DNA polymerase type-B alpha subfamily catalytic domain. Three DNA-dependent DNA polymerases type B (alpha, delta, and epsilon) have been identified as essential for nuclear DNA replication in eukaryotes. DNA polymerase (Pol) alpha is almost exclusively required for the initiation of DNA replication and the priming of Okazaki fragments during elongation. In most organisms no specific repair role, other than check point control, has been assigned to this enzyme. Pol alpha contains both polymerase and exonuclease domains, but lacks exonuclease activity suggesting that the exonuclease domain may be for structural purposes only.