Final answer:
Catabolite repression of the lac operon restricts expression when glucose is present, preventing the binding of the catabolite activator protein (CAP) and inhibiting transcription.
Step-by-step explanation:
Catabolite repression of the lac operon restricts expression when glucose is present. When glucose is present, it prevents the binding of the catabolite activator protein (CAP) to the promoter region of the lac operon, which is necessary for transcription. As a result, the lac operon remains repressed and the expression of lac genes is restricted. This regulation allows the cell to prioritize the utilization of glucose as an energy source.