DNA carries the biological informatio`n that instructs cells how to existin an ordered fashion. Accurate replication is thus one of the mostimportant events in the cell life cycle. This function is mediated byDNA-directed DNA-polymerases, which add nucleotide triphosphate (dNTP)residues to the 5'-end of the growing DNA chain, using a complementary DNA as template. Small RNA molecules are generally used as primers forchain 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 significantsimilarity to <taxon tax_id="562">Escherichia coli</taxon> polymerase I; hence family A is also known as the pol Ifamily. The bacterial polymerases also contain an exonuclease activity,which is coded for in the N-terminal portion. The yeast gamma polymerasesshow some similarity to the prokaryotic nuclear polymerases of family A,but their function is restricted to mitochondrial DNA replication and repair [<cite idref="PUB00002437"/>].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"/>]. DNA-directed DNA-polymerase, family A, mitochondria