Question

When bacteria are grown in glucose-depleted media containing high concentration of lactose, expression of lac operon genes is activated by

• A. the binding of lac repressor in the operator site and cAMP-CAP complex in the CAP site.
• B. the dissociation of bound lac repressor from the operator site and binding of cAMP-CAP complex in the CAP site.
• C. the dissociation of bound lac repressor only from the operator site.
• D. the dissociation of both bound lac repressor from operator site and cAMP-CAP complex in CAP site.

Hint:

For lac operon activation, the lac repressor must dissociate from the operator, and the cAMP-CAP complex must bind to the CAP site to promote gene expression.

Answer 1

The Correct Option is B

Step 1: Understanding lac operon regulation.
In bacteria, the lac operon is regulated by the lac repressor and the cAMP-CAP complex. When glucose is scarce, cAMP levels rise, allowing the cAMP-CAP complex to bind to the CAP site and activate transcription of the lac genes. The lac repressor, when bound to the operator site, inhibits this process.

Step 2: Analyzing the options.
(A) the binding of lac repressor in the operator site and cAMP-CAP complex in the CAP site: Incorrect — Binding of the lac repressor inhibits transcription, so this would prevent gene expression.
(B) the dissociation of bound lac repressor from the operator site and binding of cAMP-CAP complex in the CAP site: Correct — The dissociation of the lac repressor and binding of the cAMP-CAP complex allow the lac operon to be expressed.
(C) the dissociation of bound lac repressor only from the operator site: Incorrect — The cAMP-CAP complex must also bind to activate the operon.
(D) the dissociation of both bound lac repressor from operator site and cAMP-CAP complex in CAP site: Incorrect — The dissociation of the cAMP-CAP complex would prevent the operon from being activated.

Step 3: Conclusion.
The correct answer is (B) the dissociation of bound lac repressor from the operator site and binding of cAMP-CAP complex in the CAP site, as this allows the lac operon to be activated.