Example 1
Order extension, annealing and denaturation in one standard PCR cycle.
Denaturation → annealing → extension: separate strands, bind primers, then extend new strands.
Year 12 · SACE Stage 2
PCR cycles and components
Match each PCR component to its role and order the copying cycle.
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Method and worked examples
PCR copies a selected DNA region through repeated temperature cycles. Denaturation heats the sample to separate complementary strands. Annealing cools it so short DNA primers bind complementary sequences bordering the target. During extension, a heat-resistant polymerase adds free DNA nucleotides to the primers. The nucleotide pool supplies the new strand’s building blocks; primers specify starting regions rather than acting as enzymes.
Two primers allow copying from opposite strands across the selected region. Heat resistance allows polymerase to remain functional through repeated high-temperature steps. For ideal complete doubling, copies = initial target molecules ×2ᶜ, where c is the cycle count. Real amplification eventually departs from this ideal as reagents or other conditions limit copying. Amplification copies DNA; it does not translate the target into protein.
Example 1
Example 2
A reaction starts with 5 target molecules and completes 3 ideal doubling cycles. Find copies.
5×23=40 target molecules. The calculation assumes complete doubling in every cycle.
Watch out: Free nucleotides are building blocks; primers contribute targeting and polymerase catalyses extension.