Lac Operon Feedback & Glucose Effect — NEET Biology
📚 Practice ConceptIn the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?
Which of the above statements are NOT correct regarding the regulation of the lac operon? S1: A deletion mutation in the i gene would typically lead to constitutive expression of the lac operon. S2: The presence of lactose in the medium ensures that the lac repressor remains bound to the operator, inhibiting transcription. S3: High levels of glucose in the growth medium generally lead to decreased transcription of the lac operon, even if lactose is present. S4: A non-functional repressor protein due to a mutation would result in continuous synthesis of lac operon enzymes. S5: When a functional repressor is bound to the operator sequence, RNA polymerase cannot effectively initiate transcription. S6: The lac operon is considered inducible because its expression is turned off in the presence of its substrate, lactose.
View solution & NCERT explanation
Correct answer: A — S2 and S6 only
Let's evaluate each statement regarding the regulation of the lac operon: S1: If the i gene (which codes for the repressor) is deleted, no repressor protein will be produced, leading to continuous transcription of the structural genes (constitutive expression). This statement is correct. S2: Lactose acts as an inducer; it binds to and inactivates the repressor protein. This prevents the repressor from binding to the operator, thereby allowing transcription. So, the presence of lactose does not ensure the repressor remains bound to the operator to inhibit transcription. This statement is NOT correct. (Page 101) S3: Glucose is a preferred carbon source for E. coli. Its presence leads to catabolite repression, which generally decreases the transcription of the lac operon, even if lactose is also available. This statement is correct, as implied by NCERT's discussion of glucose as a preferred carbon source. (Page 101) S4: If the repressor protein is non-functional due to a mutation, it cannot bind to the operator, resulting in continuous transcription of the lac operon genes. This statement is correct. S5: When a functional repressor protein is bound to the operator, it physically blocks the RNA polymerase from moving along the DNA to transcribe the structural genes. This statement is correct. (Page 101) S6: The lac operon is inducible because its expression is turned ON (induced) in the presence of its substrate, lactose, not turned off. This statement is NOT correct. (Page 101) Therefore, statements S2 and S6 are NOT correct.
What does NCERT say about medium where E coli?
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.