Mystified at first by the resurrection of the lethal bacteria, scientists eventually suspected that the virulent property was somehow passed from the dead bacteria to the living and previously harmless strain by a "transforming principle." The transforming principle was, in essence, genetic material, carrying the trait of virulence from dead cells to living ones. If the transforming principle could be extracted and isolated, scientists would at last know what genes are made from.

The second half of the puzzle was finally solved by Oswald Avery and his coworkers in New York in 1944. They spent years grinding up bacteria, refining and purifying their extracts, and adding chemicals until everything was eliminated but the one essential transforming principle. What they ended up with was DNA. It must be DNA, they concluded, that carries hereditary information.

There was great excitement among molecular biologists during the 1930s and '40s. The physical basis of heredity was rapidly becoming better understood, and scientists felt they were close to peering inside the hidden machinery of the cells, into the "little black box" that directs what each living thing is to become. They knew that:

• heredity is controlled by discrete factors called genes
• genes are located on threadlike chromosomes found in cell nuclei
• genes are made from DNA.