<P> PCR amplifies a specific region of a DNA strand (the DNA target). Most PCR methods amplify DNA fragments of between 0.1 and 10 kilo base pairs (kbp), although some techniques allow for amplification of fragments up to 40 kbp in size . The amount of amplified product is determined by the available substrates in the reaction, which become limiting as the reaction progresses . </P> <P> A basic PCR set - up requires several components and reagents, including: </P> <Ul> <Li> a DNA template that contains the DNA target region to amplify </Li> <Li> a DNA polymerase, an enzyme that polymerizes new DNA strands; heat - resistant Taq polymerase is especially common, as it is more likely to remain intact during the high - temperature DNA denaturation process </Li> <Li> two DNA primers that are complementary to the 3' (three prime) ends of each of the sense and anti-sense strands of the DNA target (DNA polymerase can only bind to and elongate from a double - stranded region of DNA; without primers there is no double - stranded initiation site at which the polymerase can bind); specific primers that are complementary to the DNA target region are selected beforehand, and are often custom - made in a laboratory or purchased from commercial biochemical suppliers </Li> <Li> deoxynucleoside triphosphates, or dNTPs (sometimes called "deoxynucleotide triphosphates"; nucleotides containing triphosphate groups), the building blocks from which the DNA polymerase synthesizes a new DNA strand </Li> <Li> a buffer solution providing a suitable chemical environment for optimum activity and stability of the DNA polymerase </Li> <Li> bivalent cations, typically magnesium (Mg) or manganese (Mn) ions; Mg is the most common, but Mn can be used for PCR - mediated DNA mutagenesis, as a higher Mn concentration increases the error rate during DNA synthesis </Li> <Li> monovalent cations, typically potassium (K) ions </Li> </Ul> <Li> a DNA template that contains the DNA target region to amplify </Li>

What three steps are carried out during each cycle of a pcr