| Description | 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 5'-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. Three motifs, A, B and C [ ], 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 Klenlow (Escherichia coli) structure: motif A contains a strictly-conserved aspartate at the junction of a β-strand and an α-helix; motif B contains an α-helix with positive charges; and motif C has a doublet of negative charges, located in a β-turn-beta secondary structure [].DNA polymerases ( ) can be classified, on the basis of sequence similarity [ , ], into at least four different groups: A, B, C and X. Members of family X are small (about 40 Kd) compared with other polymerases and encompass two distinct polymerase enzymes that have similar functionality: vertebrate polymerase beta (same as Saccharomyces cerevisiae pol 4), and terminal deoxynucleotidyl-transferase (TdT) (). The former functions in DNA repair, while the latter terminally adds single nucleotides to polydeoxynucleotide chains. Both enzymes catalyse addition of nucleotides in a distributive manner, i.e. they dissociate from the template-primer after addition of each nucleotide. DNA-polymerases show a degree of structural similarity with RNA-polymerases. This entry represents terminal deoxynucleotidyl-transferase (TdT) and the DNA-directed DNA/RNA polymerase mu. The latter is a gap-filling polymerase involved in repair of DNA double-strand breaks by non-homologous end joining [ , ]. | Name | DNA nucleotidylexotransferase (TdT) / DNA-directed DNA/RNA polymerase mu |
| Short Name | TdT/Mu | Type | Family |
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