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A short piece of DNA… (Class 12 Biology SQP 2023-24, Q3)

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BiologyCBSE Class 12 · Sample paper, session 2023-24 · 1 mark · MCQ

CBSE Class 12 Biology sample paper 2023-24: question 3

From the official CBSE sample question paper for 2023-24, section A, question 3, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q3 · 1 mark · MCQCompetency-basedMolecular Basis of Inheritance

A short piece of DNA, having 20 base pairs, was analyzed to find the number of nucleotide bases in each of the polynucleotide strands. Some of the results are shown in the table.

AdenineCytosineGuanineThymine
Strand 144
Strand 25

(Column heading: Number of nucleotide bases)

How many nucleotides containing Adenine were present in strand 2?

  1. (A)2
  2. (B)4
  3. (C)5
  4. (D)7

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

Each strand of a 20 bp DNA has 20 nucleotides.

Hint 2

A in strand 2 equals T in strand 1.

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. Each strand has 20 nucleotides. In strand 1, C = 4, and C in strand 2 is 5, so G in strand 1 must be 5 (G pairs with C of strand 2). Then T in strand 1 = 20 − (4 + 4 + 5) = 7.

    ½ mark

  2. T in strand 1 pairs with A in strand 2, so A in strand 2 = 7.

    ½ mark

Answer: 7, option (D)

Where the marks are

  • Uses complementary base pairing to find T in strand 1 = 7 (0.5 mark)
  • Concludes A in strand 2 = 7 (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.