This document describes the polymerase chain reaction (PCR) technique. PCR is used to amplify a specific DNA sequence by repeating cycles of heating and cooling of the DNA sample. The key components of a PCR reaction are DNA template, primers, nucleotides, DNA polymerase, buffer, and magnesium ions. During each cycle, the DNA is denatured by heating, the primers anneal to the DNA at a lower temperature, and the DNA polymerase extends the DNA chain. This process is repeated many times, exponentially amplifying the target DNA sequence. PCR is a powerful, sensitive, specific and reliable method for detecting small amounts of DNA.