PCR is a cycle of three steps:
DENATURATION - the strands of the DNA are melted apart by heating to 95°C.
ANNEALING - the temperature is reduced to ~ 55°C to allow the primers to anneal to the target DNA.
POLYMERISATION/EXTENSION - the temperature is changed to the optimum temperature in order for the DNA polymerase to catalyse extension of the primers, i.e. to copy the DNA between the primers.
The cycle is repeated over and over again - as many times as needed to produce a detectable amount of product (DNA).
