Final answer:
A mutant strain that can only grow with an amino acid supplement has a mutation in the gene for synthesizing that amino acid. Beadle and Tatum's experiments with molds supported the one gene-one enzyme hypothesis and helped map specific metabolic pathways.
Step-by-step explanation:
A mutant strain that requires a specific amino acid supplement to grow typically has a mutation in a gene that's crucial for synthesizing that particular amino acid. This condition is the basis for the one gene-one enzyme hypothesis, initially proposed by scientists Beadle and Tatum. Their famous experiments with the mold Neurospora crassa involved irradiating spores to induce mutations and then growing these spores on different media to pinpoint the defects in amino acid synthesis pathways. For instance, if a mutant requires arginine to grow, it indicates a genetic defect in the arginine biosynthesis pathway. Different classes of mutants were able to grow with supplementation of intermediates like citrulline or ornithine, which are part of this pathway. Additionally, experiments with Escherichia coli mutants have also shown that supplementing the growth media with different amino acids can help overcome metabolic deficiencies caused by mutations, illustrating a connection between genes and metabolic pathways.