# Lac operon

A group of bacterial genes switched on together to digest lactose. A repressor protein blocks the genes unless lactose is present. When lactose is there, it binds the repressor, which lets go, and the genes turn on.

Biology (university) · Part 05: Gene regulation and biotechnology · Card 1 of 8 · Last updated 2026-10-19

## Analogy

A light that only switches on when someone is in the room.

## Example

**Lac operon rule**

No lactose: repressor on DNA.
Genes OFF.
Lactose present: repressor released.
Genes ON.

## In practice

E. coli makes lactose-digesting enzymes only when lactose is the available food. This saves energy.

## Practice

### Flashcards

- **What switches the lac operon on?** Lactose, which binds the repressor and makes it release the DNA.

### Interview questions

**Explain how the lac operon is regulated and why this benefits E. coli.**

- Repressor blocks genes without lactose
- Lactose binds repressor and it releases DNA
- Genes for lactose enzymes switched on
- Saves energy when lactose is absent

## Questions people ask

### Why is the lac operon called inducible?

Because the genes are normally off and are switched on by the presence of a molecule, the inducer. Here the inducer is allolactose, a form of lactose. It binds the repressor and changes its shape so it releases the DNA. Other operons, like trp, work in the opposite way and are repressible.

### Why does E. coli prefer glucose to lactose?

Glucose is quicker to use. When glucose is present, a second system lowers the level of a helper molecule called cyclic AMP, so the activator that boosts lac gene transcription does not work. Cells then use glucose first and only use lactose after the glucose is gone.

## Related concepts

- [Transcription factor](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-2)
- [Epigenetic modification](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-3)

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