Skip to content

(I) Complete the… (Class 12 Chemistry SQP 2026-27, Q19)

questionbuddy.in/q/cbse-12-chemistry-2026-27-sqp-q19-complete-sequence-nitrogenous-bases-dna

ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 2 marks · Very short answer

CBSE Class 12 Chemistry sample paper 2026-27: question 19

From the official CBSE sample question paper for 2026-27, section B, question 19, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q19 · 2 marks · Very short answerCompetency-basedBiomolecules

(I) Complete the sequence of nitrogenous bases in the given DNA strand (Figure 2) (1)
(II) Milk contains a protein called casein. How does the structure of casein change on addition of a few drops of lemon juice to milk? (1)

Figure in the paper: Figure 2: DNA double helix with 5' and 3' ends marked; base pairs partly shown with unknown bases '?' paired with C (triple bond ?≡C), A (A=?), and '?=A', A=... ; student must complete the missing complementary bases. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Count the hydrogen bonds shown between each pair.

Hint 2

A triple bond means the G–C pair; a double bond means the A–T pair.

Hint 3

For (II), think about how a change in pH affects the 2° and 3° structure of a protein.

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. (I) In DNA, G pairs with C through three hydrogen bonds and A pairs with T through two hydrogen bonds. So the missing bases are:

    • ?≡C → G (guanine)
    • A=? → T (thymine)
    • ?=A → T (thymine)
    • A=… → T

    The complementary strand runs antiparallel (3'→5').

    1 mark

  2. (II) Lemon juice (citric acid) lowers the pH of milk. This breaks the hydrogen bonds that hold the secondary and tertiary structure of casein. The globular casein unfolds and coagulates, i.e. it is denatured. The primary structure stays intact, and the milk curdles.

    1 mark

Answer: (I) G opposite C; T opposite each A; (II) Casein is denatured: its H-bonds break, the globules unfold and the milk curdles.

Where the marks are

  • Correct complementary bases using A=T and G≡C pairing (1 mark)
  • States that the lowered pH breaks H-bonds, causing unfolding/denaturation of casein (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 paperSample paper 2026-27: 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.