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Starting from a tiny trace of DNA, how can we make millions of copies in a few hours?

Describe the polymerase chain reaction and explain how it amplifies a specific DNA sequence

A focused answer to the O-Level outcome on PCR. The three temperature steps, the role of primers and a heat-stable polymerase, the doubling per cycle, and what PCR is used for.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
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Try this

What this dot point is asking

This outcome asks you to describe the polymerase chain reaction (PCR) and explain how it makes millions of copies of a chosen DNA sequence. PCR is essentially DNA replication carried out in a tube, driven by cycling the temperature, and it is one of the most important techniques in all of biotechnology.

The answer

What PCR does

PCR amplifies (makes many copies of) a specific target sequence of DNA. From a tiny starting amount, it can produce millions of copies in a few hours.

The three steps of a cycle

One cycle of PCR has three steps, set by temperature:

Denaturation (about 95 degrees Celsius). The mixture is heated so the hydrogen bonds break and the two DNA strands separate.
Annealing (about 55 degrees Celsius). The mixture is cooled so that short pieces of single-stranded DNA called primers bind, by complementary base pairing, to the ends of the target sequence on each strand.
Extension (about 72 degrees Celsius). The mixture is warmed to the best temperature for a heat-stable DNA polymerase, which adds free nucleotides to extend the primers and build new complementary strands.

Doubling each cycle

At the end of each cycle, every target molecule has become two. So the number of copies doubles each cycle. Repeating the cycle around 30 times gives millions of copies.

The key ingredients

Primers mark the start of the region to be copied and give the polymerase a place to begin.
A heat-stable DNA polymerase survives the repeated heating, so it does not need replacing each cycle.
Free nucleotides are the building blocks for the new strands.

Counting copies after several PCR cycles

You start a PCR with $1$ copy of a target sequence. Assuming the amount doubles each cycle, work out how many copies you have after $10$ cycles, and then after $20$ cycles.

Step 1: Write the rule

After $n$ cycles the number of copies is $2^{n}$ , because each cycle doubles the previous amount.

Step 2: Calculate after 10 cycles

2^{10} = 1024 \approx 1\times10^{3}\ \text{copies}

Step 3: Calculate after 20 cycles

2^{20} = 2^{10}\times2^{10} = 1024\times1024 \approx 1\times10^{6}\ \text{copies}

Step 4: Interpret the result

In just $20$ cycles a single copy becomes about a million, and $30$ cycles would give about a billion. This exponential doubling is why PCR is so powerful, and using $2^{n}$ is exactly what the question rewards.

Examples in context

Example 1. Forensic DNA from a trace. A crime scene may yield only a tiny amount of DNA. PCR amplifies it into enough material to analyse and compare, which is why PCR is central to forensic genetic profiling.

Example 2. Detecting a virus. To diagnose a viral infection, a sample is tested for the virus's genetic material. PCR amplifies any viral sequence present until it can be detected, making PCR a key tool in medical diagnostics.

Try this

Q1. Name the three steps of one PCR cycle in order. [3 marks]

Cue. Denaturation, annealing, then extension.

Q2. State the role of the primers in PCR. [1 mark]

Cue. They bind by complementary base pairing to the ends of the target sequence, giving the polymerase a starting point.

Q3. Starting from one copy, how many copies are there after $5$ cycles? [1 mark]

Cue. $2^{5} = 32$ copies.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksDescribe the three steps of one cycle of the polymerase chain reaction (PCR) and explain how repeating the cycle amplifies a DNA sequence.

Show worked answer →

Examiners want the three named steps with their temperatures and the doubling per cycle.

In denaturation, the DNA is heated to about 95 degrees Celsius, breaking the hydrogen bonds so the two strands separate.

In annealing, the mixture is cooled to about 55 degrees so that short primers bind by complementary base pairing to the ends of the target sequence on each strand.

In extension, the mixture is warmed to about 72 degrees, the best temperature for a heat-stable DNA polymerase, which adds free nucleotides to extend the primers and build new complementary strands.

Each cycle doubles the number of copies of the target sequence, so repeating the cycle many times produces millions of copies very quickly.

What markers reward: the three steps named (denaturation, annealing, extension) with approximate temperatures, the role of primers and the heat-stable polymerase, and the point that each cycle doubles the target so many cycles give millions of copies.

Original3 marksExplain why a heat-stable DNA polymerase, rather than an ordinary one, is used in PCR.

Show worked answer →

The answer should connect the high denaturation temperature to the need for a heat-stable enzyme.

PCR repeatedly heats the mixture to about 95 degrees Celsius to separate the DNA strands. An ordinary DNA polymerase would be denatured and destroyed by this high temperature, so it would have to be replaced every cycle.

A heat-stable DNA polymerase, taken from an organism that lives in very hot conditions, survives the high temperatures, so it can keep working through many cycles without being replaced. This makes PCR practical and automatic.

What markers reward: the point that the high denaturation temperature would destroy an ordinary enzyme, and that a heat-stable polymerase survives repeated heating so it can be used through all the cycles.