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A short stretch of… (Class 12 Biology SQP 2026-27, Q31A)

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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 31(A)

From the official CBSE sample question paper for 2026-27, section E, question 31(A), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(A) · 5 marks · Long answerCompetency-basedMolecular Basis of Inheritance

A short stretch of DNA strand that codes for a polypeptide is shown below:
3'—TACAAAAAGAAGAAAAAGAAGATT--5'
Due to exposure to harmful UV radiation two different types of errors occurred during DNA replication affecting their sequences of bases in the DNA in two different cells.
I. What is the term given to a segment of DNA that codes for a polypeptide?
II. Which type of mutation could have occurred in each of the following due to the error during replication?
a) 3'-- TACAAAAAGAAGAAAAAGAAAATT--5'
b) 3'-- TACAAAAAGAAGAAAAAGAGATT--5'
III. If DNA strand (a) codes for Methionine and sequences of Phenylalanine, mention the sequence of amino acids that will constitute the polypeptide chain after translation.
How many amino acids will be translated from this strand?
IV. If all the codons of the stretch code for Phenylalanine excluding the initiator codon which feature does it depict?

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

Compare each mutant strand with the original base by base, and also count the length.

Hint 2

Write the mRNA complementary to the 3'→5' template, split it into triplets and look for the stop codon.

Hint 3

Look at the codons for Phe: are they all the same?

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. A segment of DNA that codes for a polypeptide is called a cistron.

    ½ mark

  2. II(a). Only one base has changed: the 7th codon AAG has become AAA (G replaced by A), and the length is unchanged. This is a point mutation (base substitution).

    1 mark

  3. II(b). The strand has one base fewer than the original (23 instead of 24), because a single base has been deleted. This is a deletion, which causes a frameshift mutation in the reading frame.

    1 mark

  4. III. Template (a): 3'-TAC AAA AAG AAG AAA AAG AAA ATT-5'.
    mRNA: 5'-AUG UUU UUC UUC UUU UUC UUU UAA-3'.
    Polypeptide: Met–Phe–Phe–Phe–Phe–Phe–Phe (UAA is a stop codon). 7 amino acids are translated.

    2 marks

  5. IV. Different codons (UUU and UUC) code for the same amino acid, phenylalanine. This depicts the degeneracy of the genetic code.

    ½ mark

Answer: I. Cistron. II. (a) Point mutation (substitution); (b) deletion/frameshift mutation. III. Met–Phe–Phe–Phe–Phe–Phe–Phe; 7 amino acids. IV. Degeneracy of the code.

Where the marks are

  • Cistron (0.5 mark)
  • (a) Point mutation (1 mark)
  • (b) Deletion/frameshift mutation (1 mark)
  • Met followed by 6 Phe, giving 7 amino acids, with UAA as stop (2 marks)
  • Degenerate code (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 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.