<p>DNA carries the biological information 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. 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 (<taxon tax_id="562">Escherichia coli</taxon>) 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><p>DNA polymerases (<db_xref db="EC" dbkey="2.7.7.7"/>) can be classified, on the basis of sequencesimilarity [<cite idref="PUB00004647"/>, <cite idref="PUB00004955"/>], into at least four different groups: A, B, C and X. Members of family X are small (about 40 kDa) compared with other polymerases and encompass two distinct polymerase enzymes that have similar functionality: vertebrate polymerase beta (same as yeast pol 4), and terminal deoxynucleotidyl-transferase (TdT) (<db_xref db="EC" dbkey="2.7.7.31"/>). The former functions in DNA repair, whilethe latter terminally adds single nucleotides to polydeoxynucleotide chains.Both enzymes catalyse addition of nucleotides in a distributive manner, i.e. theydissociate from the template-primer after addition of each nucleotide.DNA-polymerases show a degree of structural similarity with RNA-polymerases.</p><p>Five regions of similarity are found in all the polymerases of this entry. The signature of this entry is to the conserved region, known as 'motif B' [<cite idref="PUB00004955"/>]; motif B is located in a domain which, in E. coli polA, has been shown to bind deoxynucleotide triphosphate substrates; it contains a conserved tyrosine which has been shown, by photo-affinity labelling, to be in the active site; a conserved lysine, also part of this motif, can be chemically labelled, using pyridoxal phosphate.</p>