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Answer the following. (Class 12 Biology 2025, Q22)

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BiologyCBSE Class 12 · Board exam 2025, set 57/1/1 · 3 marks · Short answer

CBSE Class 12 Biology 2025: question 22

Set in the CBSE Class 12 board exam of 2025, paper 57/1/1, section C, question 22, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q22 · 3 marks · Short answerBiotechnology: Principles and Processes

Answer the following.

  1. (a)

    Write the palindromic nucleotide sequence for following sequence of DNA segment :
    5' – GAATTC – 3'

    0 marks
  2. (b)

    Name the restriction endonuclease that recognizes this sequence.

    0 marks
  3. (c)

    How are sticky ends produced ? Mention their role.

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/1/1.

Hints

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

Hint 1

Write the complementary base for each base and keep track of the 3' and 5' ends.

Hint 2

This is the enzyme named after E. coli strain RY13.

Hint 3

Does the enzyme cut both strands at exactly the same position? What is left hanging?

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) The complementary strand is – CTTAAG – . Read , both strands give GAATTC, so the sequence is a palindrome.

    ½ mark

  2. (b) This sequence is recognised by EcoRI.

    ½ mark

  3. (c) Production: The restriction enzyme cuts each strand a little away from the centre of the palindrome, but between the same two bases on the opposite strands (between G and A). This leaves short single-stranded overhanging stretches at both ends, called sticky ends.

    1 mark

  4. Role: Sticky ends form hydrogen bonds with their complementary cut counterparts. When vector DNA and foreign DNA are cut with the same enzyme, their sticky ends pair up, and DNA ligase then joins them to form recombinant DNA.

    1 mark

Answer: (a) 3'–CTTAAG–5'; (b) EcoRI; (c) staggered cuts between the same two bases on opposite strands leave overhangs (sticky ends), which pair with complementary ends and help DNA ligase join vector and foreign DNA.

Where the marks are

  • Writes 3'–CTTAAG–5' (½ mark)
  • Names EcoRI (½ mark)
  • Explains staggered cutting that leaves single-stranded overhangs (1 mark)
  • Role: hydrogen bonding with complementary ends, helping ligase join vector and foreign DNA (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 paper2025 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.