When glucose is absent but lactose is readily available, what steps need to occu

Question

When glucose is absent but lactose is readily available, what steps need to occur in order to transcribe the structural genes from the lac operon?

A) The repressor protein, LacI, must be inactivated, and cAMP-CAP must be bound near the lac promoter.

The repressor protein, LacI, must be active, and cAMP-CAP must be bound near the lac promoter.

C) The repressor protein, LacI, must be inactivated, and CAP must be bound near the lac promoter.

D) The repressor protein, LacI, must be active, and CAP must be bound near the lac promoter.

E)The repressor protein, LacI, must bind to cAMP.

A) The repressor protein, LacI, must be inactivated, and cAMP-CAP must be bound near the lac promoter.

The repressor protein, LacI, must be active, and cAMP-CAP must be bound near the lac promoter.

C) The repressor protein, LacI, must be inactivated, and CAP must be bound near the lac promoter.

D) The repressor protein, LacI, must be active, and CAP must be bound near the lac promoter.

E)The repressor protein, LacI, must bind to cAMP.

Explanation / Answer

Lac operon is regulated in 2 ways. The repressor protein is always in the active (expressed) form. When lactose is present, allolactose binds to the repressor which in turn gets inactivated, meaning that it cannot bind to the operator. Thus the operon 'switches on'.

Another regulatory mechanism of lac operon is through catabolite activator protein (CAP). CAP binds to the operon to promote transcription.

cAMP levels rise when glucose is absent. This means that more cAMP-CAP is present. This allows for efficient binding of cAMP-CAP to the promoter.

So for both cases, lac operon is activated to transcribe lac genes. Answer is A)

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