One gene one enzyme hypothesis

• The one gene–one enzyme hypothesis, proposed by George Wells Beadle in the US in 1941, is the theory that each gene directly produces a single enzyme, which consequently affects an individual step in a metabolic pathway.
• In 1941, Beadle demonstrated that one gene in a fruit fly controlled a single, specific chemical reaction in the fruit fly, which one enzyme controlled.
• In the 1950s, the idea that genes produce enzymes that control a single metabolic step was dubbed the one gene–one enzyme hypothesis by Norman Horowitz, a professor at the California Institute of Technology (Caltech) and an associate of Beadle’s.
• This concept helped researchers characterize genes as chemical molecules, and it helped them identify the functions of those molecules.
• Geneticists George W. Beadle and E. L. Tatum exposed some of the conidia of one mating type of Neurospora to ultraviolet rays in order to induce mutations.
• Then individual irradiated spores were allowed to germinate on a “complete” medium; that is, one enriched with various vitamins and amino acids.
• Once each had developed a mycelium, it was allowed to mate with the other mating type.
• The ascospores produced were dissected out individually and each one placed on complete medium.
• After growth had occurred, portions of each culture were sub cultured on minimal medium.
• Sometimes growth continued; sometimes it didn’t.
• When it did notऽthe particular strain was then supplied with a mixture of vitamins, amino acids, etc. until growth did occur.
• Eventually each mutated strain was found to have acquired a need for one nutrient; in the example illustrated here, the vitamin thiamine.
• Beadle and Tatum reasoned that radiation had caused a gene that permits the synthesis of thiamine from the simple ingredients in minimal medium to mutate to an allele that does not. The synthesis of thiamine from sucrose requires a number of chemical reactions, each one catalyzed by a specific enzyme.
• By adding, one at a time, the different precursors of thiamine to the medium in which their mutant mold was placed, they were able to narrow down the defect to the absence of a single enzyme.
• If they added to the minimal medium any precursor further along in the process, growth occurred.
• Any precursor before the blocked step could not support growth.
• Thus, in this example, the conversion of precursor C to precursor D was blocked because of the absence of the needed enzyme (c).
• This led them to postulate the one gene – one enzyme theory: each gene in an organism controls the production of a specific enzyme. It is these enzymes that catalyze the reactions that lead to the phenotype of the organism.