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