Q.Explain how Avery, MacLeod and McCarty used protease, RNase and DNase treatment of the purified transforming principle to identify its chemical nature. What result did each enzyme treatment give?

Avery, MacLeod and McCarty (1944) identified the chemical nature of Griffith's 'transforming principle' through a purely biochemical approach. They purified the transforming principle from heat-killed S-strain bacteria and treated separate samples with three different classes of enzyme: protease (which digests protein), RNase (which digests RNA), and DNase (which digests DNA). Treatment with protease or RNase left the transforming activity completely intact — transformation of R bacteria into S bacteria still occurred — ruling out both protein and RNA as the transforming principle. Treatment with DNase, however, completely destroyed transforming activity, showing that DNA — and DNA alone — was responsible. This was the first purely biochemical, molecule-by-molecule proof that DNA is the genetic material. Even so, many biologists at the time remained skeptical, since DNA's simple four-base chemical alphabet seemed too limited to encode the vast complexity of heredity, and it took the independently designed Hershey-Chase bacteriophage experiment of 1952 to convince the wider scientific community conclusively.