DNA polymerase type-B family catalytic domain. DNA-directed DNA polymerases elongate DNA by adding nucleotide triphosphate (dNTP) residues to the 5'-end of the growing chain of DNA. DNA-directed DNA polymerases are multifunctional with both synthetic (polymerase) and degradative modes (exonucleases) and play roles in the processes of DNA replication, repair, and recombination. DNA-dependent DNA polymerases can be classified in six main groups based upon their phylogenetic relationships with E. coli polymerase I (class A), E. coli polymerase II (class B), E. coli polymerase III (class C), euryarchaeota polymerase II (class D), human polymerase beta (class x), E. coli UmuC/DinB, and eukaryotic RAP 30/Xeroderma pigmentosum variant (class Y). Family B DNA polymerases include E. coli DNA polymerase II, some eubacterial phage DNA polymerases, nuclear replicative DNA polymerases (alpha, delta, epsilon, and zeta), and eukaryotic viral and plasmid-borne enzymes. DNA polymerase is made up of distinct domains and sub-domains. The polymerase domain of DNA polymerase type B (Pol domain) is responsible for the template-directed polymerization of dNTPs onto the growing primer strand of duplex DNA that is usually magnesium dependent. In general, the architecture of the Pol domain has been likened to a right hand with fingers, thumb, and palm sub-domains with a deep groove to accommodate the nucleic acid substrate. There are a few conserved motifs in the Pol domain of family B DNA polymerases. The conserved aspartic acid residues in the DTDS motifs of the palm sub-domain is crucial for binding to divalent metal ion and is suggested to be important for polymerase catalysis.