Animation 4.2 Avery, MacLeod & McCarty
Textbook Reference: Heredity, Genes, and DNA, p. 116
In 1944, Avery, MacLeod, and McCarty performed experiments to determine the chemical nature of the transforming principle, which in today's terms is genetic material. They prepared an active transforming principle from a heat-killed S strain of Pneumococcus bacteria. A live S strain is pathogenic and kills mice.
A bioassay was performed in which this active transforming principle was added to a nonpathogenic R strain of bacteria, and then the bacteria were used to inoculate mice. In such an assay, the bacteria were transformed—they had become pathogenic.
In an important experiment, they first treated the transforming principle with a protease, a protein-destroying enzyme. Although it hydrolyzed the proteins in the bacterial extract, this treatment did not inactivate the transforming principle, which in a bioassay was still capable of transforming a nonpathogenic R strain of Pneumococcus into a pathogenic one.
Then they treated the transforming principle with a carbohydrase. This hydrolyzed the carbohydrates in the extract, but did not inactivate the transforming principle.
Treatment with RNase (ribonuclease) hydrolyzed RNAs in the extract, but did not inactivate the transforming principle.
However, treatment with DNase (deoxyribonuclease) both hydrolyzed DNA and destroyed the transforming principle. In the bioassay, the bacteria were not transformed—they did not become pathogenic. From these experiments, it certainly seems reasonable to conclude that the transforming principle is DNA.