What is The polymerase chain reaction?

The polymerase chain reaction (PCR) amplifies a chosen stretch of DNA between two primers — synthetic oligonucleotides of about twenty bases complementary to the two strands at the ends of the target — through cycles of three temperatures: $95\,{}^{\circ}\mathrm{C}$ to separate the strands, $50\text{ to }65\,{}^{\circ}\mathrm{C}$ for the primers to anneal, $72\,{}^{\circ}\mathrm{C}$ for a heat-stable polymerase (Taq, from the hot-spring bacterium Thermus aquaticus) to extend them. Each cycle doubles the number of copies of the segment between the primers, so thirty cycles turn a single molecule into a billion. Reverse transcription PCR first copies RNA into DNA and so measures transcripts or RNA viruses; quantitative PCR (qPCR) follows the amplification in real time with a fluorescent dye and reads the starting quantity from the cycle at which the signal crosses a threshold.