<p>DNA carries the biological information that instructs cells how to exist in an ordered fashion. Accurate replication is thus one of the most important events in the cell life cycle. This function is mediated by DNA-directed DNA polymerases, which add nucleotide triphosphate (dNTP) residues to the 3'-end of the growing DNA chain, using a complementary DNA as template. Small RNA molecules are generally used as primers for chain elongation, although terminal proteins may also be used.DNA-dependent DNA polymerases have been grouped into families, denoted A, Band X, on the basis of sequence similarities [<cite idref="PUB00004647"/>, <cite idref="PUB00004955"/>]. Members of family A, which includes bacterial and bacteriophage polymerases, share significant similarity to <taxon tax_id="562">Escherichia coli</taxon> polymerase I; hence family A is also known as the pol I family. The bacterial polymerases also contain an exonuclease activity,which is coded for in the N-terminal portion. Three motifs, A, B and C [<cite idref="PUB00004955"/>], are seen to be conserved across all DNA polymerases, with motifs A and C also seen in RNA polymerases. They are centred on invariant residues, and their structural significance was implied from the Klenow (E. coli) structure. Motif A contains a strictly-conserved aspartate at the junction of a beta-strand and an alpha-helix; motif B contains an alpha-helix with positive charges; and motif C has a doublet of negative charges, located in a beta-turn-beta secondary structure [<cite idref="PUB00004955"/>].</p>