The lac operon is one of the most NEET-tested examples of gene regulation in prokaryotes, and the trap that costs marks is confusing the roles of the structural genes, regulatory gene, and the two small-molecule signals — lactose and glucose.
The system. The lac operon in E. coli is an inducible operon that encodes three structural genes — lacZ (β-galactosidase), lacY (permease), and lacA (transacetylase) — arranged as a single transcription unit under the control of one promoter and one operator (NCERT Class 12 Biology Chapter 5, page 100). A separate regulatory gene (lacI), located upstream but not part of the operon itself, constitutively produces a repressor protein.
Default state (no lactose, glucose present). The repressor binds the operator, physically blocking RNA polymerase from transcribing the structural genes. The operon is OFF.
Induction (lactose present, glucose absent). Lactose (the inducer) is converted to allolactose, which binds the repressor and changes its shape so it can no longer bind the operator. RNA polymerase accesses the promoter and transcribes lacZ, lacY, and lacA as a polycistronic mRNA. The operon is ON.
The glucose layer (catabolite repression). Even when lactose is present, if glucose is also available, the operon stays largely OFF. High glucose lowers cyclic AMP (cAMP). Without cAMP, the CAP (catabolite activator protein) cannot bind upstream of the promoter to enhance transcription. Full expression requires lactose present AND glucose absent.
Watch-out for NEET. Questions frequently test whether you know (a) the repressor is the product of lacI, not of the structural genes, (b) lactose is the inducer but allolactose is the actual molecule that binds the repressor, and (c) the operon concept — a polycistronic unit with promoter + operator + structural genes — was proposed by Jacob and Monod.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The lac operon of E. coli consists of which of the following structural genes?
Answer: A. The three structural genes of the lac operon are lacZ (β-galactosidase), lacY (permease), and lacA (transacetylase), as described in NCERT Class 12 Biology Chapter 5, page 100. lacI is the regulatory gene located outside the operon.
Why B is wrong: B includes lacI, which is the regulatory gene encoding the repressor protein. It is NOT a structural gene of the operon.
Why C is wrong: C includes lacI and excludes lacY. The regulatory gene lacI is not part of the operon's structural gene cluster.
Why D is wrong: D includes lacP, which is not a gene name — the promoter is a DNA sequence, not a protein-coding gene. Also excludes lacZ.
The concept of the operon was proposed by:
Answer: B. François Jacob and Jacques Monod proposed the operon model of gene regulation in 1961, based on their work on the lac operon in E. coli (NCERT Class 12 Biology Chapter 5, page 100).
Why A is wrong: A — Watson and Crick proposed the double-helix structure of DNA, not the operon model.
Why C is wrong: C — Beadle and Tatum proposed the one gene–one enzyme hypothesis, which relates to gene function, not gene regulation.
Why D is wrong: D — Hershey and Chase experimentally confirmed DNA as genetic material using bacteriophages, not gene regulation.
Which molecule directly binds the lac repressor to inactivate it?
Answer: A. Allolactose, an isomer of lactose, is the actual inducer that binds the repressor and changes its conformation so it releases the operator (NCERT Class 12 Biology Chapter 5, pages 100–101). Lactose itself is converted to allolactose inside the cell.
Why B is wrong: B — Galactose is one of the two products of lactose hydrolysis by β-galactosidase; it does not bind the lac repressor. The inducer is lactose (acting as allolactose).
Why C is wrong: C — Glucose does not bind the repressor. Glucose affects the operon indirectly through cAMP levels and CAP activation.
Why D is wrong: D — cAMP binds CAP (catabolite activator protein), not the repressor. cAMP–CAP complex enhances transcription but does not inactivate the repressor.
In E. coli, when both glucose and lactose are present in the medium, the lac operon is:
Answer: C. When glucose is present, cAMP levels are low, so the CAP–cAMP complex cannot form and cannot enhance transcription. Even though lactose (via allolactose) removes the repressor from the operator, without CAP assistance, transcription occurs only at a low basal level (NCERT Class 12 Biology Chapter 5, page 101).
Why A is wrong: A — Full induction requires BOTH lactose present (to remove repressor) AND glucose absent (to allow cAMP–CAP activation). Glucose being present prevents full induction.
Why B is wrong: B — The repressor IS released because allolactose still binds it. The issue is not repressor binding but low cAMP preventing CAP-mediated enhancement.
Why D is wrong: D — A lacI mutation would make the operon constitutive, but the question describes a wild-type scenario with both sugars present. This option introduces a genetic change not stated in the stem.
A mutation in the lacI gene produces a defective repressor that cannot bind the operator. In the absence of lactose, the structural genes of the lac operon will be:
Answer: D. If the repressor cannot bind the operator, RNA polymerase has permanent access to the promoter regardless of whether lactose is present. The structural genes are transcribed constitutively (NCERT Class 12 Biology Chapter 5, pages 100–101). This is how constitutive mutants of the lac operon behave.
Why A is wrong: A — A free operator is what ALLOWS transcription, not what prevents it. RNA polymerase can proceed past an unblocked operator.
Why B is wrong: B — Allolactose is needed to remove a functional repressor. If the repressor is already defective and cannot bind, allolactose is irrelevant — transcription happens without it.
Why C is wrong: C — While glucose absence (via cAMP–CAP) enhances transcription level, the key point is that with a defective repressor, transcription occurs regardless of lactose. The operon is constitutive, not conditionally induced by glucose absence alone.
The product of the lacZ gene in the lac operon is:
Answer: D. lacZ encodes β-galactosidase, the enzyme that cleaves lactose into glucose and galactose (NCERT Class 12 Biology Chapter 5, page 100). lacY encodes permease and lacA encodes transacetylase.
Why A is wrong: A — Permease is the product of lacY, not lacZ. It facilitates lactose transport across the cell membrane.
Why B is wrong: B — Transacetylase is the product of lacA, not lacZ.
Why C is wrong: C — The repressor protein is encoded by lacI, the regulatory gene located outside the operon, not by any of the structural genes.
In the lac operon model, the operator region functions as:
Answer: B. The operator is the short DNA sequence between the promoter and the structural genes where the repressor binds. When the repressor occupies the operator, RNA polymerase cannot proceed (NCERT Class 12 Biology Chapter 5, pages 100–101).
Why A is wrong: A — The site where RNA polymerase first binds is the promoter, not the operator. Promoter and operator are distinct regulatory regions.
Why C is wrong: C — The repressor protein is encoded by the lacI gene, which is a separate regulatory gene. The operator is a non-coding regulatory sequence.
Why D is wrong: D — The cAMP–CAP complex binds upstream of the promoter (at the CAP-binding site), not at the operator.
Which of the following statements about the lac operon is INCORRECT?
Answer: C. The regulatory gene lacI is NOT part of the operon. It is located upstream and is transcribed independently, constitutively producing the repressor protein. The operon proper includes the promoter, operator, and the three structural genes lacZ, lacY, lacA (NCERT Class 12 Biology Chapter 5, page 100).
Why A is wrong: A — This is correct. The lac operon is switched on (induced) only when lactose is present, making it a classic inducible system.
Why B is wrong: B — This is correct. The three structural genes share one promoter and are transcribed together as a single polycistronic mRNA, which is then translated into three separate proteins.
Why D is wrong: D — This is correct. β-galactosidase (product of lacZ) has a side activity that isomerises lactose to allolactose, which then acts as the true inducer. This is an accepted biochemical detail consistent with NCERT.
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Given
A wild-type E. coli culture is grown in a medium containing lactose as the sole carbon source (no glucose). The lac operon is fully induced. You are asked: Which proteins are being actively produced, and what would happen if glucose were suddenly added to the medium?
Required
(a) Identify the proteins produced when the lac operon is fully induced.
(b) Predict the effect of adding glucose to the lactose-containing medium.
Concept
The lac operon is an inducible operon regulated at two levels: (i) negative regulation by the lacI repressor (removed by allolactose), and (ii) positive regulation by the cAMP–CAP complex (active when glucose is absent). Full transcription requires both conditions: repressor removed AND cAMP–CAP bound.
Formula / Principle
No mathematical formula applies. The regulatory logic is:• Lactose present → allolactose binds repressor → operator free → transcription possible.• Glucose absent → high cAMP → cAMP–CAP binds upstream → RNA polymerase strongly recruited → full transcription.
Substitution / Application
(a) With lactose present and glucose absent: repressor is inactivated by allolactose, and cAMP–CAP enhances transcription. All three structural genes (lacZ, lacY, lacA) are transcribed as a polycistronic mRNA. The proteins produced are β-galactosidase, permease, and transacetylase.
(b) Adding glucose: glucose lowers intracellular cAMP. Without cAMP, CAP cannot bind upstream of the promoter. Even though the repressor remains inactivated (lactose is still present), transcription drops to a low basal level because CAP-mediated enhancement is lost. This is catabolite repression — the cell preferentially uses glucose.
Calculation
Not applicable (conceptual question — no numerical computation).
Final answer
(a) β-galactosidase, permease, and transacetylase are actively produced.
(b) Adding glucose causes catabolite repression: transcription drops to basal level despite lactose still being present, because the cAMP–CAP positive regulatory mechanism is disabled.
Common trap
Students often say "the operon switches completely OFF when glucose is added." This is incorrect — the repressor is still inactivated by allolactose, so the operator remains unblocked. What is lost is the CAP enhancement. Transcription continues at a low basal rate, not zero.
Similar NEET-style question
"In an E. coli mutant where the CAP-binding site is deleted, what would be the expression status of the lac operon when only lactose is present in the medium?"
(Answer: The operon would be transcribed only at a basal level, because without the CAP-binding site, the cAMP–CAP complex cannot enhance transcription even when glucose is absent.)
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Jacob-Monod model. Lac repressor binds operator → blocks transcription. Lactose → allolactose → binds repressor → induces expression. Three structural genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase). Negative regulation; CAP-cAMP positive regulation.
-- NCERT Class 12 Biology, Ch. 5, p. 100More in Genetics and Evolution: 21 exam traps and mistakes · 4 formulas · 1 question pattern from its other lessons.
4 questions from NEET 2022, 2023, 2024, 2026. Answers verified against NTA official keys.
Which of the following statements about lac-operon is correct ?
The lactose present in the growth medium of bacteria is transported to the cell by the action of
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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