3. DNA as the transforming principle

Bacteriologists were interested in the difference between the two strains of Streptococci that Frederick Griffith had identified in 1928. The bacteriologists suspected the transforming factor was some kind of protein. The transforming principle could be involved with alcohol, which showed that it was not a carbohydrate, like the polysaccharide coat itself. But Avery and MacCarty observed that proteases, enzymes that degrade proteins, did not destroy the transforming principle. Neither did lipases, enzymes that digest lipids. They found that the transforming substance was rich in nucleic acids, but ribonuclease, which digests RNA, did not inactivate the substance. Avery and McCarty also found that the transforming principle had a high molecular weight. In 1944, Oswald Avery, Colin MacLeod and Maclyn MacCarty showed in their experiments (Figure 21.2) that DNA (not proteins) can transform the properties of cells. They had isolated DNA which was the agent to produce an enduring, heritable change in an organism. Thus, clarifying the chemical nature of the genes and proving that DNA as the "transforming principle" while studying Streptococcus pneumonia, bacteria that can cause pneumonia. Until then, biochemists had assumed that deoxyribonucleic acid was a relatively unimportant, structural chemical in chromosomes and that protein, with their greater chemical complexity, transmitted genetic traits.

Figure 21.2: Avery, Macleod and McCarty's experiment demonstrating the Griffith's transformation