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The two strands in DNA are… (Class 12 Chemistry 2025, Q21)

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ChemistryCBSE Class 12 · Board exam 2025, set 56/7/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2025: question 21

Set in the CBSE Class 12 board exam of 2025, paper 56/7/1, 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 answerBiomolecules

The two strands in DNA are not identical but complementary. Explain. What products would be formed when DNA is hydrolysed ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 56/7/1.

Hints

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

Hint 1

Which base always pairs with adenine, and which with guanine?

Hint 2

A nucleotide is made of three parts; hydrolysis gives all three back.

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. Complementary strands: the two strands are held together by hydrogen bonds formed only between specific base pairs. Adenine pairs with thymine (two H-bonds) and guanine pairs with cytosine (three H-bonds). The base sequence of one strand therefore fixes the sequence of the other. The strands are complementary, not identical.

    1 mark

  2. Products of hydrolysis of DNA: 2-deoxy-D-ribose (sugar), nitrogenous bases (adenine, guanine, cytosine, thymine) and phosphoric acid.

    1 mark

Answer: Specific base pairing (A–T, G–C) makes the strands complementary; hydrolysis gives 2-deoxyribose, nitrogenous bases (A, G, C, T) and phosphoric acid.

Where the marks are

  • H-bonds form between specific base pairs (A–T, G–C) (1 mark)
  • Products: 2-deoxyribose, bases and phosphoric acid (1 mark)

Revise the chapter

  • Chapter: Biomolecules (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Biomolecules, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 56/7/1: 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.