Lactose as Operon Inducer — NEET Biology
✅ Asked in NEET 2023Which one of the following acts as an inducer for lac operon? (NEET 2023)
Which one of the following acts as an inducer for lac operon? (NEET 2023)
Inducer for lac operon is: (NEET 2016 Phase 1)
Answer & NCERT explanation
Correct answer: A — Lactose
Lactose acts as an inducer for lac operon according to NCERT. In the presence of lactose, it binds to the lac repressor protein, causing an allosteric change that prevents the repressor from binding to the operator sequence. This derepression allows transcription of lac genes. Sucrose, glucose, and galactose do not function as inducers for the lac operon system.
Read more NCERT concept on the PYQ
Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the operon. Hence, it is termed as inducer. In the absence of a preferred carbon source such as glucose, if lactose is provided in the growth medium of the bacteria, the lactose is transported into the cells through the action of permease (Remember, a very low level of expression of lac operon has to be present in the cell all the time, otherwise lactose cannot enter the cells). The lactose then induces the operon in the following manner.
The lac operon is regulated by lactose, which acts as an inducer to switch the operon ON when lactose is present and glucose is absent. NTA tests whether students understand that lactose itself is the substrate AND the regulatory molecule. Students often confuse lactose with glucose, or forget that permease (the transport protein) requires basal expression of the operon to import lactose into cells—creating a catch-22 that's crucial for understanding regulation. Remember: lactose induces the operon by binding to the repressor protein and inactivating it, allowing transcription. The inducer concept is fundamental to gene regulation and has appeared twice in recent NEETs.
What does NCERT say about Lactose substrate for enzyme?
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Through deep analysis of NEET and NTA, 88 of 90 questions from the NEET 2026 paper were matched straight from the MedicNEET Biology question bank.
88/90MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.