Gene control
Gene control
- Not all genes are switched on all the time.
- Cells control which proteins they make, and when.
- A famous bacterial example is the lac operon.
Types of gene and enzyme
- structural genes code for useful proteins such as enzymes.
- regulatory genes control whether other genes are switched on.
- an inducible enzyme is made only when needed; a repressible enzyme is normally made but can be switched off.
Practice
A regulatory gene:
Structural genes code for proteins like enzymes; regulatory genes control whether other genes are switched on.
Practice
An inducible enzyme is one that is:
Inducible enzymes are produced only when required (e.g. the lac enzymes when lactose is present).
The lac operon
- In a bacterium, the lac operon controls the digestion of lactose:
- no lactose: a repressor protein binds the operon and blocks transcription — no enzymes made.
- lactose present: lactose binds the repressor and pulls it off, so transcription happens and the enzymes are made.
- These enzymes are therefore inducible.
Practice
In the lac operon, when there is NO lactose:
Without lactose, the repressor sits on the operator and blocks transcription of the lac genes.
Practice
When lactose IS present in the lac operon:
Lactose removes the repressor, allowing transcription — so the lac enzymes are inducible.
Control in eukaryotes
- Transcription factors are proteins that bind DNA and control gene expression (raising or lowering transcription).
- Gibberellin works this way: it causes the breakdown of DELLA repressors, which normally block the factors that switch genes on.
Practice
In eukaryotes, transcription factors:
Transcription factors bind DNA to raise or lower transcription; gibberellin acts by removing DELLA repressors.
You've got it
Key idea
- structural genes (proteins/enzymes) vs regulatory genes (control); inducible (made when needed) vs repressible (switched off)
- lac operon: no lactose → repressor blocks transcription; lactose → pulls repressor off → enzymes made (inducible)
- in eukaryotes, transcription factors control gene expression; gibberellin removes DELLA repressors