Biology
Explainer
The Lac Operon: Gene Logic in E. coli
E. coli only makes lactose enzymes when lactose is present and glucose is scarce.
Quick check · tap an answer to continue
Lesson notes
Key points
- An operon is a cluster of bacterial genes transcribed together from one promoter, controlled by a nearby operator.
- Without lactose, the LacI repressor binds the operator; allolactose binding changes its shape so it releases the DNA.
- When glucose is low, cAMP rises and the cAMP–CAP complex boosts RNA polymerase binding, strongly activating transcription.
Definitions
- Operon
- Genes sharing one promoter and operator, transcribed as a single mRNA; common in prokaryotes
Takeaway: Lac needs lactose present to lift repression and low glucose for CAP.
Quick check: What does allolactose do in the lac operon?
- Inactivates the repressor
- Activates CAP directly
- Blocks RNA polymerase
OperonRepressorCAP–cAMP
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