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A segment of DNA, 'TTG AGG… (Class 12 Biology 2025, Q24)

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BiologyCBSE Class 12 · Board exam 2025, set 57/7/1 · 3 marks · Short answer

CBSE Class 12 Biology 2025: question 24

Set in the CBSE Class 12 board exam of 2025, paper 57/7/1, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerMolecular Basis of Inheritance

A segment of DNA, 'TTG AGG GGG ATG' was translated into an oligopeptide with the amino acids, Lysine – Serine – Proline – Tyrosine.
(a) Write the codons in correct sequence for the four amino acids.
(b) If first adenine in DNA is substituted by guanine, what will be the sequence of amino acids in the new oligopeptide ?
(c) Write the anticodons for these amino acids.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/7/1.

Hints

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

Hint 1

Codons are complementary to the template DNA, with U in place of T.

Hint 2

Find the first A in the DNA sequence, change it to G, and rewrite that codon. Anticodons pair with codons.

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) The given segment is the template strand, so the codons are its complementary bases (T is replaced by A, and A is replaced by U):

    DNA: TTG AGG GGG ATG

    mRNA codons: AAC – UCC – CCC – UAC, for Lysine – Serine – Proline – Tyrosine.

    1 mark

  2. (b) When the first adenine (in AGG) becomes guanine, the DNA reads TTG GGG GGG ATG. The mRNA then reads AAC CCC CCC UAC, so the second codon changes from UCC (Ser) to CCC (Pro).

    New oligopeptide: Lysine – Proline – Proline – Tyrosine.

    1 mark

  3. (c) Anticodons on the tRNAs are complementary to the codons:

    Codons: AAC UCC CCC UAC → anticodons UUG AGG GGG AUG

    For the mutated peptide in (b), the anticodons are UUG GGG GGG AUG.

    1 mark

Answer: (a) AAC UCC CCC UAC; (b) Lysine–Proline–Proline–Tyrosine; (c) UUG AGG GGG AUG (UUG GGG GGG AUG for the mutant)

Where the marks are

  • Correct codons AAC UCC CCC UAC (1 mark)
  • Lys–Pro–Pro–Tyr after the substitution (1 mark)
  • Anticodons UUG AGG GGG AUG (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 paper2025 board paper, set 57/7/1: 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.