Skip to content

The diagram below… (Class 12 Biology SQP 2023-24, Q18)

questionbuddy.in/q/cbse-12-biology-2023-24-sqp-q18-diagram-shows-sequence-amino-acids

BiologyCBSE Class 12 · Sample paper, session 2023-24 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2023-24: question 18

From the official CBSE sample question paper for 2023-24, section B, question 18, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q18 · 2 marks · Very short answerCompetency-basedMolecular Basis of Inheritance

The diagram below shows the sequence of amino acids in part of a haemoglobin molecule.

  1. a)

    If the base T* was substituted with A, how would it affect the haemoglobin chain?

    0 marks
  2. b)

    Name the condition and the effects associated with the above substitution.

    0 marks

Figure in the paper: Haemoglobin chain: Val, His, Leu, Thr, Pro, Glu, Glu; below it the mRNA, below that DNA triplets CAT, GTA, AAT, TGA, GGA, CTT (T in the middle marked with * ), CTC. Key: Val=valine, His=histidine, Leu=leucine, Thr=threonine, Pro=proline, Glu=glutamic acid. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Write the new DNA triplet after the T is swapped for A.

Hint 2

Convert the new template triplet into its mRNA codon and look up the amino acid.

Hint 3

Which well-known disease is caused by Glu being replaced by Val in β-globin?

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 DNA triplet for glutamic acid is CTT, which gives the mRNA codon GAA. If the middle T* is replaced by A, the triplet becomes CAT. The mRNA codon then becomes GUA, which codes for valine. So in this chain, valine replaces glutamic acid (Glu → Val) at that position.

    ½ mark

  2. b) The condition is sickle-cell anaemia, caused by this point mutation (single base substitution) in the β-globin gene.

    ½ mark

  3. Effect: the mutant haemoglobin (HbS) polymerises under low oxygen tension.

    ½ mark

  4. This changes the RBC from a biconcave disc into an elongated, sickle-shaped cell.

    ½ mark

Answer: a) CTT → CAT (mRNA GAA → GUA), so glutamic acid is replaced by valine. b) Sickle-cell anaemia: mutant Hb polymerises under low O₂ and the RBCs become sickle-shaped.

Where the marks are

  • States the triplet becomes CAT/GUA and valine replaces glutamic acid (0.5 mark)
  • Names sickle-cell anaemia (0.5 mark)
  • Mutant haemoglobin polymerises (0.5 mark)
  • RBC changes from biconcave disc to sickle shape (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 2023-24: 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.