polymerase chain reaction
(Science: molecular biology, technique) The first practical system for in vitro amplification of dNA and as such one of the most important recent developments in molecular biology.
Two synthetic oligonucleotide primers, which are complementary to two regions of the target dNA (one for each strand) to be amplified, are added to the target DNA (that need not be pure), in the presence of excess deoxynucleotides and taq polymerase, a heat stable dNA polymerase. In a series (typically 30) of temperature cycles, the target DNA is repeatedly denatured (around 90-C), annealed to the primers (typically at 50-60-C) and a daughter strand extended from the primers (72-C). As the daughter strands themselves act as templates for subsequent cycles, DNA fragments matching both primers are amplified exponentially, rather than linearly.
The original dNA need thus be neither pure nor abundant and the polymerase chain reaction has accordingly become widely used not only in research, but in clinical diagnostics and forensic science.