One-gene-One-Enzyme Hypothesis:The actual function of gene became clear from the research in the 1940s on Neurospora by George Beadle and Edward Tatum. Later on they received Nobel Prize for their work. One gene one enzyme hypothesis, the name was given based on the idea that a gene act by the production of the enzymes and each gene is responsible for production of the single enzyme that in turn effect a single step in metabolic pathway.
Beadle and Tatum experiment:Beadle and Tatum analyzed mutants of Neurospora, a fungus with a haploid genome. They first irradiated Neurospora to produce mutations and when tested cultures from ascospores for interesting mutant phenotype. They detected numerous auxotrophs. In each case, the mutation was inherited as a single gene mutation: each gave a 1:1 ratio when crossed with a wild type.
One set of mutant strains required arginine to grow on minimal medium. These strains provided the focus for much of Beadle and Tatum’s further work. First they found that mutations mapped out in three different regions on separate chromosomes. Even though the supplement (arginine) satisfied the growth requirement for each mutant. Let’s call the three loci arg-1, arg-2, and arg-3 genes. Beadle and Tatum discovered that the auxotrophs for each of the three loci differ in their response to the chemical compounds ornithine and citrulline, which are related to arginine. The arg-1 mutant will grow if supplemented with either ornithine or citrulline or arginine in addition to minimal medium. The arg-2 mutant will grow only if supplemented with citrulline or arginine but not the ornithine. The arg-3 mutant will grow only if supplied with arginine.
It was already known that cellular enzymes often interconvert related compounds such as these. Based on the properties of arg mutants, Beadle and Tatum and their colleagues proposed a biochemical model for such conversion in Neurospora.
Note how this relationship easily explains the three classes of mutants. The arg-1 mutants have a defective enzyme A, so they are unable to convert precursor into ornithine as the first step in producing arginine. However, they have normal enzymes B and C, and so the arg-1 mutants are able to produce arginine if supplemented with ornithine or citrulline. The arg-2 mutants lack enzyme B and arg-3 mutants lack enzyme C; thus a mutation at a particular gene is assumed to interfere with the production of the single enzyme. The defective enzyme then creates a block in some biosynthetic pathway. The block can be overcome by supplying to the cells any compound that normally comes after the block inthe pathway.
This entire model was inferred from the properties of the mutant classes detected through genetic analysis. This model, which is known as one-gene-one-enzyme hypothesis, provides the first evidence of function of the gene; some genes were responsible for the functions of the enzymes and each gene apparently control one specific enzyme.
One-gene-one-enzyme hypothesis became one of the great unifying concept in biology because it provides a bridge that brought together the concept and research techniques of genetics and biochemistry