The Avery–MacLeod–McCarty Experiment

DNA is the transforming principle

First published: O. T. Avery, C. M. MacLeod, M. McCarty, "Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types", *J. Exp. Med.* 79 (1944): 137–158.

A purified extract from virulent pneumococci converts harmless strains into virulent ones. Destroy the DNA in the extract; the conversion stops.

Building on Griffith's 1928 demonstration that something in killed virulent pneumococci could convert harmless strains into virulent ones, Avery, MacLeod, and McCarty systematically purified the active substance. They identified it chemically and showed: enzymes that degrade protein leave the activity intact; DNase abolishes it. Conclusion: the transforming principle is DNA. The result was the first direct chemical identification of the genetic material, predating Hershey-Chase by eight years. It was, however, slow to be accepted — the dominant view remained that protein was the heritable material, partly because DNA seemed too chemically monotonous to encode genetic complexity. Watson-Crick (1953) and the molecular detail that followed eventually vindicated the result.