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(a) Why does DNA replication… (Class 12 Biology 2024, Q26)

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BiologyCBSE Class 12 · Board exam 2024, set 57/5/1 · 3 marks · Short answer

CBSE Class 12 Biology 2024: question 26

Set in the CBSE Class 12 board exam of 2024, paper 57/5/1, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerMolecular Basis of Inheritance

(a) Why does DNA replication occur within a replication fork and not in its entire length simultaneously ?
(b) "DNA replication is continuous and discontinuous on the two strands within the replication fork." Explain with the help of a schematic representation.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/5/1.

Hints

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

Hint 1

Consider the energy needed to separate two very long strands completely.

Hint 2

In which direction can DNA polymerase add nucleotides?

Hint 3

Remember that the two template strands are antiparallel.

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) Replication is an energetically very expensive process. The two strands of the long DNA cannot be separated along their entire length at once, because of the very high energy needed. So replication proceeds in a small opening of the helix called the replication fork.

    1 mark

  2. (b) DNA-dependent DNA polymerase can polymerise nucleotides only in the 5'→3' direction.

    ½ mark

  3. On the template strand with 3'→5' polarity, the new strand (the leading strand) is synthesised continuously, towards the fork.

    ½ mark

  4. On the template strand with 5'→3' polarity, the new strand (the lagging strand) is synthesised discontinuously as short Okazaki fragments, away from the fork. These fragments are later joined by DNA ligase.

    ½ mark

  5. Schematic (described): Draw a Y-shaped replication fork with the parental strands labelled 3'→5' and 5'→3'. Along the 3'→5' template, draw one continuous arrow (5'→3') moving into the fork, labelled 'leading strand – continuous'. Along the 5'→3' template, draw several short arrows, each 5'→3', pointing away from the fork, labelled 'lagging strand – Okazaki fragments (discontinuous)'. The polarity must be shown correctly.

    ½ mark

Answer: (a) Unwinding the whole DNA at once would need very high energy, so replication occurs in a small fork. (b) Polymerase works only 5'→3', so the strand on the 3'→5' template is made continuously and the strand on the 5'→3' template is made discontinuously (Okazaki fragments, joined by ligase).

Where the marks are

  • States that replication is energetically very expensive, hence the fork (1 mark)
  • Polymerase acts only 5'→3' (½ mark)
  • Continuous synthesis on the 3'→5' template (½ mark)
  • Discontinuous synthesis on the 5'→3' template (½ mark)
  • Correctly labelled schematic with polarity shown (½ mark; no marks if polarity is wrong)

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 paper2024 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.