Final answer:
The lac repressor binds as a tetramer to the operator region of the lac operon and represses transcription by RNA polymerase. When lactose is present, it binds to the repressor and causes it to release from the operator, enabling gene transcription.
Step-by-step explanation:
The lac repressor binds as a tetramer to the operator region. The binding represses the lac operon by preventing transcription initiation formation by RNA polymerase. In the absence of lactose, this repressor is actively bound to the operator, preventing RNA polymerase from transcribing the structural genes of the lac operon. However, when lactose is present, it acts as an inducer by binding to the repressor. This binding causes a conformational change in the repressor, rendering it inactive. The allosterically altered repressor then dissociates from the operator, allowing RNA polymerase to transcribe the genes necessary for the digestion of lactose.