 The cycling is often preceded by a single temperature step at a very high temperature (>), and followed by one hold at the end for final product extension or brief storage.The temperatures used and the length of time they are applied in each cycle depend on a variety of parameters, including the enzyme used for DNA synthesis, the concentration of bivalent ions and dNTPs in the reaction, and the melting temperature ("Tm") of the primers.The individual steps common to most PCR methods are as follows