AP® Biology · Unit 6 · Topic 6.5
Regulation of Gene Expression
Gene regulation controls when a gene's instructions actually get used, often by proteins that block or allow RNA polymerase from transcribing an operon depending on what the cell needs.
A bacterium doesn't waste energy building enzymes it doesn't need. If no lactose is around, there's no point making the proteins that digest lactose. Operons solve this problem: a cluster of genes with related jobs, controlled by one switch. In the lac operon, a repressor protein normally sits on a stretch of DNA called the operator, which overlaps the promoter where RNA polymerase needs to attach. With the repressor parked there, polymerase is physically blocked and transcription can't start. When lactose (or a related molecule) shows up, it acts as an inducer: it binds the repressor, changes its shape, and knocks it off the operator. Now polymerase has a clear runway, and the lactose-digesting genes get transcribed.
A gene expressed all the time is called constitutive. That's a description of its pattern, not a judgment about importance. Genes for basic cell maintenance are often constitutive because they're needed constantly. Genes like the lac operon are regulated, switched on only when lactose is present, but during that window they are just as necessary as any constitutive gene. Being 'always on' and being 'always needed' are two separate questions.
This system depends on DNA sequences doing their job correctly. If a mutation changes the operator so the repressor no longer recognizes it, the repressor can't bind at all. The operon then runs constantly, inducer or not, because the molecular check that normally holds it shut is gone. This connects forward to eukaryotic regulation, where similar logic (proteins binding DNA to control access for RNA polymerase) gets layered with more complex controls like enhancers and chromatin structure.
Key terms
- Operon
- A cluster of genes transcribed together under control of a single promoter and operator.
- Repressor
- A protein that binds an operator and blocks RNA polymerase from transcribing the operon.
- Operator
- A DNA sequence near the promoter where a repressor protein can bind to control transcription.
- Inducer
- A molecule that binds a repressor and changes its shape so it releases the operator, allowing transcription.
- Constitutive expression
- A gene expression pattern where a gene is transcribed continuously rather than switched on and off.
Watch it explained
- How it worksHow does a repressor protein sitting on DNA physically block RNA polymerase?
- Common mistakeWhy isn't a gene that is always expressed the same as a gene that is always needed?
- How it worksWalk me through an inducible gene switching on when its inducer binds the repressor.
- Going furtherWhat happens to the operon's output when the regulatory sequence itself is mutated?
Check yourself
End of topic.