How does extracellular glucose inhibit transcription of the lac operon?

• A. By strengthening the binding of the repressor to the operator.
• B. By weakening the binding of the repressor to the operator.
• C. By inhibiting RNA polymerase from opening the strands of DNA to initiate transcription.
• D. By reducing the levels of intracellular cAMP.

Answer: (D)

Verified step by step guidance

1. Understand the role of the lac operon: The lac operon is a set of genes responsible for the metabolism of lactose in bacteria. It is regulated by the presence or absence of lactose and glucose.
2. Learn about the role of cAMP: Cyclic AMP (cAMP) is a molecule that helps activate the lac operon by binding to the catabolite activator protein (CAP), which then facilitates the binding of RNA polymerase to the promoter region of the lac operon.
3. Explore the effect of glucose on cAMP levels: When extracellular glucose is present, it leads to a decrease in intracellular cAMP levels. This is because glucose inhibits the enzyme adenylate cyclase, which is responsible for converting ATP to cAMP.
4. Connect cAMP levels to lac operon transcription: Lower levels of cAMP mean that CAP cannot bind effectively to the promoter region, reducing the ability of RNA polymerase to initiate transcription of the lac operon.
5. Conclude the mechanism: Therefore, extracellular glucose inhibits transcription of the lac operon by reducing the levels of intracellular cAMP, which in turn prevents the activation of the lac operon by CAP.