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(i) Draw a schematic diagram… (Class 12 Biology 2024, Q31a)

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BiologyCBSE Class 12 · Board exam 2024, set 57/5/1 · 5 marks · Long answer

CBSE Class 12 Biology 2024: question 31(a)

Set in the CBSE Class 12 board exam of 2024, paper 57/5/1, section E, question 31(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(a) · 5 marks · Long answerBiotechnology: Principles and Processes

(i) Draw a schematic diagram of the cloning vector pBR 322 and label (1) Bam HI site (2) gene for ampicillin resistance (3) 'ori' (4) 'rop' gene.
(ii) State the role of 'rop' gene.
(iii) A cloning vector does not have a selectable marker. How will it affect the process of cloning ?
(iv) Why is insertional inactivation preferred over the use of selectable markers in cloning vectors ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/5/1.

Hints

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

Hint 1

pBR322 carries two antibiotic-resistance genes. Which one contains the BamHI site?

Hint 2

What is the job of a selectable marker?

Hint 3

Compare plating on two antibiotic plates with a simple colour test using lacZ.

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. (i) Diagram (described): Draw a circular double-stranded plasmid labelled pBR322 (4361 bp). Label (1) the BamHI site within the tet^R gene; (2) the amp^R gene (ampicillin resistance) as a separate segment; (3) ori, the origin of replication; and (4) the rop gene placed next to ori. Other sites such as EcoRI, HindIII, PstI, PvuI, PvuII and ClaI may also be shown.

    2 marks

  2. (ii) The rop gene codes for the proteins involved in the replication of the plasmid, which control its copy number.

    1 mark

  3. (iii) Without a selectable marker it would not be possible to differentiate transformants from non-transformants (or recombinants from non-recombinants). Identifying cells carrying the vector would become very difficult.

    1 mark

  4. (iv) Selection by antibiotic markers requires simultaneous plating on two plates with different antibiotics, which is cumbersome. Insertional inactivation, e.g. inserting the DNA into the lacZ gene coding for β-galactosidase, lets recombinants be identified in one step by colour (blue–white screening): recombinant colonies are white and non-recombinant colonies are blue in the presence of a chromogenic substrate.

    1 mark

Answer: (i) pBR322 with BamHI (in tet^R), amp^R, ori and rop labelled. (ii) rop codes for proteins for plasmid replication. (iii) Transformants cannot be distinguished from non-transformants. (iv) Insertional inactivation gives easy one-step colour (blue–white) selection instead of cumbersome plating on two antibiotic plates.

Where the marks are

  • Four correct labels on the pBR322 diagram (½ mark each)
  • rop codes for proteins involved in plasmid replication (1 mark)
  • Transformants or recombinants cannot be identified without a marker (1 mark)
  • Insertional inactivation allows simple one-step blue–white selection (1 mark; the scheme awards it if attempted)

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 57/5/1: 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.