The trap that costs marks here: confusing which GM strategy gives which trait. Bt crops, RNAi crops, and herbicide-tolerant crops each use a different gene and a different mechanism — NEET questions test whether you can match them correctly.
Bt crops express Cry proteins (from Bacillus thuringiensis) inside plant tissues. When a target insect — cotton bollworm (Helicoverpa armigera) for Bt cotton, corn borer for Bt corn — ingests the plant tissue, the inactive protoxin (Cry protein) is solubilised in the alkaline insect gut, converted to active toxin, and creates pores in midgut epithelial cells. The insect dies. The plant is not sprayed with anything — it produces the toxin endogenously (NCERT Class 12 Biology Chapter 10, page 179).
Specific Cry genes and their targets:
RNAi-based resistance uses a different mechanism entirely: RNA interference silences a specific nematode gene. The classic NCERT example is tobacco engineered to resist root-knot nematode (Meloidogyne incognita) by producing dsRNA complementary to a nematode mRNA. This is gene silencing, not a toxin.
Herbicide-tolerant GM crops carry genes like bar or EPSPS that allow the plant to survive broad-spectrum herbicides (e.g., glyphosate). The crop lives; the weeds die.
Watch-out: A common NEET distractor describes Bt cotton as "spraying" Cry protein externally. That confuses Bt spray (used in organic farming) with Bt transgenic crops (endogenous expression). The protein is made inside the plant cells — no spray involved.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following Cry genes codes for a protein toxic to cotton bollworm?
Answer: C. Cry1Ac produces a toxin specifically active against cotton bollworms (lepidopterans attacking cotton). NCERT Class 12 Biology Chapter 10 lists Cry1Ac and Cry2Ab as the genes used in Bt cotton.
Why A is wrong: A is wrong because Cry1Ab targets corn borer, not cotton bollworm — both are lepidopteran-targeting but crop-specific (trap: mixing up Cry gene–crop pairings).
Why B is wrong: B is wrong because Cry2Aa is not listed as a Bt cotton gene in NCERT; Cry2Ab is the second gene used alongside Cry1Ac (trap: close alphanumeric code confusion).
Why D is wrong: D is wrong because Cry1Aa is not listed among the standard Bt cotton Cry genes in the NCERT syllabus (trap: assuming all Cry1 variants target cotton).
In Bt cotton, the Cry protein is:
Answer: A. The cry gene is integrated into the cotton genome; the plant's own cells synthesise the Cry protein endogenously (NCERT Class 12 Biology Chapter 10, pages 179–180).
Why B is wrong: B is wrong because external spraying describes conventional Bt biopesticide use in organic farming, not Bt transgenic crops — the whole point of the transgene is endogenous production (trap: confusing Bt spray with Bt crop).
Why C is wrong: C is wrong because B. thuringiensis does not colonise the transgenic plant; the gene has been transferred into the plant genome via rDNA technology (trap: conflating the gene source organism with in-planta production).
Why D is wrong: D is wrong because the Cry protein is expressed continuously in plant tissues after germination, not applied as a one-time seed coat (trap: confusing seed treatment technology with transgenic expression).
RNAi-based resistance in transgenic tobacco protects against which pest?
Answer: B. The NCERT example of RNAi-based pest resistance is transgenic tobacco producing dsRNA to silence a nematode gene, specifically targeting the root-knot nematode Meloidogyne incognita (NCERT Class 12 Biology Chapter 10).
Why A is wrong: A is wrong because cotton bollworm is controlled by Bt Cry toxin in Bt cotton, not by RNAi in tobacco (trap: mixing up Bt mechanism with RNAi mechanism).
Why C is wrong: C is wrong because corn borer is targeted by Cry1Ab in Bt corn, not by RNAi in tobacco (trap: confusing crop–pest–mechanism combinations).
Why D is wrong: D is wrong because aphid resistance via RNAi in tobacco is not the standard NCERT example; the documented case is nematode resistance (trap: guessing a common pest without verifying the specific NCERT example).
A transgenic crop is engineered with the EPSPS gene. This crop would be expected to show resistance to:
Answer: A. The EPSPS gene confers tolerance to glyphosate (a broad-spectrum herbicide) by encoding a glyphosate-insensitive form of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase. The crop survives herbicide application while weeds are killed.
Why B is wrong: B is wrong because nematode resistance uses RNAi-based gene silencing, not a metabolic enzyme like EPSPS (trap: lumping all GM resistance mechanisms together).
Why C is wrong: C is wrong because insect resistance requires Cry toxin genes (Bt strategy), not EPSPS — EPSPS has no insecticidal activity (trap: conflating herbicide-tolerance and insect-resistance GM strategies).
Why D is wrong: D is wrong because EPSPS targets the shikimate pathway relevant to herbicide tolerance, not any antifungal defence pathway (trap: assuming any resistance gene works against any biotic stress).
The mechanism by which Bt toxin kills the target insect involves:
Answer: D. The inactive Cry protoxin is solubilised in the insect's alkaline gut (pH > 9.5), proteolytically cleaved to active toxin, which binds to midgut epithelial cell receptors and creates pores. Cell lysis and gut perforation kill the insect (NCERT Class 12 Biology Chapter 10).
Why A is wrong: A is wrong because acetylcholinesterase inhibition is the mechanism of organophosphate and carbamate chemical insecticides, not Bt Cry protein (trap: mixing up biopesticide mechanism with chemical insecticide mechanism).
Why B is wrong: B is wrong because chitin synthesis inhibitors (e.g., diflubenzuron) are insect growth regulators — Cry protein acts on midgut epithelium, not on moulting (trap: confusing different insect-control strategies).
Why C is wrong: C is wrong because spiracle blockage is the mechanism of horticultural oils and some physical insecticides, not a protein toxin that acts via ingestion (trap: confusing physical vs biochemical modes of action).
Which of the following correctly matches the GM strategy with its mechanism?
Answer: B. Herbicide-tolerant crops express genes like EPSPS or bar that allow the plant to survive herbicide application. This is the correct strategy–mechanism match.
Why A is wrong: A is wrong because Bt crops use Cry toxin protein (protoxin expressed endogenously), not RNA interference — RNAi is the nematode-resistance strategy (trap: swapping Bt and RNAi mechanisms).
Why C is wrong: C is wrong because RNAi tobacco uses double-stranded RNA to silence nematode genes, not Cry protein — Cry protein is the Bt strategy (trap: exact reversal of Bt vs RNAi).
Why D is wrong: D is wrong because Bt cotton expresses Cry genes (Cry1Ac, Cry2Ab) for insect resistance; bar gene confers herbicide tolerance, a completely different trait (trap: mixing up which gene goes with which trait).
A student claims: "Bt cotton and Bt biopesticide spray work the same way — both put Bt toxin on the plant surface to kill insects on contact." What is wrong with this claim?
Answer: C. In Bt cotton, the cry gene is part of the plant genome and the Cry protein is synthesised inside plant cells. Insects are killed when they ingest plant tissue containing the protoxin — it is not a surface application. Bt spray is an external application that sits on the surface and must also be ingested, but the key error in the claim is that Bt cotton has toxin "on the plant surface."
Why A is wrong: A is wrong because Bt biopesticide spray is also derived from Bacillus thuringiensis (biological, not synthetic) — the distinction is endogenous vs external, not natural vs synthetic (trap: inventing a false natural/synthetic dichotomy).
Why B is wrong: B is wrong because Bt cotton does NOT have toxin on the surface — it is produced inside the cells, which is the critical distinction the student missed (trap: accepting the surface-toxin premise while trying to differentiate by kill mechanism).
Why D is wrong: D is wrong because the student's claim conflates two fundamentally different delivery systems — endogenous transgenic expression vs external spray application (trap: accepting the claim at face value without analysing the production site).
An agricultural scientist is developing a GM crop that must simultaneously resist lepidopteran pests AND tolerate glyphosate herbicide. Which combination of transgenes would be required?
Answer: D. Lepidopteran resistance requires a Cry gene (Cry1Ac targets bollworms). Glyphosate tolerance requires EPSPS or bar gene. Combining both transgenes (gene stacking) gives the dual-trait crop. This requires identifying the correct gene for each trait and combining them.
Why A is wrong: A is wrong because the RNAi construct targets nematodes, not herbicide tolerance — this combination gives insect + nematode resistance but no herbicide tolerance (trap: confusing RNAi target with herbicide-tolerance mechanism).
Why B is wrong: B is wrong because stacking two Cry genes gives broader insect resistance (both target lepidopterans) but provides zero herbicide tolerance — neither Cry gene has anything to do with glyphosate resistance (trap: assuming more Cry genes = more trait coverage, ignoring that herbicide tolerance needs a different gene entirely).
Why C is wrong: C is wrong because the RNAi construct gives nematode resistance, not lepidopteran resistance — lepidopteran pests require Cry toxin, and RNAi cannot substitute for Bt against bollworms (trap: treating all pest-resistance mechanisms as interchangeable).
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Given
• The crop is transgenic cotton (Bt cotton).• It produces a protein lethal to cotton bollworm larvae upon ingestion.• The question asks for gene source, gene identity, kill mechanism, and distinction from Bt spray.
Required
Identify the organism, gene(s), mechanism of action, and the key difference between transgenic Bt expression and external Bt spray.
Concept
Bt cotton carries cry genes from Bacillus thuringiensis integrated into its genome. The expressed Cry protein is a protoxin that becomes active in the alkaline gut of susceptible insects.
Relevant information
• Source organism: Bacillus thuringiensis• Genes for cotton bollworm: Cry1Ac, Cry2Ab• Activation: alkaline gut pH solubilises protoxin → active toxin• Mode of killing: pore formation in midgut epithelial cells• Bt spray: external application on plant surface; Bt transgenic: endogenous production inside plant cells
Identification/Reasoning
(a) The gene comes from Bacillus thuringiensis, a soil bacterium.
(b) The specific genes are Cry1Ac and Cry2Ab — both target lepidopteran pests that attack cotton.
(c) The Cry protein is produced as an inactive protoxin inside plant cells. When a bollworm larva ingests plant tissue, the protoxin enters the larval gut (pH > 9.5), is solubilised, and proteolytic enzymes cleave it into the active Bt toxin. The active toxin binds to specific receptors on midgut epithelial cells, creating pores. This causes cell lysis, gut perforation, septicaemia, and death.
(d) In Bt spray, the toxin is applied externally and sits on the plant surface — it degrades with UV and rain. In Bt cotton, the Cry protein is synthesised endogenously within every cell of the plant, providing continuous protection without reapplication and without surface residue.
Answer assembly
| Part | Answer |
|------|--------|
| (a) Source organism | Bacillus thuringiensis |
| (b) Gene(s) | Cry1Ac, Cry2Ab |
| (c) Kill mechanism | Protoxin → alkaline gut activation → midgut pore formation → cell lysis → insect death |
| (d) Bt spray vs Bt crop | Spray = external, degrades; Bt crop = endogenous expression in plant cells, continuous |
Final answer
Bt cotton carries Cry1Ac and Cry2Ab genes from Bacillus thuringiensis. The plant produces Cry protoxin endogenously; upon ingestion by bollworm larvae, the alkaline gut activates the toxin, which creates pores in midgut epithelial cells, killing the larva. Unlike Bt spray (external, degradable), the transgenic protein is continuously produced inside plant tissues.
Common trap
Stating that Bt cotton "sprays" or "coats" the plant surface with Cry protein. This confuses Bt biopesticide spray (an organic farming input applied externally) with Bt transgenic expression (gene integrated into plant genome, protein made inside cells).
Similar NEET-style question
"Golden rice is a transgenic crop enriched with β-carotene. Which of the following correctly identifies the gene strategy — Bt toxin, RNAi, or metabolic pathway engineering?" (Answer: metabolic pathway engineering — introduction of phytoene synthase and carotene desaturase genes, not Bt or RNAi.)
---
Bt cotton: Cry1Ac (bollworm), Cry2Ab. Bt corn: Cry1Ab. RNAi (RNA interference): nematode resistance in tobacco. Pest-resistant, herbicide-tolerant (Roundup-Ready) GM crops.
-- NCERT Class 12 Biology, Chapter 10, p. 180These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Bt: insect resistance (Cry toxin). RNAi: nematode resistance via gene silencing. Herbicide-tolerant: bar/EPSPS gene.
Question on which gene/strategy gives which trait.
Cry → insects. RNAi → nematodes. EPSPS → herbicide tolerance.
Root cause: term confusion
Bt cotton expresses Cry protein INSIDE its tissues — endogenous; insects ingesting plant tissue are killed. Not externally sprayed.
More in Biotechnology and its Applications: 5 exam traps and mistakes · 1 question pattern from its other lessons.
4 questions from NEET 2023, 2024, 2025, 2026. Answers verified against NTA official keys.
The inactive form of Bt toxin is converted to the active form in the insect gut ______.
Silencing of specific mRNA is possible via RNAi because of
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