The optimal temperature for the polymerase to operate is 72℃, so at third temperature is used to achieve maximum enzyme activity. Since primers are directed to both single strands of DNA, each PCR cycle will result in a doubling in copy number of the target sequence.
The cycle of changing temperatures (95℃, 50℃ and 72℃) is then repeated and two copies become four. Another cycle and four become eight, and so on... for 40-50 cycles.
After amplifying your gene in to many millions of copies it is possible to run the amplified DNA out on a polyacrilamide gel and stain it with a dye which makes is visible.
The bigger the visible band, the more copies of your gene you have created.Hence if you are comparing 2 samples e.g. one from a healthy patient and one from a cancer patient, you can see in which sample your gene of interest was expressed most highly.
