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Study the diagram given below… (Class 12 Biology 2024, Q30)

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BiologyCBSE Class 12 · Board exam 2024, set 57/1/1 · 4 marks · Case-based

CBSE Class 12 Biology 2024: question 30

Set in the CBSE Class 12 board exam of 2024, paper 57/1/1, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedBiotechnology: Principles and Processes

Study the diagram given below that shows the steps involved in the procedure of selecting transformed bacteria and answer the questions that follow :

  1. (a)

    Identify the colony that has got transformed. Justify your answer.

    1 mark
  2. (b)

    What are the sites in a plasmid called where ampicillin and tetracycline resistance genes are inserted ? State their role in genetic engineering.

    2 marks
  3. (c)

    Name two enzymes playing an important role in genetic engineering.

    1 mark

  4. OR

    (c)

    State the role of β-galactosidase in insertional inactivation.

    1 mark

Figure in the paper: (i) Plasmid with gene for ampicillin resistance and gene for tetracycline resistance. (ii) Plasmid with recombinant DNA inserted. (iii) Plate M: colonies derived from single bacteria growing on agar, numbered 1 to 5 (colony 3 is oval/different). Replica plating arrow to (iv) Plate N: agar with tetracycline and ampicillin, showing growth of colonies 1, 2, 4(?) and 5 positions but an oval colony 3 position shown differently; colonies 1, 2, 4, 5 grow on Plate N except one position. See it in the official paper.

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

Hints

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

Hint 1

Compare plates M and N: which colony is missing on the plate containing tetracycline?

Hint 2

Where was the foreign DNA inserted, and what happens to a gene when DNA is inserted into it?

Hint 3

For enzymes, think of 'molecular scissors' and 'molecular glue'.

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) Colony 4 is the transformed colony with recombinant DNA. It grows on ampicillin (plate M) but fails to grow on the tetracycline-containing plate N.

    ½ mark

  2. (a) Justification: the foreign DNA was inserted within the tetracycline resistance gene. This inactivated the gene (insertional inactivation), so recombinants lose tetracycline resistance but keep ampicillin resistance.

    ½ mark

  3. (b) The ampicillin and tetracycline resistance genes are selectable markers. The sites within them where foreign DNA is inserted are restriction (recognition) sites, e.g. BamHI in tet and PstI in amp of pBR322.

    1 mark

  4. (b) Role: selectable markers help identify and select transformants from non-transformants. Insertion of foreign DNA into one marker inactivates it, which helps distinguish recombinants from non-recombinants.

    1 mark

  5. (c) [First alternative] Two enzymes: restriction endonuclease (cuts DNA at specific sites) and DNA ligase (joins DNA fragments). Taq DNA polymerase, used in PCR, is also important.

    1 mark

  6. (c) [Second alternative] The recombinant DNA is inserted within the coding sequence of β-galactosidase, which inactivates it. With a chromogenic substrate, non-recombinant colonies with active enzyme appear blue, while recombinant colonies appear colourless (white). This identifies recombinants.

    1 mark

Answer: (a) Colony 4 – grows on ampicillin but not tetracycline due to insertional inactivation of tet^R; (b) selectable markers (restriction sites within them) – help select transformants/recombinants; (c) restriction endonuclease and DNA ligase / inactivated β-galactosidase gives colourless recombinant colonies

Where the marks are

  • Identifies colony 4 (½ mark)
  • Justification: no resistance to tetracycline because of insertion (½ mark)
  • Selectable markers / restriction sites, with their role in selecting transformants (2 marks)
  • Restriction endonuclease and ligase (1 mark) OR colourless colonies when β-galactosidase is inactivated (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 paper2024 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.