BiologyISC Class 12 · Biotechnology and its Applications

Bt Cotton and Pest-Resistant Plants: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Bt cotton carries cry genes from Bacillus thuringiensis that code for an inactive protoxin. In the alkaline midgut of an insect larva the crystals dissolve and the toxin becomes active, binds midgut cells, makes pores and kills the insect. cryIAc and cryIIAb control cotton bollworms. Tobacco resists the nematode Meloidogyne incognita through RNA interference.

Understand Bt Cotton and Pest-Resistant Plants

Why pest-resistant crops. Genetically modified (GM) crops reduce reliance on chemical pesticides, which are costly and harmful. Plants, bacteria, fungi and animals whose genes have been altered by manipulation are genetically modified organisms (GMOs). Other GM benefits: tolerance to abiotic stresses (cold, drought, salt, heat), fewer post-harvest losses, efficient mineral use, enhanced nutrition (golden rice).

1. Bt crops.
Source. Bt toxin is produced by the soil bacterium Bacillus thuringiensis (Bt). Some strains make proteins that kill certain insects:

Insect group	Examples
lepidopterans	tobacco budworm, armyworm (and bollworms)
coleopterans	beetles
dipterans	flies, mosquitoes

How the toxin works (flow chart). During a particular phase of growth B. thuringiensis forms protein crystals containing a toxic insecticidal protein → the toxin exists as an inactive protoxin → an insect larva eats it → the alkaline pH of the insect gut solubilises the crystals and converts protoxin to the active toxin → the active toxin binds to the surface of midgut epithelial cells → it creates pores → cells swell and lyse → the insect dies.
Why the bacterium is not killed: inside the bacterium the toxin is an inactive protoxin; activation needs the insect's alkaline gut.

cry genes and specificity. The toxin is coded by genes named cry. Most Bt toxins are insect-group specific, so the choice of gene depends on the crop and its pest.

Gene	Target pest
cryIAc and cryIIAb	cotton bollworms
cryIAb	corn borer

Specific Bt toxin genes are isolated and incorporated into crops such as cotton; other Bt crops include corn, rice, tomato, potato and soyabean. In effect, the plant makes its own biopesticide.

2. Pest-resistant plants by RNA interference (RNAi).
The pest. The nematode Meloidogyne incognita infects the roots of tobacco plants and greatly reduces yield.
The principle. RNAi occurs in all eukaryotes as a method of cellular defence. A complementary double-stranded RNA (dsRNA) binds to a specific mRNA and prevents its translation (silencing). Natural sources of such RNA: infection by viruses with RNA genomes, or transposons that replicate through an RNA intermediate.
The method (flow chart). Using Agrobacterium vectors, nematode-specific genes were introduced into tobacco → the introduced DNA produced both sense and anti-sense RNA in the host cells → the two complementary RNAs formed dsRNA → dsRNA initiated RNAi → the specific mRNA of the nematode was silenced → the parasite could not survive in the transgenic host → the plant was protected.

Comparison.

	Bt cotton	RNAi tobacco
Pest	insect larvae (bollworms)	nematode (Meloidogyne incognita)
Gene source	bacterium (B. thuringiensis)	the nematode itself
Plant makes	Bt protein (protoxin)	sense + anti-sense RNA → dsRNA
Kills by	pores in midgut cells	silencing an essential nematode mRNA

Diagram checklist (Bt mode of action). Draw a larva's gut: crystals of protoxin entering → 'alkaline pH' label → active toxin → binding to midgut epithelial cells → pores → swollen, lysed cells. Label each arrow.

Bt Cotton and Pest-Resistant Plants: key facts

Bt activation: 1.Protoxin crystals eaten by the larva
2.alkaline gut pH solubilises crystals and activates the toxin
3.toxin binds midgut epithelial cells
4.pores form; cells swell and lyse
5.insect dies; Inactive inside the bacterium.
cry genes: cryIAc cotton bollworm
cryIIAb cotton bollworm
cryIAb corn borer; Gene names are italicised, small letters for cry.
RNAi: 1.Agrobacterium delivers nematode-specific genes
2.tobacco cells make sense and anti-sense RNA
3.the two pair into dsRNA
4.RNAi silences the nematode mRNA
5.Meloidogyne incognita cannot survive; Gene delivered by Agrobacterium.
Insect groups killed: Lepidopterans tobacco budworm, armyworm, bollworms
Coleopterans beetles
Dipterans flies, mosquitoes; Different strains, different targets.

cryIAc	cotton bollworm
cryIIAb	cotton bollworm
cryIAb	corn borer

Lepidopterans	tobacco budworm, armyworm, bollworms
Coleopterans	beetles
Dipterans	flies, mosquitoes

How to answer Bt Cotton and Pest-Resistant Plants questions

ISC asks the target pest of a named cry gene, the mechanism of Bt toxin, why the bacterium is safe from its toxin, and RNAi against Meloidogyne incognita.

1Match gene to pest exactly: cryIAc → cotton bollworm; cryIAb → corn borer.
2Describe Bt action in order: protoxin crystals, alkaline gut, activation, binding to midgut cells, pores, lysis, death.
3For 'why not toxic to the bacterium', say it is stored as an inactive protoxin.
4For RNAi, give the nematode, the host (tobacco), the vector (Agrobacterium), sense and anti-sense RNA forming dsRNA, and mRNA silencing.
5Name the advantage over chemical pesticides: fewer sprays, less pollution, specific action.
6Use full scientific names: Bacillus thuringiensis, Meloidogyne incognita.

Bt Cotton and Pest-Resistant Plants in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Specificity of Bt toxins

Suppose a farmer who grows Bt cotton notices that bollworm damage has fallen sharply, but aphids (sap-sucking insects) still attack the same plants.

Explain both observations.

Try it first, then show the solutionSolution

The cotton carries cry genes such as cryIAc whose toxin is specific to lepidopteran larvae like the bollworm.
Bollworm larvae chew the plant, ingest the protoxin, and their alkaline gut activates it, so they die.
Bt toxins are insect-group specific; the toxin in this cotton does not act on aphids, which belong to a different group and feed differently.
The farmer still needs another control, for example ladybird beetles as biocontrol for aphids.

Answer: The cry toxin kills bollworms specifically; aphids are not its target, so other control is needed.

Memory hook: Right gene, right pest.

Tricks to remember Bt Cotton and Pest-Resistant Plants

Short memory aids. Say each one aloud once, then use it on a question the same day.

Crystal, Alkali, Pore
C-A-P: Crystal protoxin, Alkaline gut activates, Pores kill.
Use it when: Bt mechanism.
Ac for cotton, Ab for corn
cryIAc: 'c' for cotton; cryIAb: corn borer.
Use it when: Gene-pest pairs.
Sense + anti-sense = silence
Two RNAs pair up and hush the worm's message.
Use it when: RNAi.

Bt Cotton and Pest-Resistant Plants: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Target pests · 2 marks · Short answerBoard level

Name the target pests of the proteins coded by (i) cryIAc and (ii) cryIAb.

Try it first, then show the solutionSolution

(i) Cotton bollworm (1 mark).
(ii) Corn borer (1 mark).

Answer: Cotton bollworm; corn borer.

Example 2: Mode of action · 3 marks · Short answerBoard level

Explain how Bt toxin kills an insect larva.

Try it first, then show the solutionSolution

The toxin is present as inactive protoxin in protein crystals; the larva ingests it with plant tissue (1 mark).
The alkaline pH of the insect's gut solubilises the crystals and converts protoxin into active toxin (1 mark).
The active toxin binds to midgut epithelial cells, creates pores, the cells swell and lyse, and the insect dies (1 mark).

Answer: Protoxin → activated in alkaline gut → pores in midgut cells → lysis and death.

Example 3: Why the bacterium survives · 2 marks · Short answerHarder board level

Bacillus thuringiensis makes a protein that kills insects. Why does the protein not kill the bacterium itself? Why does it not harm humans who eat Bt crops?

Try it first, then show the solutionSolution

In the bacterium the toxin exists as an inactive protoxin in crystals; it is activated only by the alkaline pH of the insect gut (1 mark).
The human stomach is acidic, so the protoxin is not activated in the same way (and the active toxin needs specific binding sites on insect midgut cells) (1 mark).

Answer: Inactive protoxin in the bacterium; activation needs the alkaline insect gut, which humans lack.

Example 4: RNAi in tobacco · 3 marks · Short answerHarder board level

How was tobacco made resistant to the nematode Meloidogyne incognita?

Try it first, then show the solutionSolution

Nematode-specific genes were introduced into tobacco using Agrobacterium vectors (1 mark).
The introduced DNA produced both sense and anti-sense RNA, which paired to form double-stranded RNA (1 mark).
dsRNA started RNA interference, silencing a specific nematode mRNA, so the parasite could not survive in the transgenic plant (1 mark).

Answer: Agrobacterium-delivered nematode genes → sense + anti-sense RNA → dsRNA → RNAi silences nematode mRNA.

Example 5: Bt versus RNAi · 3 marks · Short answerHarder board level

Give three differences between the strategies used in Bt cotton and in nematode-resistant tobacco.

Try it first, then show the solutionSolution

Pest: Bt cotton targets insect larvae (bollworms); RNAi tobacco targets a nematode (1 mark).
Gene source: Bt genes come from a bacterium; RNAi uses genes of the nematode itself (1 mark).
Mechanism: Bt protein makes pores in midgut cells; RNAi silences an essential mRNA of the parasite (1 mark).

Answer: Different pest, gene source and mechanism (pores versus mRNA silencing).

Example 6: Resistance in bollworms · 4 marks · Short answerHOTS / competency

Suppose after many years of growing only Bt cotton in a district, bollworms that survive Bt toxin become common. (a) Explain this using natural selection. (b) Suggest two farm practices to slow it. (c) Why might a crop carrying two different cry genes (for example cryIAc and cryIIAb) be more durable?

Try it first, then show the solutionSolution

(a) A few bollworms with natural variation that resists the toxin survived and reproduced every season; their resistant offspring increased (directional selection) (1½ marks).
(b) Grow non-Bt 'refuge' plants nearby so susceptible insects survive and mate with resistant ones; rotate crops and combine other controls (1½ marks).
(c) An insect would need resistance to both toxins at once, which is much rarer, so resistance spreads more slowly (1 mark).

Answer: Selection of rare resistant bollworms; refuges and rotation; two toxins make resistance rarer.

Where students lose marks in Bt Cotton and Pest-Resistant Plants

The slips examiners see most often, and what to write instead.

Mistake: Incomplete name of the target pest
In 2024 examiners reported 'cotton worm', 'cotton borer', just 'bollworm' or 'corn bollworm' for the target of cryIAc.
Do this: Write 'cotton bollworm' in full.
Usually costs: 1 mark.
Mistake: Mixing up cryIAc and cryIAb
Examiners (2024) suggested clarifying the difference between cryIAc and cryIAb, the two gene names most easily confused.
Do this: Ac: cotton bollworm; Ab: corn borer.
Usually costs: 1 mark.
Mistake: Saying the toxin is active in the bacterium
Answers often omit the protoxin stage and the alkaline gut.
Do this: Always include 'inactive protoxin' and 'alkaline pH of gut'.
Usually costs: 1 mark.
Mistake: Writing that RNAi kills the plant pest by a toxin
RNAi works by silencing mRNA, not by a toxic protein.
Do this: Say dsRNA silences the nematode's mRNA.
Usually costs: 1 mark.

Exam tips

Write cry gene names exactly: cryIAc, cryIIAb, cryIAb.
Include the words 'midgut epithelial cells' and 'pores'.
Name Agrobacterium as the vector in the RNAi example.
Use 'biopesticide' to describe what the Bt plant effectively makes.
Mention insect-group specificity when asked about choice of genes.

Practice questions on Bt Cotton and Pest-Resistant Plants

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The Bt toxin becomes active in the insect gut because of:
(a) acidic pH (b) alkaline pH (c) high temperature (d) digestive lipases
Hint
Solubilises crystals.
Answer
(b) alkaline pH.
Q2 · 2 marks · Short answerBoard level
Name the nematode that infects tobacco roots and the technique used to protect the plant.
Hint
Root-knot.
Answer
Meloidogyne incognita; RNA interference (RNAi) using nematode-specific genes delivered by Agrobacterium.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): RNA interference occurs naturally in eukaryotes.
Reason (R): It is a method of cellular defence in which dsRNA silences a specific mRNA.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Why does it exist?
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
Name three insect groups killed by Bt toxins, with one example each.
Hint
Moths, beetles, flies.
Answer
Lepidopterans (tobacco budworm, armyworm); coleopterans (beetles); dipterans (flies, mosquitoes).
Q5 · 2 marks · Short answerHarder board level
What are natural sources of the complementary RNA that triggers RNAi in cells?
Hint
Viruses and transposons.
Answer
Infection by viruses with RNA genomes, and mobile genetic elements (transposons) that replicate via an RNA intermediate.
Q6 · 2 marks · Short answerHarder board level
Give two advantages of pest-resistant GM crops over spraying chemical pesticides.
Hint
Cost, environment, specificity.
Answer
They reduce the amount and cost of pesticide sprays; they cause less pollution of soil and water and harm fewer non-target organisms because the toxin is specific and stays in the plant.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a scientist wants to protect brinjal from a beetle pest using Bt technology. What should she consider in choosing the cry gene, and how would she get it into the plant?
Hint
Group specificity; vector.
Answer
Bt toxins are insect-group specific, so she must choose a cry gene whose protein acts on coleopterans (beetles), not a bollworm gene such as cryIAc. The gene can be inserted into a disarmed Ti plasmid of Agrobacterium tumefaciens (or delivered by a gene gun) to transform brinjal cells, which are then regenerated into whole plants by tissue culture.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose two tobacco fields are infected by Meloidogyne incognita. Field P has ordinary plants; field Q has transgenic plants that make both sense and anti-sense RNA of a nematode gene.
(a) Predict the condition of the roots in each field. (1)
(b) What molecule forms in Q's cells that protects them? (1)
(c) Why can the nematode not survive on Q's plants? (1)
Hint
Silencing.
Answer
(a) P: roots heavily infested, yield greatly reduced; Q: roots largely healthy. (b) Double-stranded RNA (dsRNA) formed by the sense and anti-sense RNAs. (c) The dsRNA triggers RNAi that silences a specific essential mRNA of the nematode, so its protein is not made and the parasite dies.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What does Bt stand for?Show answer
Bacillus thuringiensis.
2.Target of cryIIAb?Show answer
Cotton bollworms.
3.In what form is Bt toxin stored in the bacterium?Show answer
As an inactive protoxin in protein crystals.
4.Which plant was protected from Meloidogyne incognita by RNAi?Show answer
Tobacco.
5.What forms when sense and anti-sense RNA pair?Show answer
Double-stranded RNA (dsRNA).
6.Name a Bt crop other than cotton.Show answer
Bt corn (also rice, tomato, potato, soyabean).

Bt Cotton and Pest-Resistant Plants: frequently asked questions

What is Bt cotton?

Cotton carrying cry genes from Bacillus thuringiensis, so the plant makes a protein toxic to bollworms and needs fewer insecticide sprays.

How does Bt toxin kill insects?

The inactive protoxin is activated by the alkaline insect gut, binds to midgut epithelial cells, makes pores, and the cells swell and burst, killing the insect.

Which cry gene controls the corn borer?

cryIAb.

What is RNA interference?

A natural cellular defence in eukaryotes in which a complementary double-stranded RNA silences a specific mRNA by preventing its translation.

Why is Bt toxin harmless to the bacterium?

Inside the bacterium it is an inactive protoxin; it becomes active only in the alkaline gut of an insect.