[Music] in 1941 George Beetle and Edward Tatum through experiments on the red bread mold neurospora crasa show that each gene encodes one enzyme which acts by regulating distinct chemical events their experiments prove the sir archal Garrett's 1902 theory that hereditary diseases are in Born errors of metabolism which are missing or having false steps in the body's chemical Pathways for most of its life cycle neurospora is a haid organism which means that there is only one copy of each gene so they didn't worry about dominant and recessive alal as mle had in the lab neurospora grows well on minimal AAR that contains only a few simple sugars inorganic salts and the vitamin biotin neurospora have enzymes that convert these simple substances into the amino acids and vitamin necessary for growth Beetle and Tatum reason that if they mutate any one of the genes that make an enzyme for example gen a they should get a neurospin that cannot grow on minimal medium the mutant would be able to grow if enzyme product is added as a supplement both set out to find these nutritional mutants in 1927 Herman Mueller showed that xrays cause mutations in genes so they radiated a neurospora culture with x-ray they expected to get some rare mutants that would not grow on minimal medium the grew The Offspring of the irradiated neurospora on complete medium that contained all the vitamins and amino acids next the test of the ability of each of these cultures to grow on minimal medium most of these cultures grew on minimal medium meaning they didn't have a genetic of the kind they were looking for however the 299th culture did not grow on minimal medium they then tried growing culture 299 on minimal medium supplemented with either amino acids or vitamins they found that culture 299 did not grow on minimal medium with amino acid supplements but did grow on minimal medium with vitamin supplements therefore culture 299 must not be able to make one of the vitamins they then had to figure which vitamin was missing in culture 299 they did this by testing culture 299 ability to grow on minimal medium supplemented with single vitamins they found that culture 299 grew only if they provided the vitamin B6 it was their first neurospora mutant it could not make vitamin B6 on its own because one of the enzymes in B6 synthesis pathway must be affected thus the gene making this enzyme must have been mutated by x-rays by adding vitamin B6 as a supplement to the minimal medium the mutation could be compensated for and culture 299 could grow using this method of selection and supplementation they isolated many different types of neurospora mutants genetic mutation affect metabolic pathways and they confirm the synthesis pathway for many vitamins and amino acids the amino acid arginine is synthesized in a stepwise process catalyzed by enzymes a precursor molecule is converted into ornithine the citrine and finally Arginine if one gene makes on enzyme there should be a genetic mutation for each step of the synthesis Pathways among the Arginine mutants there should be strains that need either ornithine or citrine or arginina supplements in 1944 their colleagues Adrien SRB and Norman Horowitz found these mutant strains they started with neurospora strains that needed Arginine as a supplement these strains had mutations in different genes muttin one couldn't make ornithine so the gene that makes the enzyme for ornithine synthesis must have been mutated if ornithine is added to the media citrine and then Arginine would be made and mutant one could grow similarly a genetic mutation in mutant 2 affected the enzyme that makes the Arginine precursor citrine adding citrine as a supplement complemented the mutation and drove Arginine synthesis to completion and a genetic mutation in mutant 3 affected the final step of arginine synthesis the conversion of citrine to Arginine by adding Arginine as a supplement the mutation was complemented and mutant 3 could grow with each mutated Gene only one step of the metabolic pathway is affected there for one gene is responsible for one enzyme or protein they had biochemical proof of Sir archal Garrett's 1908 proposal of the inborn errors of metabolism who proposed that a gene mutation causes a specific defect in the biochemical Pathways Beetle and Tatum reasoned that each mutation must inactivate the enzyme protein needed to synthesize the nutrient one gene carries the directions for making one protein [Music]