operon

Meaning: An operon is a cluster of related genes in bacteria that share one promoter and one operator, so they are transcribed together as a single polycistronic mRNA and switched on or off as a unit. A separate regulatory gene encodes a repressor that binds the operator and blocks RNA polymerase. In an inducible operon such as lac, the repressor is active by default, and an inducer (allolactose) binds it and releases the operator. In a repressible operon such as trp, the repressor is inactive until the end product (tryptophan, acting as a corepressor) binds it.

On the MCAT: Passages give a mutant (operator, repressor or promoter) or a growth condition and ask whether the genes are expressed. For lac, expression is high when lactose is present and glucose is low, because low glucose raises cAMP, which activates CAP (catabolite activator protein) and enhances transcription. An operator mutation that blocks repressor binding, or a repressor mutation that blocks operator binding, gives constitutive expression. A promoter mutation that weakens RNA polymerase binding lowers expression.

Common confusion: The repressor binds the operator, not the promoter, and the inducer binds the repressor, not the DNA. Operons are characteristic of prokaryotes; eukaryotic genes generally have their own promoters and separate mRNAs.