Polymerase chain reaction

PCR's power lies in its ability to easily isolate particular regions of DNA sequence from whole genomic material. Many techniques need a pool of DNA molecules isolated from a particular DNA fragment, and the use of PCR has enabled these techniques more widespread in usage. Because PCR also amplifies the isolated region, the techniques are more powerful, applicable to samples otherwise too small for analysis.

PCR is used to amplify specific regions of a DNA strand. This can be a single gene, just a part of a gene, or a non-coding sequence. Most PCR methods typically amplify DNA fragments of up to 10 kilo base pairs (kb), although some techniques allow for amplification of fragments up to 40 kb in size.[2]
PCR, as currently practiced, requires several basic components [3]. These components are:
DNA template that contains the region of the DNA fragment to be amplified
One or more primers, which are complementary to the DNA regions at the 5' and 3' ends of the DNA region that is to be amplified.
a DNA polymerase (e.g. Taq polymerase or another DNA polymerase with a temperature optimum at around 70°C), used to synthesize a DNA copy of the region to be amplified
Deoxynucleotide triphosphates, (dNTPs) from which the DNA polymerase builds the new DNA
Buffer solution, which provides a suitable chemical environment for optimum activity and stability of the DNA polymerase
Divalent cations, magnesium or manganese ions; generally Mg2+ is used, but Mn2+ can be utilized for PCR-mediated DNA mutagenesis, as higher Mn2+ concentration increases the error rate during DNA synthesis [4]
Monovalent cation potassium ions
The PCR is carried out in small reaction tubes (0.2-0.5 ml volumes), containing a reaction volume typically of 15-100 μl, that are inserted into a thermal cycler. This is a machine that heats and cools the reaction tubes within it to the precise temperature required for each step of the reaction. Most thermal cyclers have heated lids to prevent condensation on the inside of the reaction tube caps. Alternatively, a layer of oil may be placed on the reaction mixture to prevent evaporation.