Bt Toxin cry Genes — cryIAc Controls Cotton Bollworm, cryIAb Controls Corn Borer
✅ Asked in NEET 2026Toxin proteins from Bacillus thuringiensis coded by which genes control cotton bollworms and corn borer respectively?
Toxin proteins from Bacillus thuringiensis coded by which genes control cotton bollworms and corn borer respectively?
Answer & NCERT explanation
Correct answer: A — cryIAc and cryIAb
cryIAc (and cryIIAb) toxin proteins control cotton bollworms, while cryIAb controls corn borer.
Read more NCERT concept on the PYQ
Specific Bt toxin genes were isolated from Bacillus thuringiensis and incorporated into the several crop plants such as cotton. The choice of genes depends upon the crop and the targeted pest, as most Bt toxins are insect-group specific. The toxin is coded by a gene cryΑc named cry. There are a number of them, for example, the proteins encoded by the genes cryΑAc and cryΙΑb control the cotton bollworms, that of cryΑAb controls corn borer.
Specific Bt toxin genes were isolated from Bacillus thuringiensis (Bt) and incorporated into crop plants. The toxin proteins are encoded by cry genes. The proteins encoded by cryIAc (and cryIIAb) control cotton bollworms, while cryIAb controls corn borer. Most Bt toxins are insect-group specific — the choice of gene depends on the crop and the targeted pest. Bt cotton uses cryIAc to protect against Helicoverpa armigera (cotton bollworm). The crystal toxin protein (Cry protein) is a protoxin activated in the alkaline insect gut, which then creates pores in midgut epithelium, killing the insect.
NEET 2026 answer A specifies the exact gene-pest pairing from NCERT. Students confuse cryIAc with cryIAb. Memory trick: 'C' for cotton (cryIAc for Cotton bollworm), 'b' for borer (cryIAb for corn Borer). The mechanism of Bt toxin: in insect gut, alkaline pH activates the protoxin → active toxin binds to specific membrane receptors in midgut epithelium → creates pores → cell swells and lyses → insect dies. Humans are unaffected because our gut pH is acidic and lacks the specific receptors.
cryIAb controls cotton bollworm and cryIAc controls corn borer.
cryIAc (+ cryIIAb) → cotton bollworm. cryIAb → corn borer. The 'c' helps remember Cotton.
cryIAc = C for Cotton bollworm. cryIAb = b for corn Borer. Never reverse these.
- cryIAc (and cryIIAb): control cotton bollworm (Helicoverpa armigera) — Lepidopteran pest.
- cryIAb: controls corn borer (Ostrinia nubilalis) — also Lepidopteran.
- Bt toxin mechanism: protoxin → alkaline insect gut → active toxin → midgut receptor binding → pore formation → cell lysis → death.
- Humans safe: acidic stomach pH + no specific receptors → protoxin not activated → no effect.
Regarding Bt toxin and its applications in crop protection: S1: The genes cryIAc (and cryIIAb) encode proteins that control cotton bollworms. S2: The gene cryIAb encodes a protein that controls corn borer. S3: Bt toxin is directly toxic to insects when ingested — the protoxin requires no activation. S4: Bt toxins are insect-group specific and the choice of cry gene depends on the target pest. S5: Humans are not affected by Bt toxin because our gut lacks the specific alkaline pH and receptors needed for activation. Which statements are CORRECT?
View bonus solution & explanation
Correct answer: B — S1, S2, S4 and S5
S1 CORRECT: cryIAc + cryIIAb → cotton bollworm (NCERT explicit, NEET 2026). S2 CORRECT: cryIAb → corn borer. S3 WRONG: Bt toxin is a PROTOXIN — it requires activation by the alkaline insect gut pH to become active. Not directly toxic as ingested. S4 CORRECT: Bt toxins are insect-group specific — hence different genes for different pests. S5 CORRECT: Human stomach is acidic and lacks specific receptors — protoxin not activated, hence safe.
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