There are 5 known Prokaryotic DNA polymerases:

Pol I: implicated in DNA repair; has 5'->3' polymerase activity, and both 3'->5' exonuclease activity (proofreading) and 5'->3' exonuclease activity (RNA primer removal).
Pol II: involved in repairing damaged DNA; has 3'->5' exonuclease activity.
Pol III: the main polymerase in bacteria (responsible for elongation); has 3'->5' exonuclease activity (proofreading).
Pol IV: a Y-family DNA polymerase.
Pol V: a Y-family DNA polymerase; participates in bypassing DNA damage.

There are at least 15 Eukaryotic DNA polymerase:

POLA1, POLA2: Pol Î± (also called RNA primase): forms a complex with a small catalytic (PriS) and a large noncatalytic (PriL) subunit, with the Pri subunits acting as a primase (synthesizing an RNA primer), and then with DNA Pol Î± elongating that primer with DNA nucleotides. After around 20 nucleotides elongation is taken over by Pol Îµ (on the leading strand) and Î´ (on the lagging strand).

POLB: Pol Î²: Implicated in repairing DNA, in base excision repair and gap-filling synthesis.

POLG, POLG2: Pol Î³: Replicates and repairs mitochondrial DNA and has proofreading 3'->5' exonuclease activity.

POLD1, POLD2, POLD3, POLD4: Pol Î´: Highly processive and has proofreading 3'->5' exonuclease activity. Thought to be the main polymerase involved in lagging strand synthesis, though there is still debate about its role.

POLE, POLE2, POLE3: Pol Îµ: Also highly processive and has proofreading 3'->5' exonuclease activity. Highly related to pol Î´, and thought to be the main polymerase involved in leading strand synthesis, though there is again still debate about its role.

POLH, POLI, POLK, : Î·, Î¹, Îº, and Rev1 are Y-family DNA polymerases and Pol Î¶ is a B-family DNA polymerase. These polymerases are involved in the bypass of DNA damage.

There are also other eukaryotic polymerases known, which are not as well characterized:
- POLQ: 'Î¸
- POLL: Î»
- ?: Ï†
- ?: Ïƒ
- POLM: Î¼

DNA polymerase familiesBased on sequence homology, DNA polymerases can be further subdivided into seven different families: A, B, C, D, X, Y, and RT. Family_A">Family APolymerases contain both replicative and repair polymerases. Replicative members from this family include the extensively-studied T7 DNA polymerase, as well as the eukaryotic mitochondrial DNA Polymerase Î³. Among the repair polymerases are Escherichia coli DNA pol I, Thermus aquaticus pol I, and Bacillus stearothermophilus pol I. These repair polymerases are involved in excision repair and processing of Okazaki fragments generated during lagging strand synthesis.

Family B

Polymerases mostly contain replicative polymerases and include the major eukaryotic DNA polymerases Î±, Î´, Îµ, (see Greek letters) and also DNA polymerase Î¶. Family B also includes DNA polymerases encoded by some bacteria and bacteriophages, of which the best-characterized are from T4, Phi29, and RB69 bacteriophages. These enzymes are involved in both leading and lagging strand synthesis during replication. A hallmark of the B family of polymerases is their highly faithful DNA synthesis during replication. While many have an intrinsic 3'-5' proofreading exonuclease activity, eukaryotic DNA polymerases Î± and Î¶ are two examples of B family polymerases lacking this proofreading activity.

Family_C">Family CPolymerases are the primary bacterial chromosomal replicative enzymes. DNA Polymerase III alpha subunit from E. coli is the catalytic subunit [1] and possesses no known nuclease activity. A separate subunit, the epsilon subunit, possesses the 3'-5' exonuclease activity used for editing during chromosomal replication. Recent research has classified Family C polymerases as a subcategory of Family X[citation needed]. Family_D">Family DPolymerases are still not very well characterized. All known examples are found in the Euryarchaeota subdomain of Archaea and are thought to be replicative polymerases. Family_X">Family XContains the well-known eukaryotic polymerase pol Î², as well as other eukaryotic polymerases such as pol Ïƒ, pol Î», pol Î¼, and terminal deoxynucleotidyl transferase (TdT). Pol Î² is required for short-patch base excision repair, a DNA repair pathway that is essential for repairing abasic sites. Pol Î» and Pol Î¼ are involved in non-homologous end-joining, a mechanism for rejoining DNA double-strand breaks. TdT is expressed only in lymphoid tissue, and adds "n nucleotides" to double-strand breaks formed during V(D)J recombination to promote immunological diversity. The yeast Saccharomyces cerevisiae has only one Pol X polymerase, Pol IV, which is involved in non-homologous end-joining. Family_Y">Family YY Polymerases differ from others in having a low fidelity on undamaged templates and in their ability to replicate through damaged DNA. Members of this family are hence called translesion synthesis (TLS) polymerases. Depending on the lesion, TLS polymerases can bypass the damage in an error-free or error-prone fashion, the latter resulting in elevated mutagenesis. Xeroderma pigmentosum variant (XPV) patients for instance have mutations in the gene encoding Pol Î· (eta), which is error-free for UV-lesions. In XPV patients, alternative error-prone polymerases, e.g., Pol Î¶ (zeta) (polymerase Î¶ is a B Family polymerase a complex of the catalytic subunit REV3L with Rev7, which associates with Rev1[2]), are thought to be involved in mistakes that result in the cancer predisposition of these patients. Other members in humans are Pol Î¹ (iota), Pol Îº (kappa), and Rev1 (terminal deoxycytidyl transferase). In E. coli, two TLS polymerases, Pol IV (DINB) and Pol V (UmuD'2C), are known.

Family RT

The reverse transcriptase family contains examples from both retroviruses and eukaryotic polymerases. The eukaryotic polymerases are usually restricted to telomerases. These polymerases use an RNA template to synthesize the DNA strand.

Variety across Species:DNA polymerases have highly-conserved structure, which means that their overall catalytic subunits vary, on a whole, very little from species to species. Conserved structures usually indicate important, irreplaceable functions of the cell, the maintenance of which provides evolutionary advantages.

Some viruses also encode special DNA polymerases, such as Hepatitis B virus DNA polymerase. These may selectively replicate viral DNA through a variety of mechanisms. Retroviruses encode an unusual DNA polymerase called reverse transcriptase, which is an RNA-dependent DNA polymerase (RdDp). It polymerizes DNA from a template of RNA.

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