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A farmer grew two varieties… (Class 12 Biology 2026, Q27)

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BiologyCBSE Class 12 · Board exam 2026, set 57/1/1 · 3 marks · Short answer

CBSE Class 12 Biology 2026: question 27

Set in the CBSE Class 12 board exam of 2026, paper 57/1/1, section C, question 27, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27 · 3 marks · Short answerCompetency-basedBiotechnology and its Applications

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.

  1. (a)

    Name the gene in the spores responsible for control of pest.

    0 marks
  2. (b)

    What effect will the spores of Bt have on insect pest ?

    0 marks
  3. (c)

    How has corn plants of field B developed resistance against this pest ? Explain.

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Different cry genes act on different pests. Which one acts on corn borer?

Hint 2

The toxin is inactive inside the bacterium. What condition in the insect gut activates it?

Hint 3

For field B, think about where the cry gene now sits.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (a) The corn borer is controlled by the protein encoded by the gene *cry*IAb.

    ½ mark

  2. (b) The Bt spores, sprayed with water on field A, contain inactive protoxin crystals. These crystals are eaten by the insect larvae along with the leaves.

    ½ mark

  3. In the alkaline pH of the insect gut, the crystals dissolve and the inactive protoxin is converted into the active toxin.

    ½ mark

  4. The active toxin binds to the midgut epithelial cells and creates pores. This causes swelling and lysis of the cells, and the insect dies.

    ½ mark

  5. (c) The corn in field B is genetically modified: the cry gene from Bacillus thuringiensis has been introduced into it. These Bt corn plants produce the Bt toxin themselves, so a corn borer feeding on them takes in the toxin and is killed.

    1 mark

Answer: (a) cry IAb; (b) larvae eat the spores, the alkaline gut activates the toxin, it makes pores in the gut cells and the insect dies; (c) Bt corn carries the cry gene and produces its own Bt toxin, which kills the borer

Where the marks are

  • Names cry IAb (½ mark)
  • Spores or crystals ingested by larvae (½ mark)
  • Alkaline gut activates the protoxin (½ mark)
  • Toxin creates pores in midgut cells, so the insect dies (½ mark)
  • GM corn expresses the Bt cry gene and produces the toxin that kills the pest (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Biotechnology and its Applications, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 57/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.