Read PCR by Temperature and Doubling
PCR repeats denaturation, primer annealing and extension; ideal DNA quantity doubles each cycle.
PCR is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Follow the cycle
High temperature separates DNA strands by disrupting hydrogen bonds, not the phosphodiester backbone. Primers anneal to complementary sequences and provide free 3′-OH ends; a thermostable polymerase extends them. Annealing commonly occurs around 40–60°C.
Calculate ideal yield
Starting with two DNA molecules, 2×2⁸ = 512 after eight cycles. PCR can amplify pathogen-specific DNA for detection.
Primer-dimer formation
Complementary 3′ ends of forward and reverse primers can anneal and be extended, creating a short primer-dimer product that competes with the target amplicon. Reduce 3′ complementarity and optimise primer design/conditions.
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- ✓Denature → anneal → extend.
- ✓Only hydrogen bonds separate during denaturation.
- ✓Ideal yield: N₀×2ⁿ.
- ✓Two molecules to 512 → eight cycles.
Do not say PCR breaks the DNA backbone at denaturation temperature.
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