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Explain how PCR… (Class 12 Biology SQP 2024-25, Q27)

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BiologyCBSE Class 12 · Sample paper, session 2024-25 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2024-25: question 27

From the official CBSE sample question paper for 2024-25, section C, question 27, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27 · 3 marks · Short answerBiotechnology: Principles and Processes

Explain how PCR technique can be used for amplification of a small amount of DNA template.

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

Expand PCR and list the ingredients in the reaction tube.

Hint 2

Each cycle involves separating strands, attaching short sequences and building new strands.

Hint 3

Why must the enzyme withstand high temperature?

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. PCR (Polymerase Chain Reaction) makes many copies of a gene or DNA of interest in vitro. It uses two sets of primers (small, chemically synthesised oligonucleotides complementary to the ends of the target region), DNA polymerase, nucleotides and the genomic DNA as template.

    1 mark

  2. Each cycle has three steps:

    • Denaturation: heating separates the double-stranded DNA into single strands.
    • Annealing: the primers bind to their complementary regions on each strand.
    • Extension: DNA polymerase extends the primers using the nucleotides, copying the template.
    1 mark

  3. The cycle is repeated many times, and the DNA doubles each time, so the segment is amplified about a billion times. This is possible because a thermostable DNA polymerase (Taq polymerase, from Thermus aquaticus) stays active through the high temperature of denaturation. The amplified DNA can then be ligated into a vector for cloning.

    1 mark

Answer: PCR amplifies DNA in vitro using two primers and thermostable Taq polymerase through repeated cycles of denaturation, annealing and extension, giving about a billion copies.

Where the marks are

  • Explains PCR as in vitro synthesis of copies using primers, DNA polymerase and template (1 mark)
  • Names the three steps: denaturation, annealing, extension (1 mark)
  • Repeated cycles give about a billion copies; thermostable Taq polymerase used (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 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.