non-virulent bacteria pick up virulence from heat killed virulent S-type bacteria and transform into virulent forms. It was this experiment which indicated presence of a ‘transforming principle’ which was later found out to be DNA, by Avery et al.