Biology – Gene control | e-Consult

In the absence of lactose, the lac operon is switched off. This regulation is primarily achieved through the repressor protein. The repressor protein is normally bound to the operator sequence within the lac operon. This binding physically blocks RNA polymerase from accessing the promoter region, thus preventing transcription of the genes required for lactose metabolism (lacZ, lacY, and lacA).

When lactose is present, a small amount is converted to allolactose, which acts as an inducer. Allolactose binds to the repressor protein, causing a conformational change. This change in shape renders the repressor unable to bind to the operator sequence. Consequently, RNA polymerase can now access the promoter and transcribe the lac operon genes, leading to the production of enzymes needed to metabolize lactose.

Therefore, the absence of lactose leads to repressor binding to the operator, preventing transcription, while the presence of lactose leads to allolactose binding to the repressor, causing it to detach from the operator and allow transcription.