In 1944 Oswald Avery, Colin MacLeod and Maclyn McCarty, gave an experimental demonstration, that DNA is the substance that causes bacterial transformation, when it had been widely believed that it was proteins that served the function of carrying genetic information. The experiment was conducted at Rockefeller Institute for Medical Research, New York. This was done to determine if rough bacteria could be transformed into smooth bacteria, hence passing along the genetic information causing the transformation. By isolating and purifying this chemical component, they could deduce if it had characteristics of a protein or DNA molecule.
Two sets of bacteria were taken one smooth (virulent) and one rough (non-virulent).
(i) They first injected deadly encapsulated bacteria into the mouse - the mouse dies at the end.
(ii) They then injected non-encapsulated, nonvirulent bacteria into the mouse – the mouse lives.
(iii) Next, they heated the virulent bacteria at a temperature that kills them and injected these bacteria into the mouse - the mouse lives.
(iv) After that, they then have the denatured fatal bacteria mix into the living non-encapsulated, non-fatal bacteria. The mixture was then injected into the mouse - the mouse dies.
(v) Finally, they mix the live, non-encapsulated harmless bacteria with the DNA that was extracted from the heated, lethal bacteria. These "harmless" bacteria injected to the mouse after being mixed - the mouse dies. This showed that non-virulent bacteria become deadly after mixing with the DNA of the virulent bacteria. Such a demonstration shows that nonvirulent bacteria became virulent because of the genetic information that originally came from the virulent bacteria. The protein from the virulent bacteria was already denatured during step 3. Thus, it was DNA and not protein that transferred the genetic information to the nonvirulent bacteria.
