→ Prior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44), the genetic material was thought to be a protein.
→ They worked to determine the biochemical nature of 'transforming principle' in Griffith's experiment.
→ They purified biochemicals (proteins, DNA, RNA, etc.) from the heat-killed S cells to see which ones could transform live R cells into S cells.
→ They discovered that DNA alone from S bacteria caused R bacteria to become transformed.
→ They also discovered that protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not affect transformation, so the transforming substance was not a protein or RNA.
→ Digestion with DNase did inhibit transformation, suggesting that the DNA caused the transformation.
→ They concluded that DNA is the hereditary material, but not all biologists were convinced.