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Gene Technology

The polymerase chain reaction

  • 3 things to know
  • Unit A2 2

Overview

Purpose of the polymerase chain reaction, primer in the polymerase chain reaction and why two different primers are required.

What you need to know

Answer each one before you reveal it.

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Key term

In the polymerase chain reaction, to what temperature is the mixture heated in the first step, and what does this achieve?

Key term

What is a primer in the polymerase chain reaction?

Key term

Explain why two different primers are required in the polymerase chain reaction

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is the purpose of the polymerase chain reaction?

    Model answer

    The polymerase chain reaction amplifies a tiny sample of DNA, producing millions of copies of a chosen target sequence in a few hours.

  2. 2

    Name the three steps of one cycle of the polymerase chain reaction in the order they occur.

    Model answer

    Each cycle consists of heating to separate the strands, cooling to allow the primers to anneal, and warming to allow the polymerase to extend the primers.

  3. 3

    To what temperature is the reaction mixture heated in the first step, and what does this achieve?

    Model answer

    The mixture is heated to about 95 °C, which breaks the hydrogen bonds between the base pairs so the double helix separates into two single strands.

  4. 4

    Why is the mixture then cooled to around 55 °C?

    Model answer

    Cooling allows the short primers to bind by complementary base pairing to the sequences at each end of the target region, which they could not do at the higher temperature.

  5. 5

    What is a primer in the polymerase chain reaction?

    Model answer

    A primer is a short single strand of DNA with a base sequence complementary to one end of the target region, which marks where copying is to begin.

  6. 6

    Explain why two different primers are required.

    Model answer

    The two strands of the target run in opposite directions, so one primer is needed to bind at each end and each is complementary to a sequence on a different strand.

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