Practice question

Auxotrophic mutants fail on minimal medium because structural gene encoding enzyme for specific anabolic step carries lesion abolishing catalytic activity. Without functional enzyme pathway halts, intracellular pool of needed amino acid or vitamin depletes and protein synthesis stops preventing colony formation. Wild-type prototroph synthesises metabolite endogenously whereas mutant requires supplementation bypassing block. Beadle and Tatum isolated such mutants to prove each gene controls one reaction, with auxotrophy revealing gene-controlled biochemical lesion rather than defects in DNA replication or cell division machinery itself.