lac operon

Meaning: An operon is a group of genes under one promoter, transcribed as a single mRNA, typical of prokaryotes. The lac operon holds lacZ (β-galactosidase, which splits lactose into glucose and galactose), lacY (permease, which imports lactose) and lacA (transacetylase). The repressor, encoded by lacI with its own promoter, binds the operator and blocks RNA polymerase. Allolactose, an isomer of lactose, binds the repressor and releases it from the operator, so transcription proceeds (negative, inducible control). Low glucose raises cAMP, and the cAMP–CAP complex binds near the promoter and increases transcription (positive control).

On the MCAT: A passage gives a condition (glucose and lactose present or absent) or a mutant and asks whether the genes are transcribed. Lactose with low glucose gives high expression. Lactose with high glucose gives low expression. No lactose gives very little. A loss-of-function lacI mutation, or an operator mutation that blocks repressor binding, gives constitutive expression. IPTG is a non-metabolizable inducer used in experiments.

Common confusion: Lactose alone does not guarantee maximal transcription. If glucose is also high, cAMP is low and CAP cannot activate. The inducer binds the repressor protein, not the DNA.