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Answer the following. (Class 12 Biology 2023, Q33b)

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BiologyCBSE Class 12 · Board exam 2023, set 57/5/1 · 5 marks · Long answer

CBSE Class 12 Biology 2023: question 33(b)

Set in the CBSE Class 12 board exam of 2023, paper 57/5/1, section E, question 33(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(b) · 5 marks · Long answerBiotechnology: Principles and Processes

Answer the following.

  1. (i)

    ‘EcoRI’ has played very significant role in r-DNA technology.
    (I) Explain the convention for naming EcoRI.
    (II) Write the recognition site and the cleavage sites of this restriction endonuclease.

    3 marks
  2. (ii)

    What are the protruding and hanging stretches of DNA produced by these restriction enzymes called ? Describe their role in formation of r-DNA.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/5/1.

Hints

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

Hint 1

The letters of a restriction enzyme name come from the bacterium it was isolated from, and the Roman numeral has a meaning too.

Hint 2

EcoRI recognises a six-base palindrome that begins with G and ends with C. Find where the cut falls between two of those bases.

Hint 3

The overhangs can pair with complementary overhangs. Which enzyme then seals the joined fragments?

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)(I) Naming convention of EcoRI:

    • E = genus Escherichia
    • co = species coli
    • R = strain RY 13
    • I (Roman numeral) = the order in which the enzyme was isolated from that strain (the first one)
    2 marks

  2. (i)(II) Recognition site: the palindromic sequence

    5′–G A A T T C–3′

    3′–C T T A A G–5′

    ½ mark

  3. Cleavage site: EcoRI cuts between G and A on both strands (5′–G↓AATTC–3′ and 3′–CTTAA↑G–5′). This leaves 5′ AATT overhangs.

    ½ mark

  4. (ii) The protruding, single-stranded hanging stretches are called sticky ends.

    1 mark

  5. Role in r-DNA formation:

    • The vector and the foreign DNA are cut with the same enzyme, so their sticky ends are complementary. They pair with each other through hydrogen bonds.
    • This stickiness holds the fragments together and makes it easier for the enzyme DNA ligase to join them, which forms the recombinant DNA.
    1 mark

Answer: (i)(I) E = Escherichia (genus), co = coli (species), R = strain RY13, I = first enzyme isolated from that strain. (II) Recognition site 5′GAATTC3′ / 3′CTTAAG5′; cut between G and A on both strands. (ii) Sticky ends; they hydrogen-bond with complementary ends and help DNA ligase join the DNA to form r-DNA.

Where the marks are

  • E = genus, co = species, R = strain, I = order of isolation (½ × 4 = 2 marks)
  • Correct palindromic recognition site GAATTC/CTTAAG (½ mark)
  • Cleavage between G and A on both strands (½ mark)
  • Names sticky ends (1 mark)
  • Sticky ends hydrogen-bond with complementary cut ends (½ mark)
  • This facilitates the action of DNA ligase to form r-DNA (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 paper2023 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.