Skip to content

Why does the… (Class 12 Biology SQP 2025-26, Q20B)

questionbuddy.in/q/cbse-12-biology-2025-26-sqp-q20b-does-insertional-inactivation-method-detect

BiologyCBSE Class 12 · Sample paper, session 2025-26 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2025-26: question 20(B)

From the official CBSE sample question paper for 2025-26, section B, question 20(B), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20(B) · 2 marks · Very short answerBiotechnology: Principles and Processes

Why does the 'insertional inactivation' method to detect recombinant DNA is preferred to 'antibiotic resistance' procedure?

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

How many plates does the antibiotic method need?

Hint 2

Think of blue-white screening with a chromogenic substrate.

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. Selecting recombinants by inactivating an antibiotic resistance gene is cumbersome. It needs simultaneous replica plating on two plates with different antibiotics (e.g., ampicillin and tetracycline).

    1 mark

  2. In insertional inactivation, the foreign DNA is inserted within the coding sequence of β-galactosidase, which inactivates the enzyme. On a medium with a chromogenic substrate, non-recombinants give blue colonies, while recombinants give colourless/white colonies. Recombinants are thus identified on a single plate.

    1 mark

Answer: Antibiotic-based selection needs two plates (cumbersome); insertional inactivation of β-galactosidase gives blue (non-recombinant) vs colourless (recombinant) colonies on one plate.

Where the marks are

  • States antibiotic resistance method is cumbersome due to plating on two antibiotic plates (1 mark)
  • Explains β-galactosidase inactivation and blue/colourless colony selection (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 paperSample paper 2025-26: 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.