Lac Operon Positive and Negative Gene Regulation
Flashcards on lac operon regulation cover how E. coli controls lactose metabolism genes through both negative repressor control and positive CAP-cAMP activation, a model system tested on AP Biology exams, MCAT, and USMLE Step 1. Distinguishing the two regulatory mechanisms is a frequent source of confusion and exam errors.
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5 CardsCAP-cAMP role in lac operon
Lac operon state with lactose only
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What is the difference between positive and negative regulation of the lac operon?
Negative regulation involves the lac repressor blocking transcription when lactose is absent. Positive regulation involves CAP-cAMP enhancing transcription when glucose is absent.
- Negative: repressor acts as an off switch
- Positive: CAP-cAMP acts as an on switch
Why is the lac operon off when both glucose and lactose are present?
When glucose is available, cAMP levels drop, preventing CAP-cAMP from activating the promoter. Even without the repressor (lactose present), transcription is very low because catabolite repression favors glucose use over lactose.
How do I remember inducible vs repressible operons?
Inducible operons (like lac) are normally OFF — an inducer molecule removes the repressor to turn ON transcription. Repressible operons (like trp) are normally ON — a corepressor binds the repressor to turn OFF transcription.