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Oswald Avery, Colin MacLeod, and Maclyn McCarty show that DNA is the transforming material in cells. They demonstrate that the transformation of Streptococcus pneumoniae from an avirulent type to a virulent type is the result of the transfer of DNA from dead smooth organisms to live rough ones. They also show that the transforming principle is destroyed by pancreatic deoxyribonuclease —an enzyme that hydrolyzes DNA —but is not affected by pancreatic ribonuclease or enzymes that destroy proteins.
Avery, O. T., C. M. Macleod, and M. McCarty. 1944. Studies on the chemical nature of the substance inducing transformation of pneumonococcal types. Induction of transformation by a deoxyribo-nucleic acid fraction isolated from pnuemococcus type III. J. Exp. Med. 79: 137-157. In Microbiology: A Centenary Perspective, edited by Wolfgang K. Joklik, ASM Press. 1999, p.116 [pdf]