Life Science Practice Exam

Operons regulate transcription in bacteria by controlling access of RNA polymerase to a promoter through an operator, with regulatory proteins that respond to specific molecules to turn transcription on or off. In the lac operon, a promoter guides transcription of lacZ, lacY, and lacA. An operator sits near this promoter and can be bound by the lac repressor. When lactose is absent, the repressor binds the operator and blocks transcription. When lactose is present, the inducer (allolactose) binds the repressor, causing it to release the operator and permit transcription. Glucose levels influence transcription as well: high glucose lowers cAMP, reducing CAP activator binding, which can further dampen transcription even if lactose is present. This combined regulation at the transcription initiation level, mediated by promoter, operator, and regulatory proteins responding to lactose and glucose, is the hallmark of how operons function in bacteria.