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The image below shows… (Class 12 Biology SQP 2023-24, Q20)

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BiologyCBSE Class 12 · Sample paper, session 2023-24 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2023-24: question 20

From the official CBSE sample question paper for 2023-24, section B, question 20, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20 · 2 marks · Very short answerCompetency-basedBiotechnology: Principles and Processes

The image below shows the result of plating bacteria in chromogenic medium after incorporating the gene of interest in plasmid. Some plates had blue colonies; some plates had white colonies. A single bacterium extracted from Plate I,II,III is shown below:

On the basis of your observations

  1. a)

    Identify the plate(s) which is/are white. Give a reason.

    0 marks
  2. b)

    Identify the plate(s) which is/are blue. Give a reason.

    0 marks

Figure in the paper: Bacterium + plasmid + gene of interest. Bacteria from plate I contains a plasmid with the gene of interest inserted (green bar on plasmid); bacteria from plate II contains a plasmid without the insert; bacteria from plate III contains no plasmid. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Which enzyme produces the blue colour with the chromogenic substrate?

Hint 2

What happens to that enzyme's gene when foreign DNA is inserted into it?

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) Plate I has white colonies. The gene of interest is inserted inside the lacZ (β-galactosidase) gene of the plasmid. This causes insertional inactivation, so no β-galactosidase is made. The chromogenic substrate is not acted on and the colonies stay white (these are the recombinants).

    1 mark

  2. b) Plate II has blue colonies. The plasmid has no insert, so its lacZ gene is intact. β-galactosidase is made and converts the chromogenic substrate into a blue product (non-recombinants).

    ½ mark

  3. Plate III also has blue colonies. The bacterium has no plasmid (no foreign DNA), so its own β-galactosidase activity is unaffected and the colonies appear blue.

    ½ mark

Answer: a) Plate I is white: insertional inactivation of β-galactosidase. b) Plates II and III are blue: plasmid without insert / no plasmid, so β-galactosidase is active.

Where the marks are

  • Plate I white because the insert inactivates the β-galactosidase gene (1 mark)
  • Plate II blue because the plasmid lacks the insert (0.5 mark)
  • Plate III blue because there is no plasmid (0.5 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 paperSample paper 2023-24: 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.