Pathways Gene A Gene B Enzyme A Enzyme B Substrate  intermediate  product Most often the final product of the biochemical pathway is something essential to life, like amino acids, nucleotides, etc.

Pathways One-gene-one-enzyme hypothesis –George Beadle and Edward Tatum Neurospora

Pathways Wild type Neurospora (all genes intact), will grow on minimal media of mineral salts, glucose and biotin –Has all the genes to make all the proteins to make everything the beasty needs to live and survive

Pathways Auxotrophs (mutation in one or more genes), will not grow on minimal media of mineral salts, glucose and biotin –Will grow on minimal media only if supplemented with one or more specific nutrients that it cannot make

Arginine auxotroph

Pathways Gene A Gene B Enzyme A Enzyme B Substrate  intermediate  product Most often the final product of the biochemical pathway is something essential to life, like amino acids, nucleotides, etc.

Pathways Gene A Mutant Gene B Enzyme A No Enzyme B Substrate  intermediate No Product Can use mutants to work out pathways, and identify which gene catalyzes which step

+ = growth - = no growth Minimal + ornithine + citrulline + arginine No. +’s Wild- type Mut Mut Mut No. 3No. 2No. 1 argininecitrullineornithine   substrate

Unknown Pathway Mutant No. Meth- ionine Homo- serine CysteineHomo- cysteine Cysta- thionine     No. 1No. 3No. 2 No. 5 No. 4 methioninehomocysteinecystathionine cysteine homoserine