63.
In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?
The addition of lactose induces the lac-operon as the lactose acts as inducer by binding to the repressor and preventing it from binding to the operator. Hence RNA polymerase can access the promoter and complete transcription.
But with time all the lactose is hydrolysed by the action of enzymes . Therefore in the absence of any more lactose the lac-operon shuts down as the repressor binds to the operator and blocks RNA polymerase from accessing the promoter and transcribing the operon.
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