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Draw a labelled diagram of a… (Class 12 Biology 2026, Q23)

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BiologyCBSE Class 12 · Board exam 2026, set 57/5/1 · 3 marks · Short answer

CBSE Class 12 Biology 2026: question 23

Set in the CBSE Class 12 board exam of 2026, paper 57/5/1, section C, question 23, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q23 · 3 marks · Short answerMolecular Basis of Inheritance

Draw a labelled diagram of a 'replicating fork' showing the polarity. Why does DNA replication occur within such 'fork' ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/5/1.

Hints

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

Hint 1

DNA polymerase works only in the 5'→3' direction. Decide which new strand can be made continuously and which must be made in pieces.

Hint 2

For the reason, think about how long DNA is and what it costs to break all the hydrogen bonds at once.

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. Diagram (described): Draw a Y-shaped replication fork. The parental double helix (one strand 5'→3', the other 3'→5') opens at the fork point (helicase site).

    • On the template with 3'→5' polarity, the new leading strand is drawn as one continuous arrow, synthesised 5'→3' towards the fork.
    • On the template with 5'→3' polarity, the new lagging strand is drawn as short separate pieces (Okazaki fragments), each synthesised 5'→3' away from the fork and joined by DNA ligase.
    • Label the 5' and 3' ends of all four strands, mark DNA polymerase, and show the direction of replication with arrows.
    2 marks

  2. DNA is a very long molecule. Separating its two strands along the entire length at once would need a very high amount of energy. Replication therefore occurs within a small opening of the helix, the replication fork, where the strands unwind only a little at a time.

    1 mark

Answer: Labelled fork showing 3'→5' and 5'→3' templates, continuous leading strand, discontinuous lagging strand (Okazaki fragments). A fork is used because separating the full length of the long DNA molecule at once needs very high energy.

Where the marks are

  • Correct fork diagram showing template polarity, continuous leading strand and discontinuous lagging strand with labels (2 marks)
  • Reason: unwinding the full length of the long DNA needs too much energy, so replication proceeds in a small fork (1 mark)

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 57/5/1: 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.