Observe the given picture and answer the following questions.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Regulation of the lac operon by the repressor is referred to as negative regulation.
Reason (R): The repressor of the lac operon is synthesised all the time (constitutively) from the i gene.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
The lac operon has one regulatory gene (i) and three structural genes (z, y and a). The i gene makes the repressor, and the repressor keeps the operon switched off until an inducer arrives.
Assertion (A): Regulation of the lac operon by the repressor is referred to as negative regulation. — True. NCERT uses exactly this term for control by the repressor.
Reason (R): The repressor of the lac operon is synthesised all the time (constitutively) from the i gene. — True. The repressor is made all the time, whether lactose is present or not.
A is about the kind of control: the regulatory protein works by turning transcription off. R is about how the repressor is supplied: it is made constantly from the i gene.
Why R does not explain A? The control is called negative because the repressor binds the operator and stops RNA polymerase from transcribing the operon. That the repressor is made constitutively only tells us that it is always available. It does not say what the repressor does to transcription, so it is not the reason for the name. So both statements are correct, but R is not the explanation of A.
NCERT Extract:
The Lac Operon – “… The lactose then induces the operon in the following manner. The repressor of the operon is synthesised (all-the-time – constitutively) from the i gene. The repressor protein binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon.”
The Lac Operon – “… Regulation of lac operon by repressor is referred to as negative regulation. Lac operon is under control of positive regulation as well, but it is beyond the scope of discussion at this level.”