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A-Level Biology October/November 2024 Q8(c): The polymerase chain reaction (PCR) is used to make many copies of a gene. Three temper…
A-Level Biology · Paper 9700/41 · October/November 2024 · Question 8(c) · [3 marks]
The polymerase chain reaction (PCR) is used to make many copies of a gene. Three temperatures are used in a PCR cycle. State the three temperatures that are used, and outline what happens at each temperature during a PCR cycle.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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The three stages of a single PCR cycle use three different temperatures:
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95°C (Denaturation): The mixture is heated to a high temperature, typically between 90-98°C. This breaks the hydrogen bonds between the two strands of the target DNA, causing them to denature and separate into single strands.
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55°C (Annealing): The temperature is lowered to a range of 50-65°C. This allows the short, single-stranded DNA primers to bind (anneal) to their specific complementary sequences on the now single-stranded DNA templates.
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72°C (Extension): The temperature is raised to the optimal temperature for the polymerase, usually 68-75°C. The heat-stable Taq polymerase enzyme attaches to the primers and synthesises a new complementary strand of DNA by adding free nucleotides.
How the marks are awarded
- B1 — The first mark is awarded for stating a temperature in the range 90-98°C and correctly identifying the process as denaturation, where the DNA strands separate.
- B1 — The second mark is awarded for stating a temperature in the range 50-65°C and explaining that this is when primers bind or anneal to the DNA.
- B1 — The third mark is awarded for stating a temperature in the range 68-75°C and describing how Taq polymerase (or DNA polymerase) synthesises the new DNA strand.
Common mistakes
- Mixing up the order of the stages or assigning the wrong temperature to a stage (e.g., stating annealing occurs at 95°C).
- Using vague terminology, such as 'the DNA breaks' instead of 'denatures' or 'strands separate', or 'primers join' without specifying they anneal to the DNA template.
- Incorrectly naming the enzyme, for example, using 'DNA helicase' (which separates strands in vivo) or 'DNA ligase' instead of the heat-stable Taq polymerase.
- Providing incorrect temperature ranges, such as 37°C for extension, which is the optimum for human enzymes but not for Taq polymerase.
Examiner tip: For process-based questions, memorise key terms, conditions (like temperature), and enzymes for each stage, and practice writing them out in the correct sequence.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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