In the lac operon, why are the lactose-digesting enzymes made only when lactose is present?
Normally a repressor protein sits on the operator and blocks transcription, so no enzymes are made.
Normally a repressor protein sits on the operator and blocks transcription, so no enzymes are made. When lactose is available a little of it (as allolactose) binds the repressor and changes its shape, so it releases the operator. RNA polymerase can then transcribe the z, y and a genes that digest lactose. Remove the lactose and the repressor rebinds, switching the operon off -- so enzymes are produced only when their substrate is around.
Key point
Lactose inactivates the repressor, freeing the operator so the enzymes are made only when needed.
Common mistake
thinking lactose acts as a positive builder rather than by removing the repressor.
Memory tip
lactose is the key that pulls the 'off' repressor off the operator.
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