Concept 17 A gene is made of DNA.
In the 1920s, experiments showed that a harmless strain of bacteria can become infectious when mixed with a virulent strain of bacteria that had been killed. The dead bacteria apparently provide some chemical that "transforms" the harmless bacteria to infectious ones. This so-called "transforming principle" appeared to be a gene.
A team of scientists led by Oswald Avery at the Rockefeller Institute, rigorously followed up on these experiments in the 1940's. They found that a pure extract of the "transforming principle" was unaffected by treatment with protein-digesting enzymes but was destroyed by a DNA-digesting enzyme. This showed that the transforming principle is DNA — and, by extension, a gene is made of DNA. Still, many scientists were slow to accept this clear proof that DNA, not protein, is the genetic molecule.