In 1944 Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty of the Rockefeller Institute in New York published a paper in the Journal of Experimental Medicine that was to become a classic. Scientists had suspected for some time that DNA was the genetic material, but it had not been experimentally established unequivocally. The question was, Is it really DNA that carries information from one generation to the next?
There are two forms of the pneumococcus bacterium, called R and S. R is a nonencapsulated and nonvirulent form, while S is an encapsulated, virulent (infectious) form. It was known that the R form could be transformed into the S form in an animal by injecting a heat-killed preparation of S and a small quantity of living R. Somehow the R form that does not cause disease was being transformed into the S form that causes pneumonia. Avery, MacLeod, and McCarty wanted to find out the nature of the “transforming” principle. Was it DNA, or was it protein? The question is not as simple as it sounds, because DNA in the nucleus of cells has some proteins associated with it.
They purified the “transforming” principle, the chemical material that they could show was bringing about the R to S transformation, and tested it using a number of methods to help characterize its nature. The methods included chemical analysis, enzyme digestion, and serological reactions. Chemical analysis indicated that the material was consistent with the known composition of DNA but not of protein. Using different enzymes to digest the material (in order to try to destroy the transforming activity), the researchers showed that enzymes that disrupt DNA disrupted the transforming activity. Finally, the serological reaction testing showed that the cell’s non-nuclear material was not involved in the transformation.
The results of the experiments showed convincingly that DNA was responsible for determining whether the cell was R or S. The DNA was dictating the cell type and thus its disease-causing ability. Thus, DNA was the genetic material, the material that controlled cell function. By extension DNA was thus the material that carries the information for dictating cell function from generation to generation, because DNA is what passes from generation to generation. Many other experiments by many scientists have confirmed these findings.
