MacLeod, McCarty and Avery worked to determine the ‘biochemical nature’ of the ‘transforming principle’ in Griffith’s experiment. They extracted purified DNA, RNA, proteins and other materials from heat-killed ‘S’ bacteria nucleoid and mixed ‘R’ bacteria with these different materials, and only those mixed with DNA were transformed into ‘S’ bacteria.
This experiment strongly implied that DNA is the “transforming factor” and not proteins or other materials and by this it was demonstrated what is known to us as the transforming principle - that genes are made of DNA and DNA is the hereditary material.
