Skip to content

Read the following passage… (Class 12 Biology 2026, Q30)

questionbuddy.in/q/cbse-12-biology-2026-57-5-1-q30-know-plasmids-bacteriophages-most-commonly

BiologyCBSE Class 12 · Board exam 2026, set 57/5/1 · 4 marks · Case-based

CBSE Class 12 Biology 2026: question 30

Set in the CBSE Class 12 board exam of 2026, paper 57/5/1, 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

Read the following passage and answer the questions that follow :

We know that plasmids and bacteriophages are the most commonly used vectors in biotechnology experiments. If we can link an alien piece of DNA to the plasmid DNA, the alien DNA can be multiplied equal to the copy number of the plasmid. Engineered vectors are used these days. Study the diagram of the E. coli cloning vector pBR322 and answer the questions that follow :

  1. (a)

    Why are plasmids and bacteriophages used as cloning vectors ?

    1 mark
  2. (b)(i)

    Identify :
    (I) The gene in the cloning vector that controls the copy number of the vector.
    (II) The restriction site – C in the 'rop' gene.

    1 mark

  3. OR

    (b)(ii)

    Identify and name two selectable markers shown in the diagram.

    1 mark
  4. (c)

    Name the two restriction sites each in the two genes you have identified as selectable markers.

    2 marks

Figure in the paper: Circular map of E. coli cloning vector pBR322 with labelled sites: Cla I and Hind III at top, letters F, E, D, A, B, C marking restriction sites/genes around the ring, inner labels G and H (genes), and 'ori' and 'rop' labelled near the bottom. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/5/1.

Hints

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

Hint 1

What property must a vector have so that DNA joined to it is copied many times?

Hint 2

Recall the NCERT pBR322 map: the ori region, the rop gene and the two antibiotic-resistance genes.

Hint 3

Two sites lie in the ampicillin-resistance gene and two in the tetracycline-resistance gene.

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) Plasmids and bacteriophages can replicate within bacterial cells independently of the chromosomal DNA, and an alien piece of DNA can be ligated to them. The linked foreign DNA is therefore multiplied along with them, up to their copy number.

    1 mark

  2. (b)(i) (I) The gene controlling copy number is ori (origin of replication). (II) The restriction site C in the rop gene is Pvu II.

    1 mark

  3. (b)(ii) G is ampR (ampicillin-resistance gene). H is tetR (tetracycline-resistance gene). Both are selectable markers.

    1 mark

  4. (c) Restriction sites in ampR: Pst I and Pvu I. Restriction sites in tetR: BamH I and Sal I.

    2 marks

Answer: (a) They replicate independently in bacteria and can carry linked foreign DNA; (b)(i) ori, Pvu II; (b)(ii) G = ampR, H = tetR; (c) ampR: Pst I, Pvu I; tetR: BamH I, Sal I

Where the marks are

  • (a) Ability to replicate independently within bacteria, carrying ligated alien DNA (1 mark)
  • (b)(i) ori (½ mark); Pvu II (½ mark)
  • (b)(ii) ampR (½ mark); tetR (½ mark)
  • (c) Pst I, Pvu I, BamH I, Sal I (½ mark each = 2 marks)

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 paper2026 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.