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A student placed DNA… (Class 12 Biology SQP 2026-27, Q32B)

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BiologyCBSE Class 12 · Sample paper, session 2026-27 · 5 marks · Long answer

CBSE Class 12 Biology sample paper 2026-27: question 32(B)

From the official CBSE sample question paper for 2026-27, section E, question 32(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(B) · 5 marks · Long answerCompetency-basedBiotechnology: Principles and Processes

A student placed DNA from three different sources: bacteria, plant tissue, and fungal cells in a test tube A, B and C respectively. During the process of isolating and purifying DNA, the student added cellulase to all test tubes.

  1. I.

    Predict and explain in which trial(s) DNA isolation would fail, and why? Also suggest how it could be corrected.

    2 marks
  2. II.

    After rectifying the mistake made in 'a', outline the steps to obtain visible spooled DNA from the sample.

    2 marks
  3. III.

    Suggest one application of the spooled DNA in biotechnology.

    1 mark

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Think about what each organism's cell wall is made of.

Hint 2

Cellulase works only on cellulose; recall which enzymes break peptidoglycan and chitin.

Hint 3

After lysis, remove RNA and proteins with specific enzymes, then use chilled ethanol to precipitate the DNA.

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. I. Prediction: Isolation will fail in test tube A (bacteria) and test tube C (fungus), and will succeed in test tube B (plant). Cellulase can only digest cellulose, which is found in plant cell walls. The bacterial wall is made of peptidoglycan and the fungal wall is made of chitin, so cellulase cannot break these cells open and their DNA is never released.

    1 mark

  2. Correction: Treat each sample with the enzyme that suits its wall: lysozyme for bacteria (tube A), cellulase for plant tissue (tube B) and chitinase for fungi (tube C).

    1 mark

  3. II. Steps to obtain spooled DNA:
    1. Once the wall is digested, the cells break open. DNA is released along with RNA, proteins, polysaccharides and lipids.
    2. Add ribonuclease to digest the RNA and protease to digest the proteins. Remove polysaccharides and lipids by suitable treatments.

    1 mark

  4. 3. Add chilled ethanol to the purified solution. The DNA precipitates as fine white threads, which can be wound (spooled) around a glass rod.

    1 mark

  5. III. Application: The spooled DNA can be cut with restriction enzymes and joined to a vector to make recombinant DNA, which is genetic engineering. It can also be used for PCR amplification, DNA fingerprinting or sequencing. (Any one.)

    1 mark

Answer: I. Fails in A (bacteria, peptidoglycan wall) and C (fungus, chitin wall); succeeds in B (plant). Correct by using lysozyme for A and chitinase for C (cellulase for B). II. Lyse cells → RNase + protease treatment → remove polysaccharides/lipids → add chilled ethanol → spool white DNA threads. III. Recombinant DNA/genetic engineering (or PCR, DNA fingerprinting, sequencing).

Where the marks are

  • Identifies failure in A and C and gives the reason based on wall composition (1 mark)
  • Suggests the correct enzymes: lysozyme, cellulase and chitinase (1 mark)
  • Cell lysis followed by RNase and protease treatment and removal of other contaminants (1 mark)
  • Precipitation with chilled ethanol and spooling of DNA threads (1 mark)
  • Any one valid biotechnological application (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Biotechnology: Principles and Processes, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: 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.