Biology › Section H · Gene expression and its regulation › Topic H.5
Gene regulation and the lac operon
A bacterium that transcribed every gene all the time would starve itself making useless enzymes. Prokaryotes solve this with the operon: a cluster of genes for one pathway, wired to a single switch. The trp operon shuts off when tryptophan is plentiful; the lac operon switches on only when glucose is gone and lactose is on the menu. One promoter, one operator, and a handful of proteins that either block or boost the polymerase — the whole logic of bacterial gene control in a few moving parts.
In this topic
- Why bacteria only tune transcription
- Anatomy of an operon
- Three kinds of regulatory molecules
- The trp operon: on until told otherwise
- CAP: turning genes up, not just off
- The lac operon: two keys, one switch
- Worked example: the four-scenario table, traced
- Worked example: predicting three mutations
- Why trp is on by default and lac is off
- Common mistakes to avoid
- Key takeaways
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