PCR cycling

In theory, each PCR cycle doubles the amount of amplicon in the reaction. Therefore, 10 cycles multiply the amplicon by a factor of ~1000 and so on.

Each PCR cycle consists of template denaturation, primer annealing and primer extension. If the temperatures for annealing and extension are similar, these two processes can be combined. Each stage of the cycle must be optimized in terms of time and temperature for each template and primer pair combination.

After the required number of cycles has been completed (see table Guidelines for determining the number of PCR cycles for further information), the amplified product may be analyzed or used in downstream applications.

Guidelines for determining the number of PCR cycles
Amount of 1 kb
DNA fragment
Amount of
E. coli DNA
Amount of
human DNA  
Number of
single-copy targets
Number of
PCR cycles
0.0.1–0.11 fg 0.05–0.56 pg 36–360 pg 10–100 40–45
0.11–1.1 fg 0.56–5.56 pg 0.36–3.6 ng 100–1000 35–40
1.1–5.5 fg 5.56–278 pg 3.6–179 ng 1 x 103 – 5 x 104 30–35
>5.5 fg >278 pg >179 ng >5 x 104 25–35

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