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