Edward Lawrie Tatum's experiments with simple organisms demonstrated that cell processes can be studied as chemical reactions and that such reactions are governed by genes. With George Beadle, he offered conclusive proof in 1941 that each biochemical reaction in the cell is controlled via a catalyzing enzyme by a specific gene. The "one gene-one enzyme" theory changed the face of biology and gave it a new chemical expression. Tatum, collaborating with Joshua Lederberg, demonstrated in 1947 that bacteria reproduce sexually, thus introducing a new experimental organism into the study of molecular genetics. Spurred by Tatum's discoveries, other scientists worked to understand the precise chemical nature of the unit of heredity called the gene. This study culminated in 1953, with the description by James Watson and Francis Crick of the structure of DNA (deoxyribonucleic acid). Tatum's use of microorganisms and laboratory mutations for the study of biochemical genetics led directly to the...