Figure 28. The principle of PCR (Polymerase Chain Reaction).

The reaction mixture for a PCR test contains the target viral DNA to be amplified, two primers, and heat-stable Taq polymerase. Nucleic acid from the virus (in this example, double-stranded DNA) is denatured by heating to 95°C. Two short single-stranded primers (DNA fragments that are complementary to the viral DNA) bind to the separated DNA strands when the temperature is lowered to 55°C. The primers are extended with Taq polymerase which brings in new nucleotides to build new DNA strands at 72°C. The first synthesis cycle results in two copies of the target DNA sequence. The cycle is repeated (denaturation, annealing, and extension), and the second synthesis cycle results in four copies of the target DNA sequence. The billions of copies of the new DNA strands that are produced during 40 PCR cycles are detected using gel electrophoresis (Courtesy V. Dolja).