Molecular biology: gene regulation: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Molecular biology: gene regulation

Sample item

In the lac operon of E. coli, what happens when glucose is absent and lactose is present?

AThe operon is expressed only when both glucose and lactose are presentBAllolactose inactivates the repressor and cAMP-CAP enhances transcription, so the operon is strongly expressedCThe repressor stays bound and the operon is silentDTranscription is high because glucose activates CAP

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Allolactose inactivates the repressor and cAMP-CAP enhances transcription, so the operon is strongly expressed

Allolactose binds the repressor, releasing it from the operator. Low glucose raises cAMP, and the cAMP-CAP complex enhances RNA polymerase binding. Both conditions together give maximal expression.

Option by option

A. The operon is expressed only when both glucose and lactose are present
High glucose reduces expression even if lactose is present.
B. Allolactose inactivates the repressor and cAMP-CAP enhances transcription, so the operon is strongly expressed (correct)
Correct. Both positive and negative control favour transcription.
C. The repressor stays bound and the operon is silent
The repressor is released by allolactose in the presence of lactose.
D. Transcription is high because glucose activates CAP
Glucose lowers cAMP and so reduces CAP activity.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Allolactose inactivates the repressor and cAMP-CAP enhances transcription, so the operon is strongly expressed. Allolactose binds the repressor, releasing it from the operator.

Why is "The operon is expressed only when both glucose and lactose are present" not the answer?

High glucose reduces expression even if lactose is present.

Why is "The repressor stays bound and the operon is silent" not the answer?

The repressor is released by allolactose in the presence of lactose.

Why is "Transcription is high because glucose activates CAP" not the answer?

Glucose lowers cAMP and so reduces CAP activity.

What concept does this question test?

It tests Molecular biology: gene regulation in Biochemistry. Low glucose raises cAMP, and the cAMP-CAP complex enhances RNA polymerase binding. Both conditions together give maximal expression.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Molecular biology: gene regulation?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.