Edward Lawrie Tatum (1909 –1975) shared half of the Nobel Prize in Physiology or Medicine in 1958 with George Wells Beadle for showing that genes control cellular metabolism. Tatum and Beadle used X-rays to induce mutations in strains of Neurospora crassa (a type of red bread mold) and showed that these mutations were in fact genetic defects. Their research proved that when a genetic mutation affected a specific chemical reaction, the enzyme catalyzing that reaction was altered or missing. Thus, each gene determined the structure of a single, specific enzyme. This became known as the one gene-one enzyme hypothesis. Tatum and Joshua Lederberg later discovered genetic recombination between Escherichia coli bacteria. Largely because of their efforts, bacteria became the primary experimental organism for investigating the genetic control of biochemical processes in cells.