FIG. 3.2

What is the nature of the genetic material?

BACKGROUND Oswald Avery, Colin MacLeod, and Maclyn McCarty also studied virulence in pneumococcal bacteria. They recognized the significance of Griffith’s experiments (see Fig. 3.1) and wanted to identify the molecule responsible for transforming nonvirulent bacteria into virulent ones.

EXPERIMENT Avery, MacLeod, and McCarty prepared an extract from virulent bacteria that could transform nonvirulent bacteria into virulent ones. This extract allowed them to perform a series of tests and controlled experiments. To identify what caused transformation, they separated the extract into its macromolecular components. The transforming activity remained associated with the DNA; however, the DNA preparation also contained trace amounts of RNA and protein. They treated this preparation with enzymes that destroyed one of the three molecules. Their hypothesis was that transformation would not occur if they destroyed the molecule responsible for it.

RESULTS

CONCLUSION DNA is the molecule responsible for transforming nonvirulent bacteria into virulent bacteria. This experiment provided a key piece of evidence that DNA is the genetic material.

FOLLOW-UP WORK These experiments were followed up by Alfred Hershey and Martha Chase, who used a different system to confirm that DNA is the genetic material.

SOURCE Avery, O., C. MacLeod, and M. McCarty. 1944. “Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types.” Journal of Experimental Medicine 79:137–158.