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A culture plate of… (Class 12 Biology SQP 2024-25, Q20)

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BiologyCBSE Class 12 · Sample paper, session 2024-25 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2024-25: question 20

From the official CBSE sample question paper for 2024-25, 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 answerBiotechnology: Principles and Processes

A culture plate of Lactobacillus shows blue-coloured colonies and colourless colonies. Explain the principle involved in the formation of such variance in the colour of colonies.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

This is a selection method that uses an enzyme gene in the vector.

Hint 2

What happens to an enzyme's gene when foreign DNA is inserted inside it?

Hint 3

A chromogenic substrate gives colour only if the enzyme is active.

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. The difference in colony colour is due to the principle of insertional inactivation (blue–white selection).

    ½ mark

  2. The foreign DNA is inserted within the coding sequence of the enzyme β-galactosidase in the vector, and this inactivates the enzyme.

    ½ mark

  3. The medium contains a chromogenic substrate. Bacteria whose plasmid has no insert have an active β-galactosidase, which acts on the substrate and produces blue colonies (non-recombinants).

    ½ mark

  4. Bacteria with an insert have an inactivated β-galactosidase, cannot act on the substrate and form colourless (white) colonies. These are the recombinant colonies.

    ½ mark

Answer: Insertional inactivation of β-galactosidase: blue colonies are non-recombinants; colourless colonies are recombinants.

Where the marks are

  • Names insertional inactivation (0.5 mark)
  • Explains that the insert in the β-galactosidase coding sequence inactivates the enzyme (0.5 mark)
  • Blue colonies with chromogenic substrate when there is no insert (0.5 mark)
  • Colourless colonies are recombinants (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 2024-25: 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.