6.5 Regulation of Gene Expression

Turning genes on and off

Cells don't express every gene at once — they regulate gene expression. In prokaryotes, related genes are grouped into operons (like the lac operon, switched on by lactose, and the trp operon, switched off when tryptophan is plentiful).

Eukaryotic control

Eukaryotes use transcription factors that bind DNA regions like promoters and enhancers to turn genes on or off. Epigenetic changes also regulate expression: DNA methylation usually silences genes, and histone modifications can open or close DNA. This lets one genome produce many cell behaviors.

Key terms

Gene regulation

Controlling which genes are expressed and when, so cells use only what they need.

Operon

A cluster of prokaryotic genes controlled together (e.g., lac, trp).

Transcription factor

A protein that binds DNA to turn a gene's transcription on or off.

Promoter / enhancer

DNA regions where regulatory proteins bind to control transcription.

Epigenetics (methylation)

Reversible marks like DNA methylation that regulate expression without changing the sequence.

Try an AP-style question

What happens to lac operon expression?

1. It stays off, because the repressor remains bound to the operator
2. It turns on, because lactose binds the promoter
3. It stays off, because RNA polymerase is destroyed
4. It stays on, because the repressor cannot bind the operator