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5' – G↓AATTC – 3' 3'… (Class 12 Biology 2024, Q18)

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BiologyCBSE Class 12 · Board exam 2024, set 57/5/1 · 2 marks · Very short answer

CBSE Class 12 Biology 2024: question 18

Set in the CBSE Class 12 board exam of 2024, paper 57/5/1, section B, question 18, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q18 · 2 marks · Very short answerBiotechnology: Principles and Processes

5' – G↓AATTC – 3'
3' – CTTAA↑G – 5'
(the arrows mark a cut on each strand: after G on the top strand and before G on the bottom strand)

  1. (a)

    Name the restriction enzyme that recognises the given specific sequence of bases. What are such sequence of bases referred to as ?

    1 mark
  2. (b)

    What are the arrows in the given figure indicating ? Write the result obtained thereafter.

    1 mark

Figure in the paper: Double-stranded sequence 5'-GAATTC-3' / 3'-CTTAAG-5' with a downward arrow after G on the top strand and an upward arrow between A and G on the bottom strand. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/5/1.

Hints

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

Hint 1

GAATTC is the classic recognition site of an enzyme isolated from E. coli RY13.

Hint 2

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

Hint 3

Because the cuts on the two strands are staggered, what kind of ends are left behind?

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 enzyme that recognises 5'-GAATTC-3' is the restriction endonuclease EcoRI.

    ½ mark

  2. (a) Such sequences read the same on both strands in the 5'→3' direction. They are called palindromic nucleotide sequences (palindromes).

    ½ mark

  3. (b) The arrows mark the restriction (recognition) sites, i.e. the exact points where EcoRI cuts the sugar-phosphate backbone of each strand, between G and A.

    ½ mark

  4. (b) The cuts are slightly away from the centre of the palindrome. This leaves single-stranded overhanging stretches called sticky ends, which can join complementary ends with the help of DNA ligase.

    ½ mark

Answer: (a) EcoRI; palindromic nucleotide sequence. (b) The cutting (restriction) sites of EcoRI on the two strands; cutting produces sticky ends.

Where the marks are

  • Names EcoRI (½ mark)
  • Calls the sequence a palindrome or palindromic sequence (½ mark)
  • Arrows show the cutting or restriction sites of the enzyme (½ mark)
  • Result: sticky ends are formed (½ 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 paper2024 board paper, set 57/5/1: 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.