There are three steps of PCR
a) Denaturation. The reaction combination is heated to 95°C for a short time period (about 15-30 sec) to denature the goal DNA into one strands which can act as templates for DNA synthesis.
b) Primer annealing. The combination is rapidly cooled to a described temperature that allows the two primers to bind to the sequences on each of the two strands flanking the goal DNA. This annealing temperature is measured carefully to ensure in which the primers bind only to the desired DNA sequences. Every primer binds to each strand. Two parental strands do not reanneal with each other because the primers are in huge excess over parental DNA.
c) Elongation. The temperature of the mixture is raised to 72°C (commonly) and kept at this temperature for a preset period of time to permits DNA polymerase to elongate each primer by copying the single-stranded templates. At the last of this incubation both single-stranded template strands have been made partially double stranded. The new strand of each double-stranded DNA extends for a variable distance downstream