Practice question

Knockout of essential enzyme encoding committed step eliminates catalytic conversion preventing formation of downstream metabolite required for growth. Without gene product intermediate before block accumulates while product downstream depleted causing auxotrophy or lethality depending on essentiality and alternative routes. Growth on minimal medium fails unless final product supplied exogenously through medium. Such loss-of-function phenotype directly links gene to metabolite demonstrating causality in biochemical pathway. Complete deletion produces more severe effect than missense often fully abolishing product accumulation and revealing essential role of gene.