Left: A PCR mix is made with primers, free deoxy-nucleotides (dNTPs), a DNA template and a DNA polymerase. This is heated to make the DNA single stranded (denature the DNA). The sample is then cooled to enable the primers to bind to the single-stranded template DNA. The sample is then heated to the optimal temperature for the DNA polymerase (typically 72°C for Taq), which enables the polymerase to extend from the primer, copying the DNA. This process is then repeated (or cycled through) resulting in the exponential increase of the concentration of DNA. Right: A graph showing the exponential increase in DNA concentration compared with each repeat (cycle) of PCR.