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 116). 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?
The concept of the operon was proposed by:
Which molecule directly binds the lac repressor to inactivate it?
In *E. coli*, when both glucose and lactose are present in the medium, the lac operon is:
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:
The product of the *lacZ* gene in the lac operon is:
In the lac operon model, the operator region functions as:
Which of the following statements about the lac operon is INCORRECT?
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.) ---
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. 116F2 ratio in monohybrid cross — products of independent assortment of two alleles per locus.
| Symbol | Quantity | SI Unit |
|---|---|---|
| ratio | F2 progeny ratio | - |
F2 ratio in dihybrid cross with two independently-segregating loci, complete dominance, no linkage.
| Symbol | Quantity | SI Unit |
|---|---|---|
| ratio | F2 phenotype ratio | - |
In an idealised population (no mutation, drift, selection, gene flow, random mating), allele and genotype frequencies remain constant.
| Symbol | Quantity | SI Unit |
|---|---|---|
| p | freq of dominant allele A | - |
| q | freq of recessive allele a | - |
Proportion of recombinant offspring measures genetic distance between linked loci. Capped at 50% (independent assortment).
| Symbol | Quantity | SI Unit |
|---|---|---|
| RF | recombination frequency | % |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Sign Convention
DNA polymerase synthesises only 5'→3'. Leading strand: continuous, same direction as fork. Lagging: discontinuous (Okazaki), opposite to fork.
Question on Okazaki, leading vs lagging, primer direction.
Reading template 3'→5'; synthesising 5'→3'. Lagging strand needs short fragments because it can't run continuously against fork direction.
Category: Negative Marking
Five forces disturb HW: mutation, gene flow, drift, selection, non-random mating. ANY of these violates equilibrium.
Question asks which factor maintains/disturbs HW.
Random mating + no other forces → equilibrium. ANY of mutation/migration/drift/selection/assortative mating → disequilibrium.
Category: Similar Terms
Monohybrid: genotype 1:2:1 (AA:Aa:aa); phenotype 3:1.
Question asks for one ratio while presenting cross details.
Always note dominance: phenotype merges Aa + AA; genotype keeps them separate.
Category: Similar Terms
X-linked recessive (haemophilia, colour-blindness): affects males predominantly; carrier mother → 50% sons affected; affected father → all daughters carriers but not affected.
Pedigree question; carrier vs affected.
Sex chromosomes: XX vs XY. Recessive on X needs both copies (XaXa) in female, only one (XaY) in male.
Category: Similar Terms
Allopatric: geographic isolation. Sympatric: same area, no physical barrier (e.g. polyploidy in plants, host-shift).
Question gives speciation scenario and asks which type.
If geographic barrier mentioned → allopatric. If population overlaps → sympatric.
Root cause: formula misuse
Genotype frequencies: p² (AA) + 2pq (Aa) + q² (aa) = 1. Allele frequencies: p + q = 1. Don't drop 2pq.
Root cause: concept gap
ALL DNA pol synthesises 5'→3'. Lagging strand uses Okazaki fragments to APPEAR to extend toward fork while each fragment grows 5'→3'.
Root cause: concept gap
Frameshift = ANY indel NOT divisible by 3 (since codons are triplets). Indels of 3, 6, 9... preserve reading frame.
Root cause: term confusion
Start codon: AUG (Met / fMet). Stop codons: UAA (ochre), UAG (amber), UGA (opal/umber).
51 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Identify the statement that is NOT correct.
Which factor is important for termination of transcription?
What is the pattern of inheritance for polygenic trait?
Which of the following statement is correct regarding the process of replication in E.coli?
Which one of the following factors will not affect the Hardy-Weinberg equilibrium?
The phenomenon of pleiotropism refers to
What is the role of RNA polymerase III in the process of transcription in Eukaryotes?
Among eukaryotes, replication of DNA takes place in :
Expressed Sequence Tags (ESTs) refers to
Unequivocal proof that DNA is the genetic material was first proposed by
Broad palm with single palm crease is visible in a person suffering from-
Select the correct group/set of Australian Marsupials exhibiting adaptive radiation.
XO type of sex determination can be found in :
The process of translation of mRNA to proteins begins as soon as :
Which of the following occurs due to the presence of autosome linked dominant trait ?
Detritivores breakdown detritus into smaller particles. This process is called:
If the length of a DNA molecule is 1.1 metres, what will be the approximate number of base pairs?
Which of the following is not a desirable feature of a cloning vector?
Mutations in plant cells can be induced by: 130. Match List-I with List-II.
Which of the following RNAs is not required for the synthesis of protein?
•ŸÈ‹ πŸ ∑ § ‚◊ÿ «UË.∞Ÿ.∞. ∑§Ë ∑È §«U‹Ë ∑§Ê πÊ ‹Ÿ ◊ ∑§ÊÒŸ‚Ê
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
similar term confusion
Biology relies on precise terminology; close terms tempt selection.
Test yourself on this topic with real past-paper questions:
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