Polymerase Chain Reaction. A technique for amplifying a specific DNA segment in a complex mixture. Also present in the DNA mixture are short oligonucleotide primers to the DNA segment of interest and reagents for DNA synthesis. PCR relies on the ability of DNA to separate into its two complementary strands at high temperature (a process called denaturation) and for the two strands to anneal at an optimal lower temperature (annealing). The annealing phase is followed by a DNA synthesis step at an optimal temperature for a heat-stable DNA polymerase. After multiple rounds of denaturation, annealing, and DNA synthesis, the DNA sequence specified by the oligonucleotide primers is amplified.