Polymerase chain reaction

Method for the in vitro amplification (propagation) of a defined DNA fragment. By means of PCR, a specific DNA fragment can be enriched millions of times within a few hours from a very small amount of heterogeneous DNA.

DNA is broken down into its individual strands using heat. Two chemically synthesized oligonucleotides (primers) are hybridized to the denatured DNA (hybridization). The sequence of these primers is chosen to be complementary to one of these regions, which accompany the DNA to be propagated. The resulting short double-stranded DNA regions with the 3'OH ends of the primers pointing in the direction of the DNA to be amplified are substrate for a DNA polymerase. Under temperature conditions favourable for the enzyme, the primers are extended with 2-deoxyribonucleoside-5-phosphates. The resulting DNA strands can now serve as a template for the other primer. Repeated heat denaturation of the DNA double strands, anhybridization of the primers and extension by DNA polymerase (this is called a temperature cycle) leads to an enormous enrichment of a double-stranded (ds-)DNA fragment. After 20 cycles, starting from one DNA molecule, about 1 million copies are obtained.