Negative Regulation of Lac Operon — NEET Biology
📚 Practice ConceptRegulation 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.
Which of the above statements are correct regarding the regulation of the lac operon? S1: The i gene is responsible for the constitutive synthesis of the repressor protein. S2: When lactose is absent, the active repressor binds to the operator, physically blocking RNA polymerase access to the promoter. S3: Lactose or allolactose serves as an inducer by binding to the repressor, which then dissociates from the operator. S4: The lac operon is primarily regulated by positive control mechanisms involving the repressor protein. S5: The structural genes (z, y, a) of the lac operon are transcribed into separate mRNA molecules. S6: The presence of glucose in the medium promotes the transcription of the lac operon when lactose is also available.
View solution & NCERT explanation
Correct answer: C — S1, S2, and S3 only
The lac operon is a classic example of gene regulation. Let's evaluate each statement: S1: The i gene constitutively codes for the repressor protein of the lac operon. This is correct as stated in NCERT. (Page 100) S2: In the absence of lactose, the repressor protein binds to the operator region, preventing RNA polymerase from transcribing the operon. This statement is correct. (Page 100-101) S3: Lactose or allolactose acts as an inducer. It binds to the repressor, inactivating it and preventing its binding to the operator, thereby allowing transcription. This statement is correct. (Page 101) S4: Regulation of the lac operon by the repressor is referred to as negative regulation, not positive regulation. Thus, this statement is incorrect. (Page 101) S5: The lac operon consists of a polycistronic structural gene, meaning the z, y, and a genes are transcribed into a single mRNA molecule. Thus, this statement is incorrect. (Page 100) S6: Glucose is a preferred carbon source. Its presence generally inhibits the lac operon (catabolite repression), even if lactose is present. Glucose does not promote lac operon transcription. Thus, this statement is incorrect. (Page 101, 'In the absence of a preferred carbon source such as glucose...') Therefore, only S1, S2, and S3 are correct.
What does NCERT say about Regulation lac operon by?
Has this concept appeared in NEET?
Which chapter is this from?
Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.