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If the base adenine… (Class 12 Biology 2024, Q21)

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BiologyCBSE Class 12 · Board exam 2024, set 57/5/1 · 2 marks · Very short answer

CBSE Class 12 Biology 2024: question 21

Set in the CBSE Class 12 board exam of 2024, paper 57/5/1, section B, question 21, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21 · 2 marks · Very short answerMolecular Basis of Inheritance

If the base adenine constitutes 31% of an isolated DNA fragment, then write what will be the expected percentage of the base cytosine in it. Explain how did you arrive at the answer given.

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

In double-stranded DNA, which base always pairs with adenine?

Hint 2

The four bases together make 100%. Find what is left after A and T.

Hint 3

The remainder is shared equally between G and C.

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. By Chargaff's rule for double-stranded DNA, A = T and G = C, i.e. A/T = G/C = 1 and A + G = C + T.

    ½ mark

  2. Since A = 31%, T = 31%. So A + T = 31% + 31% = 62%.

    ½ mark

  3. G + C = 100% − 62% = 38%.

    ½ mark

  4. Since G = C, cytosine = = 19%.

    ½ mark

Answer: Cytosine = 19%

Where the marks are

  • States Chargaff's rule A = T and G = C (½ mark)
  • T = 31%, so A + T = 62% (½ mark)
  • G + C = 38% (½ mark)
  • C = 19% (½ 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 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.