Biology · Biotechnology and Its Applications · NEET
Two genes: cryIAc and cryIIAb. Both of these code for Bt toxin proteins that kill the cotton bollworm. This is directly from NCERT: 'the proteins encoded by the genes cryIAc and cryIIAb control the cotton bollworms.' In matching questions, both Cry1Ac and Cry2Ab must be linked to bollworm.
The gene cryIAb controls the corn borer. Be careful: cryIAb (with a 'b') is corn borer, but cryIAc (with a 'c') is cotton bollworm. Only one letter changes, so read the last letter slowly in the exam.
They differ by the final letter, and this changes the target pest. cryIAc controls cotton bollworms. cryIAb controls corn borer. A simple trick: c in cryIAc = Cotton bollworm; b in cryIAb = Borer (corn borer).
Because most Bt toxins are insect-group specific. Each crop has different target pests, so a different cry gene is chosen for each pest. NCERT says 'the choice of genes depends upon the crop and the targeted pest.' That is why cotton uses cryIAc/cryIIAb and corn uses cryIAb.
The gene is named cry because the Bt toxin is stored as inactive crystal (Cry) protein inside the bacterium. Cry = crystal protein gene. It stays as an inactive protoxin until an insect eats it and the alkaline pH of the insect gut activates it.
The toxin proteins isolated from Bacillus thuringiensis, coded by which of the following genes would control cotton bollworms and corn borer, respectively?
Match List I with List II. List I: A. α-1 antitrypsin, B. Cry 1Ab, C. Cry 1Ac, D. Enzyme replacement therapy. List II: I. Cotton bollworm, II. ADA deficiency, III. Emphysema, IV. Corn borer.
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; after ingestion the inactive protoxin gets converted into active form due to acidic pH of the insect gut. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Cry genes code for the Bt toxin, a crystal (Cry) protein produced by Bacillus thuringiensis. This protein kills specific insect pests when they eat it, so cry genes are used to make pest-resistant crops like Bt cotton.
No. The cry gene is the DNA that carries the code. The Bt toxin is the protein made from that gene. Different cry genes make slightly different toxin proteins that target different pests.
cryIAc and cryIIAb are both linked to controlling cotton bollworms in NCERT. cryIAc is the most commonly asked one for Bt cotton in NEET.
The toxin is eaten as an inactive protoxin (crystal). The alkaline pH of the insect gut dissolves the crystal and converts it into the active toxin, which then makes pores in gut cells and kills the insect.
Inside the bacterium the toxin stays as an inactive crystalline protoxin. It only becomes active in the alkaline gut of an insect, so it does not damage the bacterium that makes it.
