🧪 General Principles🟢 BasicGene Regulation

The Lac Operon

A cluster of bacterial genes, switched on and off together, that lets a bacterium make the enzymes to use lactose only when lactose is present.

Key points

An operon is a group of genes controlled as a single unit; the lac operon of E. coli carries three genes needed to take up and break down lactose.
A regulatory gene makes the lac repressor protein, which binds the operator and blocks RNA polymerase, so the genes are normally switched off.
When lactose is present its derivative allolactose binds the repressor and pulls it off the operator, allowing transcription, so lactose acts as the inducer.
This is an example of negative control, because the default state is off and a repressor must be removed before the genes can be expressed.
Glucose is preferred over lactose: when glucose is low, cyclic AMP rises and, with its binding protein, boosts transcription, so the operon runs hardest when glucose is absent but lactose is present.
The lac operon is the classic model of how a cell switches genes on only when their product is needed, saving energy.

Lactose lifts the lock: the repressor sits on the operator until allolactose removes it; low glucose plus high cyclic AMP then turns the volume up.

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🧪 General Principles🟢 BasicGene Regulation

A cluster of bacterial genes, switched on and off together, that lets a bacterium make the enzymes to use lactose only when lactose is present.

An operon is a group of genes controlled as a single unit; the lac operon of E. coli carries three genes needed to take up and break down lactose.
A regulatory gene makes the lac repressor protein, which binds the operator and blocks RNA polymerase, so the genes are normally switched off.
When lactose is present its derivative allolactose binds the repressor and pulls it off the operator, allowing transcription, so lactose acts as the inducer.
This is an example of negative control, because the default state is off and a repressor must be removed before the genes can be expressed.
Glucose is preferred over lactose: when glucose is low, cyclic AMP rises and, with its binding protein, boosts transcription, so the operon runs hardest when glucose is absent but lactose is present.
The lac operon is the classic model of how a cell switches genes on only when their product is needed, saving energy.

Lactose lifts the lock: the repressor sits on the operator until allolactose removes it; low glucose plus high cyclic AMP then turns the volume up.