Under what conditions are the genes involved in lactose metabolism in E. coli expressed and when are they turned off?

• A. Expressed when lactose is absent; turned off when glucose is absent.
• B. Expressed when lactose is present and glucose is absent; turned off when lactose is absent.
• C. Expressed when both lactose and glucose are present; turned off when both are absent.
• D. Expressed when glucose is present; turned off when lactose is present.

Answer: (B)

Verified step by step guidance

1. Understand the role of the lac operon in E. coli, which is responsible for the metabolism of lactose. The lac operon includes genes that encode enzymes necessary for lactose digestion.
2. Recognize that the lac operon is regulated by the presence or absence of lactose and glucose. Lactose acts as an inducer, while glucose affects the operon's activity through catabolite repression.
3. When lactose is present, it binds to the repressor protein, causing it to detach from the operator region of the lac operon, allowing transcription of the genes involved in lactose metabolism.
4. When glucose is absent, cyclic AMP (cAMP) levels increase, which binds to the cAMP receptor protein (CRP). The CRP-cAMP complex enhances the binding of RNA polymerase to the promoter, increasing transcription of the lac operon.
5. The lac operon is turned off when lactose is absent because the repressor binds to the operator, blocking transcription. Additionally, when glucose is present, it inhibits the formation of the CRP-cAMP complex, reducing transcription even if lactose is present.