OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(d) the principles of the polymerase chain reaction (PCR) and its…
Full specification wording
Manipulating genomes: (d) the principles of the polymerase chain reaction (PCR) and its application in DNA analysis
PCR is an in vitro method that copies a target DNA sequence exponentially.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 6 of 10
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
PCR is an in vitro method that copies a target DNA sequence exponentially. A reaction mixture contains the DNA template, two primers complementary to the flanking sequences, free nucleotides, a thermostable DNA polymerase such as Taq, and buffer. Each cycle has three temperature stages: denaturation near 95 °C separates the strands; annealing around 55–65 °C lets primers bind; extension near 72 °C lets polymerase add nucleotides 5′ to 3′. Repeating cycles doubles the target each time, giving 2ⁿ copies after n cycles. In DNA analysis PCR provides enough DNA for profiling, sequencing or cloning from tiny or degraded samples, and can be made quantitative.
What earns the marks
- PCR is an in vitro amplification of a target sequence using primers flanking the region of interest.
- The mixture needs template DNA, two primers, free nucleotides, thermostable DNA polymerase and buffer.
- Cycles alternate denaturation (strand separation), primer annealing and extension, with temperatures chosen for each stage.
- Each cycle doubles the target, so n cycles give 2ⁿ copies; a thermostable polymerase survives the high denaturation temperature.
- Applications include producing sufficient DNA for profiling, sequencing or cloning from small samples, and quantitative PCR for measuring amounts.
Common mistakes and how to put them right
Confusing PCR with DNA profiling.
Put it right by stating that PCR amplifies DNA whereas profiling compares amplified fragments.
Thinking primers are the enzyme or that DNA polymerase builds RNA.
Put it right by naming primers as short complementary sequences and polymerase as the enzyme adding DNA nucleotides.
Believing copies increase linearly.
Put it right by using the exponential relationship 2ⁿ after n cycles.
Exam tips
- Name each of the three stages with its approximate temperature and purpose in one sentence.
- State the direction of synthesis as 5′ to 3′ and explain why a thermostable polymerase is required.
- Apply the 2ⁿ relationship to a stated cycle number to show quantitative understanding.
Try the lesson
The start of the Manipulating genomes: (d) the principles of the polymerase chain reaction (PCR) and its application in DNA analysis lesson, made from checked teaching notes for this specification point.
Have a go first
What is the role of the primers in a PCR reaction?
Listen
PCR is an in vitro method that copies a target DNA sequence exponentially. A reaction mixture contains the DNA template, two primers complementary to the flanking sequences, free nucleotides, a thermostable DNA polymerase such as Taq, and buffer. Each cycle has three temperature stages: denaturation near 95 °C separates the strands; annealing around 55–65 °C lets primers bind; extension near 72 °C lets polymerase add nucleotides 5′ to 3′. Repeating cycles doubles the target each time, giving 2ⁿ copies after n cycles. In DNA analysis PCR provides enough DNA for profiling, sequencing or cloning from tiny or degraded samples, and can be made quantitative. A strong answer earns marks for these points. PCR is an in vitro amplification of a target sequence using primers flanking the region of interest. The mixture needs template DNA, two primers, free nucleotides, thermostable DNA polymerase and buffer. Cycles alternate denaturation (strand separation), primer annealing and extension, with temperatures chosen for each stage. Each cycle doubles the target, so n cycles give 2ⁿ copies; a thermostable polymerase survives the high denaturation temperature. A common slip: Confusing PCR with DNA profiling. Instead, stating that PCR amplifies DNA whereas profiling compares amplified fragments. Exam tip: Name each of the three stages with its approximate temperature and purpose in one sentence.
Key words
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In vitro exponential amplification
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Primer design and thermostable polymerase
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Thermal cycling stages
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Applications in DNA analysis and quantification
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See every definition
- In vitro exponential amplification
- A method that copies a target DNA sequence outside a living organism, doubling the target each cycle so that n cycles give 2ⁿ copies.
- Primer design and thermostable polymerase
- Two primers complementary to the flanking sequences bind the target, and a thermostable DNA polymerase such as Taq survives the high denaturation temperature.
- Thermal cycling stages
- Each cycle has three temperature stages: denaturation near 95 °C separates the strands, annealing around 55–65 °C lets primers bind, and extension near 72 °C lets polymerase add nucleotides 5′ to 3′.
- Applications in DNA analysis and quantification
- PCR provides enough DNA for profiling, sequencing or cloning from tiny or degraded samples, and can be made quantitative.
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