A farmer grew two varieties of corn crop in field A and B. He grew normal corn crops in field A and GM corn crops in field B. He observed that corn borers attacked only in field A. To control it, spores of Bt were sprayed on field A.
(a) Name the gene in the spores responsible for control of pest. [½]
(b) What effect will the spores of Bt have on insect pest? [1½]
(c) How has corn plants of field B developed resistance against this pest? Explain. [1]
A farmer grew two varieties of corn crop in field A and B. He grew normal corn crops in field A and GM corn crops in field B. He observed that corn borers attacked only in field A. To control it, spores of Bt were sprayed on field A.
(a) Name the gene in the spores responsible for control of pest. [½]
(b) What effect will the spores of Bt have on insect pest? [1½]
(c) How has corn plants of field B developed resistance against this pest? Explain. [1]
The pest control here works through the Cry (Bt) toxin produced by Bacillus thuringiensis.
(a) Gene responsible:
- The cry gene (for corn borer, specifically cryIAb) present in Bacillus thuringiensis.
(b) Effect of Bt spores on the insect pest:
- Bt spores contain the toxin as an inactive protoxin (crystal protein).
- When the corn borer ingests it, the alkaline pH of the insect's midgut solubilises the crystals and activates the toxin.
- The activated toxin binds to the surface of midgut epithelial cells, creating pores that cause the cells to swell, lyse and die — the insect stops feeding and dies.
(c) Resistance of field B (GM corn):
- The cry gene has been introduced into the corn genome by genetic engineering, making it a transgenic Bt corn.
- The plant itself synthesises the Cry protein in its tissues, so any corn borer feeding on it is killed — the crop is inherently pest-resistant and needs no spraying.
Marking Scheme
- 1(a) ½ mark: cry gene (cryIAb for corn borer) of Bacillus thuringiensis.
- 2(b) 1½ marks: protoxin ingested → alkaline gut pH activates toxin → binds midgut epithelial cells → creates pores/cell lysis → insect dies (any three linked points).
- 3(c) 1 mark: cry gene has been incorporated into the corn genome (transgenic Bt corn), so the plant itself produces Cry protein that kills the feeding borer.
Hint
Bt = Bacillus thuringiensis; the cry gene makes an inactive protoxin that activates in the insect's alkaline gut. Field B is transgenic — it carries the cry gene inside the plant.
Quick Oral Answer
The cry gene of Bacillus thuringiensis makes an inactive Cry protoxin; in the corn borer's alkaline gut it is activated, binds midgut cells and forms pores that lyse them, killing the insect — and in field B the cry gene is engineered into the plant, making it a transgenic Bt corn.
Analysis & Explanation
Concept:
The soil bacterium Bacillus thuringiensis (Bt) forms protein crystals during sporulation. These Cry proteins are protoxins — harmless until activated inside a susceptible insect.
Mechanism of action (why it is selective):
- The protoxin is inactive as eaten.
- The insect midgut is strongly alkaline, which solubilises the crystal and converts it to the active toxin.
- The toxin binds midgut epithelial receptors and creates pores → osmotic imbalance → cell lysis → insect death.
- Human/animal guts are acidic and lack these receptors, so the toxin is harmless to us — this is why it is a targeted biopesticide.
Gene specificity:
- cryIAb controls the corn borer; cryIAc and cryIIAb control cotton bollworms.
Exam trap:
- Field A is sprayed with Bt spores (external biopesticide); field B is transgenic — the difference is that field B carries the cry gene inside the plant, not sprayed on it.
Real-world:
Bt cotton, carrying cry genes, was the first genetically modified crop commercialised in India (2002), reducing insecticide use against bollworm.
Common Mistakes
- 1Saying the Bt toxin is active when sprayed — it is an inactive protoxin that is activated only by the alkaline pH of the insect's gut.
- 2Confusing field A and field B: field A is sprayed externally with Bt spores, while field B is transgenic with the cry gene inside the plant.
- 3Writing the wrong cry gene — cryIAb controls corn borer, whereas cryIAc/cryIIAb control cotton bollworms.
Interesting Facts
The Bt crystal protein is a protoxin that becomes active only above about pH 9.5, found in the alkaline midgut of caterpillars — this is why it is harmless to humans, whose gut is acidic.
Bt cotton was the first GM crop approved for commercial cultivation in India, in 2002.
Different cry genes are highly host-specific: cryIAb targets corn borers while cryIAc and cryIIAb target cotton bollworms.
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Frequently Asked Questions
Why is the Bt toxin harmful to insects but safe for humans?
The Bt Cry protein is eaten as an inactive protoxin. It is activated only in the highly alkaline midgut of insects like caterpillars, where it binds specific epithelial receptors and forms lethal pores. The human gut is acidic and lacks these receptors, so the protoxin is not activated and causes no harm.
What is the difference between spraying Bt and growing Bt crops?
Spraying Bt applies the bacterial spores/toxin externally on the plant surface, where it can wash off and degrade. In a Bt crop, the cry gene is engineered into the plant's own genome, so every cell continuously produces the toxin, giving built-in, long-lasting pest resistance without spraying.