Primer are short oigonucliotids used in polymerase chain reaction (PCR) after the denaturing of ds - DNA, 2 Primers are added on each strand at 3 position temperature is decreased to 50° - 70°C for 30° - 40° sec this allow the annealing of primer on each strands. They are joined to amplified the DNA molecules as they are named so, they do priming on each DNA strand and later elongation takes place to form complete ds - DNA direction of priming always 5' to 3'.