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The schematic… (Class 12 Biology SQP 2024-25, Q18A)

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BiologyCBSE Class 12 · Sample paper, session 2024-25 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2024-25: question 18(A)

From the official CBSE sample question paper for 2024-25, section B, question 18(A), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q18(A) · 2 marks · Very short answerMolecular Basis of Inheritance

The schematic representation given below shows a DNA strand and two types of mutations in the DNA strand.

(i) Identify the type of mutation exhibited in I and II.
(ii) Which of the above mutation is more harmful? Give reason.

Figure in the paper: Three sequences. Original template: A U G C A G A C A U C U U A G with codons Met, Gln, Thr, Ser, Stop. Mutation I: A U G A A G A C A U C U U A G with Met, Lys, Thr, Ser, Stop. Mutation II: A U G A G A C A U C U U A G (one base deleted) with Met, Arg, His, Leu, and no stop shown. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Compare the number of bases in each mutated sequence with the original.

Hint 2

In one sequence a single codon changes; in the other every codon after one point changes.

Hint 3

Think about what a shifted reading frame does to the protein.

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) In I, a single base is substituted (C → A in the second codon, so CAG/Gln becomes AAG/Lys) and the rest is unchanged. This is a point mutation (base substitution).

    ½ mark

  2. (i) In II, one base is deleted, so the reading frame shifts and every codon after it changes (Arg, His, Leu…). This is a frameshift mutation (deletion).

    ½ mark

  3. (ii) Mutation II (frameshift) is more harmful, because it changes all the codons downstream of the deletion, not just one.

    ½ mark

  4. (ii) Reason: many incorrect amino acids are inserted and the stop codon is lost or misread, so the polypeptide length and composition change. This gives a non-functional protein with severe phenotypic effects.

    ½ mark

Answer: (i) I – point (substitution) mutation; II – frameshift (deletion) mutation. (ii) II is more harmful, as it alters all downstream codons and gives a non-functional protein.

Where the marks are

  • Identifies I as point mutation and II as frameshift mutation (0.5 + 0.5 = 1 mark)
  • States II is more harmful (0.5 mark)
  • Gives a valid reason, e.g. many codons altered or premature/absent stop giving a non-functional protein (0.5 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 paperSample paper 2024-25: 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.