The general reaction mechanism catalyzed by DNA polymerases includes an elementary step involving the nucleophilic attack of the O3' atom of the terminal primer nucleotide and the P of the incoming nucleotide. This leads to the formation of an O-P bond and pyrophosphate (PPi), followed by PPi unbinding and translocation of the DNA to make space for the next nucleotide to come in and for further DNA elongation. The active site of most DNA polymerases, where this reaction occurs, are generally similar. They usually consist of the terminal primer nucleotide, the incoming nucleotide, conserved carboxylates (Asp and Glu) and metallic co-factors. In most DNA polymerases, it is seen that this reaction is facilitated by two metallic co-factors .