Amplifying DNA by the polymerase chain reaction

Microbiology, Immunity and Forensics · Decay, DNA techniques and forensics · note 2 of 5

Spec 6.17

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Amplifying DNA by the polymerase chain reactionSpec 6.17

In short

The polymerase chain reaction amplifies a DNA sequence in a thermal cycler. Each cycle has denaturation at about 95 °C, annealing of primers at about 55 to 65 °C and extension by Taq polymerase at about 72 °C. The number of copies doubles each cycle.

The polymerase chain reaction (PCR) makes millions of copies of a particular piece of DNA from a very small sample, such as a hair root at a crime scene. The tube contains:

the DNA template (the sample)
two primers: short single-stranded DNA sequences complementary to the start of each strand of the target sequence
free DNA nucleotides (dATP, dTTP, dCTP, dGTP)
Taq polymerase, a DNA polymerase from the bacterium Thermus aquaticus that is heat-stable and so is not denatured at 95 °C
a buffer solution with magnesium ions

Denaturation: heat to about 95 °C. Hydrogen bonds between the complementary base pairs break, separating the two strands.
Annealing: cool to about 55 to 65 °C. Primers bind (anneal) by hydrogen bonds to their complementary sequences at the ends of the target DNA on each strand.
Extension: heat to about 72 °C, the optimum for Taq polymerase. It adds free nucleotides to the 3' end of each primer, building a complementary strand, to make two double-stranded copies.
The cycle is repeated, typically 25 to 35 times in an automated thermal cycler. The amount of DNA doubles each cycle, so after n cycles there are 2ⁿ copies of the target from each original molecule.

The primers make the technique specific for one sequence, and the use of a heat-stable enzyme means that the polymerase does not have to be added again each cycle. A very small sample can be amplified enough to be analysed. Contamination with other DNA is a risk, so great care is needed.

Amplification

How many copies of a DNA sequence would there be after 25 cycles of PCR starting from one molecule?

The number doubles each cycle, so copies = 2ⁿ.
2²⁵ = 33 554 432.

Answer: About 3.4 × 10⁷ copies

One cycle of PCR: denaturation at about 95 °C, annealing of primers at about 55 °C and extension at about 72 °C by Taq polymerase, with the copies of the target sequence doubling each cycle. — One cycle of **PCR**: the number of copies doubles every cycle.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

How do microorganisms recycle carbon?

Saprotrophic bacteria and fungi secrete extracellular enzymes that digest dead organisms and waste, then absorb the soluble products. They respire some of this, releasing carbon dioxide into the atmosphere. Producers can then take up this carbon dioxide in photosynthesis, so carbon returns to the living world.

How does PCR work?

PCR copies a section of DNA using repeated cycles. At about 95 °C the strands separate, at about 55 to 65 °C primers anneal, and at about 72 °C Taq polymerase builds new strands from free nucleotides. Each cycle doubles the amount of DNA, giving millions of copies after 30 cycles.

How does gel electrophoresis separate DNA?

DNA is negatively charged because of its phosphate groups. In an electric field it moves through an agarose gel towards the positive electrode. The gel is a mesh, so shorter fragments move faster and further than longer ones, separating the fragments by length into bands.

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