Eukaryotic DNA polymerases
Five different eukaryotic DNA polymerases have been identified, from materials like yeast cells, rat liver, human cell cultures and tumour cells. These include the following : (i) DNA potymerase a, a relatively high molecular weight enzyme, also called cytoplasmic polymerase (also appropriately called large polymerase, since it is also found in the nucleus and since another small nuclear polymerase is also available in cytoplasm); (ii) DNA polymerase β or nuclear polymerase (also called small polymerase), found only in vertebrates, and not in budding yeast nor in other lower eukaryotes; (iii) DNA polymerase γ or mitochondrial polymerase, encoded in the nucleus; (iv) DNA polymerase δ, another novel enzyme reported from mammalian cells, is PCNA dependent for DNA-synthetic processivity (PCNA = proliferating cell nuclear antigen; processivity = property of continuous synthesis); (v) DNA polymerase ε (earlier described as DNA polymerase δ11), also purified and reported from mammalian Hela cells and budding yeast, is PCNA independent. In some other cases, a distinction between DNA polymerase δ and DNA polymerase ε has not been possible. Another calf thymus enzyme designated as DNA polymerase 61 represented DNA polymerase α with 3'-5' exonuclease activity. The large DNA polymerase a is the predominant DNA polymerase in eukaryotic cells and was believed for long time to be the only enzyme involved in DNA replication.