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Answer the following. (Class 12 Biology 2026, Q32a)

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BiologyCBSE Class 12 · Board exam 2026, set 57/1/1 · 5 marks · Long answer

CBSE Class 12 Biology 2026: question 32(a)

Set in the CBSE Class 12 board exam of 2026, paper 57/1/1, section E, question 32(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(a) · 5 marks · Long answerMolecular Basis of Inheritance

Answer the following.

  1. (i)

    Watson and Crick's discovery of double helical structure was based on which two findings. Also mention the name of the scientists associated with these findings.

    2 marks
  2. (ii)

    Write down the salient features of double helix structure of DNA (any three points).

    3 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

One finding came from X-ray images of DNA, the other from chemical analysis of base ratios.

Hint 2

For features, think about backbone, polarity, base pairing and the dimensions of the helix.

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) Finding 1: X-ray diffraction data of DNA, produced by Maurice Wilkins and Rosalind Franklin.

    1 mark

  2. Finding 2: Chargaff's rules, by Erwin Chargaff: in double-stranded DNA, the ratios A/T and G/C are constant and equal to 1.

    1 mark

  3. (ii) Feature 1: DNA is made of two polynucleotide chains. The sugar–phosphate backbone lies outside and the nitrogen bases project inwards.

    1 mark

  4. Feature 2: the two chains have antiparallel polarity (5'→3' and 3'→5').

    1 mark

  5. Feature 3: bases on the two strands pair by hydrogen bonds: A pairs with T by two H-bonds, and G pairs with C by three H-bonds. Purines always pair with pyrimidines, keeping the distance between the strands uniform. (Other accepted features: the chains are coiled in a right-handed helix with a pitch of 3.4 nm, about 10 bp per turn and 0.34 nm between base pairs; base-pair planes stack over one another.)

    1 mark

Answer: (i) X-ray diffraction data (Wilkins and Franklin) and the base-equivalence rule A/T = G/C = 1 (Chargaff); (ii) two chains with an outer sugar–phosphate backbone; antiparallel polarity; complementary base pairing A=T, G≡C by H-bonds (also right-handed coil, 3.4 nm pitch, 10 bp per turn)

Where the marks are

  • X-ray diffraction, Wilkins and Franklin (½ + ½ mark)
  • Chargaff's rule A/T = G/C = 1, Erwin Chargaff (½ + ½ mark)
  • Any three salient features of the double helix (1 × 3 marks)

Revise the chapter

Practise this chapterBoard-style questions from Molecular Basis of Inheritance, 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.