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Study the sequence of bases… (Class 12 Biology 2026, Q19)

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BiologyCBSE Class 12 · Board exam 2026, set 57/1/1 · 2 marks · Very short answer

CBSE Class 12 Biology 2026: question 19

Set in the CBSE Class 12 board exam of 2026, paper 57/1/1, 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 answerBiotechnology: Principles and Processes

Study the sequence of bases in a DNA molecule that is shown below and answer the questions.
5' – GAATTC – 3'
3' – CTTAAG – 5'

  1. (a)

    What are such sequences called ? Name the enzyme used that recognises such nucleotide sequences.

    1 mark
  2. (b)

    How do these enzymes function ?

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/1/1.

Hints

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

Hint 1

Read both strands from 5' to 3'. What do you notice?

Hint 2

Molecular scissors that recognise GAATTC were first isolated from E. coli RY13.

Hint 3

Note where the cut falls on each strand and what kind of ends this leaves.

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) These are palindromic nucleotide sequences: they read the same on both strands when each is read in the same direction (5'→3').

    ½ mark

  2. They are recognised by a restriction endonuclease, here EcoRI.

    ½ mark

  3. (b) The restriction endonuclease binds its specific recognition (palindromic) site. It cuts each strand a little away from the centre of the palindrome, between the same two bases on the opposite strands; EcoRI cuts between G and A in 5'-GAATTC-3'. This leaves single-stranded overhanging stretches called sticky ends on each fragment.

    1 mark

Answer: (a) Palindromic sequences; restriction endonuclease (EcoRI); (b) it cuts both strands between G and A, away from the centre of the palindrome, producing sticky ends

Where the marks are

  • Names palindromic sequences (½ mark)
  • Names restriction endonuclease/EcoRI (½ mark)
  • Explains cutting at the specific site between G and A, off-centre, producing sticky ends (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Biotechnology: Principles and Processes, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 57/1/1: 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.