Polymerase chain reaction, or PCR, is a way to copy a selected stretch of DNA many times. A tiny starting amount can become large enough for a laboratory to analyze. The choice of primers, short pieces of DNA that mark the target, determines what the reaction is set up to copy.
Three steps repeated
Each cycle separates the DNA strands with heat, lets primers bind as the mixture cools, and uses a DNA polymerase to extend new strands. Repeating the cycle can increase the target dramatically. If the starting material is RNA, a separate reverse-transcription step first converts it to DNA.
What a result means
PCR answers a targeted question about genetic material in a sample. It does not by itself tell whether an organism is alive, how severe an illness is, or what every part of a genome contains. Contamination, sampling, and assay design matter, so laboratories use controls and interpret results in context.
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