Question

How does the presence of lactose, switch on the operator?

Answer 1

In the absence of lactose, the lac-operon repressor binds to the operator and blocks transcription of the structural genes. When lactose is present, some enters the bacterial cell and binds to the repressor (more precisely, allolactose, formed from lactose, acts as the inducer). This changes the repressor’s shape so that it can no longer bind to the operator. The operator is consequently free, allowing RNA polymerase to transcribe the structural genes for lactose metabolism.

Thus, lactose switches the lac operon on indirectly by inactivating the repressor.