The elongation of the primers completes one PCR cycle. The primer molecules are elongated by the action of DNA polymerase. Deoxynucleosides triphosphates(dNTPs) required for the synthesis of DNA are present in large excess, so the synthesis step can be repeated a lot of times. To withstand the repeated exposure to high temperatures, a thermostable DNA polymerase is used for PCR - usually Taq polymerase. Taq polymerase works best at around 75 degrees centigrade. The time required for this stage depends on the length of the target sequence (for example the rate of primer elongation by Taq polymerase is about 50 - 100 nucleotides/sec).