The cellular process that PCR is modeled after is DNA replication. The polymerase chain reaction (PCR) technique, developed by Kary Mullis in 1985, is extremely powerful. It generates micro-gram quantities of DNA copies (up to billion copies) of the desired DNA segment. The PCR utilizes the following: (a) a DNA preparation containing the desired fragments to be amplified, (b) two nucleotide primers specific, i.e., complementary, to the two 3'-borders(the sequences present at the 3'-ends of the two strands) of the desired segment, (c) the four deoxynucleoside triphosphates, and (d) a heat -stable DNA polymerase. At the start of PCR, the DNA from which a segment is to be amplified, an excess of the two primer molecules, the four deoxyriboside triphosphates and the DNA polymerase are mixed together in the reaction mixture that has appropriate quantities of Mg++.