The PCR chain reaction contains 20−30 cycles following each other. Every cycle contain three steps:
i.: The double stranded DNA is heated to 94−96 °C for separating the strands. This step is the so called denaturation step, when the hydrogen bonds which holding the two strands together are broken.
ii.: After separating the two DNA strands the temperature is lowened, so the primers can connect to the DNA strands. This is the so called annealing or connecting step. This temperature depends on the primers used in the PCR reaction (usually 5 °C lower than the melting point of the primers.
iii.: Finally the DNA polymerase enzyme sythetises the complementary strand of DNA beginning from the primer. This is the so called elongation step. This temperature depends on the polymerase enzyme used in the reaction.