How does the presence of lactose affect the lac operon?

The presence of lactose affects the lac operon by preventing the repressor from inhibiting transcription. In the absence of lactose, the repressor protein binds to the operator region of the lac operon, blocking RNA polymerase from accessing the promoter and thus inhibiting transcription of the genes needed for lactose metabolism. When lactose is present, a metabolite of lactose called allolactose binds to the repressor, causing a conformational change. This binding prevents the repressor from attaching to the operator, allowing RNA polymerase to access the promoter and initiate transcription of the operon's genes.

This mechanism is a key aspect of gene regulation in prokaryotes, allowing the cell to efficiently regulate the expression of genes in response to environmental changes. The other options do not accurately describe the specific role that lactose plays within the lac operon: it does not activate the repressor, bind directly to the operator, or increase enzyme levels directly. Instead, its role in binding to the repressor is crucial for facilitating transcription of the genes necessary for lactose utilization.