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 glucose and lactose.
2. Recognize the role of cAMP: Cyclic AMP (cAMP) is a molecule that, when bound to the catabolite activator protein (CAP), enhances the transcription of the lac operon by facilitating the binding of RNA polymerase to the promoter.
3. Identify the effect of glucose on cAMP levels: When extracellular glucose levels are high, the intracellular concentration of cAMP decreases. 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: Low levels of cAMP mean that CAP cannot bind effectively to the promoter region of the lac operon. Without CAP, RNA polymerase has a reduced ability to bind to the promoter, leading to decreased 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 reduces the binding of CAP to the promoter, ultimately decreasing transcription.