Biology · Biotechnology and Its Applications · NEET
Bt toxin is a poison protein made by the soil bacterium Bacillus thuringiensis (Bt). During one growth phase, the bacterium forms protein crystals that contain this insecticidal protein, which kills insects like caterpillars (lepidopterans), beetles (coleopterans), and flies/mosquitoes (dipterans). Memory hook: "Bt = Bacteria that Terminate insects" — it makes a crystal (Cry) protein coded by the cry gene.
Bacillus thuringiensis stores Bt toxin as inactive protoxin crystals coded by the cry gene. Inside an insect's alkaline gut the protoxin becomes active toxin, forms pores in midgut cells, and kills the insect. It is insect-group specific.
Your doubts, answered
Is Bt toxin a chemical pesticide or a protein?
It is a protein, not a chemical. Bt toxin is an insecticidal protein made by a living bacterium, Bacillus thuringiensis. This is why it is called a biological method of pest control and is safer for humans and the environment than chemical insecticides. For NEET, remember: Bt toxin = insecticidal crystal PROTEIN, coded by a gene.
Why does Bacillus thuringiensis form protein crystals?
During a particular phase of its growth, B. thuringiensis makes and stores the toxin as solid protein crystals. These crystals hold the toxic insecticidal protein in an inactive, stored form. Because it is packed as a crystal (inactive protoxin), it stays safe inside the bacterium and only becomes active later inside an insect's gut.
What does the cry gene code for?
The gene that codes for the Bt toxin protein is called the cry gene (from 'crystal'). Different cry genes make toxins that target different pests. NCERT names examples: cryIAc and cryIIAb control cotton bollworms, and cryIAb controls corn borer. For NEET, link the gene name (cry) to the crystal protein it produces.
Which insects does Bt toxin kill?
NCERT lists three insect groups: lepidopterans (like tobacco budworm and armyworm), coleopterans (beetles), and dipterans (flies and mosquitoes). Each Bt toxin is usually insect-group specific — it does not kill every insect, only its target group. This specificity is a common NEET point.
Is Bt toxin the same thing as Bt cotton?
No. Bt toxin is the insecticidal protein itself. Bt cotton is a genetically modified crop where the cry gene from B. thuringiensis has been inserted so the cotton plant makes the toxin on its own to resist pests like bollworm. Bt toxin is the cause; Bt cotton is one application of it.
⚠️ The NEET trap ✗ Bt toxin is a chemical insecticide sprayed on crops, and it directly kills all insects. ✓ Bt toxin is an insecticidal PROTEIN (crystal protein) made by the bacterium Bacillus thuringiensis, coded by the cry gene. It is insect-group specific (lepidopterans, coleopterans, dipterans), not a chemical and not universal. 🧠 If the option says 'chemical' or 'kills all insects', it is wrong. Bt toxin = specific crystal PROTEIN from a gene.
Real NEET questions
NEET 2019
What triggers activation of protoxin to active Bt toxin of Bacillus thuringiensis in boll worm?
A · Body temperature
B · Moist surface of midgut
C · Alkaline pH of gut ✓
D · Acidic pH of stomach
Solution: The Bt toxin is stored as an inactive protoxin (crystal protein). When the bollworm eats it, the alkaline pH of the insect gut solubilises the crystals and converts the protoxin into the active toxin. The active toxin then binds midgut epithelial cells and makes pores, causing lysis and death. NCERT Ch 10, p.179.
NEET 2024
Statement I: Bt toxins are insect group specific and coded by a gene cry 1Ac. Statement II: Bt toxin exists as inactive protoxin in B. thuringiensis. However, after ingestion by the insect the inactive protoxin gets converted into active form due to acidic pH of the insect gut. Choose the correct answer.
A · Both Statement I and Statement II are false
B · Statement I is true but Statement II is false ✓
C · Statement I is false but Statement II is true
D · Both Statement I and Statement II are true
Solution: Statement I is true — Bt toxins are insect-group specific and coded by cry genes (e.g. cryIAc). Statement II is false because conversion of protoxin to active toxin happens due to the ALKALINE pH of the insect gut, not acidic pH. So Statement I true, Statement II false. NCERT Ch 10, p.179.
NEET 2026
The toxin proteins isolated from Bacillus thuringiensis, coded by which of the following genes would control cotton bollworms and corn borer, respectively?
A · cryIAc and cryIAb ✓
B · cryIAc and cryIIAb
C · cryIIAb and cryIAc
D · cryIAc and cryIIIAb
Solution: Proteins encoded by cryIAc (and cryIIAb) control cotton bollworms, while the protein of cryIAb controls corn borer. For cotton bollworm and corn borer respectively, the matching pair is cryIAc and cryIAb. NCERT Ch 10, p.180.
Solved Biotechnology and Its Applications NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Bt toxin is a poison protein made by the soil bacterium Bacillus thuringiensis. It kills certain insect pests, so it is used as a natural, biological way to protect crops instead of chemical insecticides.
What is the full form of Bt?
Bt stands for Bacillus thuringiensis, the name of the bacterium that makes the toxin.
Why is it called a Cry protein?
The toxin is stored as protein CRYstals inside the bacterium, and the gene that codes for it is called the cry gene. So the protein is often called the Cry protein.
Does Bt toxin harm humans?
Bt toxin is insect-group specific. It needs the alkaline gut and specific binding sites found in target insects, so it does not harm humans, which makes it safer than chemical pesticides. Note: NCERT focuses on its insect specificity.
Which three insect groups are killed by Bt toxins?
NCERT lists lepidopterans (like tobacco budworm and armyworm), coleopterans (beetles), and dipterans (flies and mosquitoes).
