Eukaryotic cells contain numerous types of DNA polymerases from the different DNA polymerase families.
These include the β, λ, σ, μ and ζ forms of polymerase and terminal deoxynucleotidyl transferase (TdT) that make up family X; the α, δ and ε polymerases that make up family B and the η, ι and κ polymerases that make up family Y.
Eukaryotes also contain telomerase and the γ and θ forms of polymerase that make up family A. Finally, eukaryotes contain reverse transcriptase, a DNA polymerase that assembles a new strand of DNA based on a template of RNA.
Eukaryotic cells contain at least 15 DNA polymerases. None of these polymerases can remove primers and the DNA sequence can only be proofread by the elongation polymerases.
The family X polymerases are mostly found in vertebrates and rarely in plants and fungi. Polymerases from this family contain highly conserved regions that include two helix-hairpin-helix domains that are essential to their interaction with DNA. One of these motifs is found in a domain that interacts with downstream DNA and the other is found in the thumb domain which works together with the primer strand to help begin the process of DNA replication.
Pol β is encoded by the POLB gene. This polymerase performs short patch repair of damaged DNA by fixing alkylated, oxidized or abasic sites that have formed due to DNA damage.
Pol λ and Pol μ are encoded by the POLL and POLM genes, respectively, and are involved in the rejoining of breaks that have occurred in double strands of DNA due to hydrogen peroxide (in the case of Pol λ) and ionizing radiation (in the case of Pol μ). TdT is only found in lymphoid tissue and adds non-templated nucleotides at V(D)J junctions, to provide diversity.
The main polymerases involved in DNA replication are the family B members Pol α, Pol δ, and Pol ε. Pol α associates with DNA primase to give the Pol α–DNA primase complex. This is made up of four subunits that include the POLA1 (catalytic subunit), POLA2 (regulatory subunit) and the two primase subunits PRIM1 and PRIM2. The DNA primase creates an RNA primer, which Pol α then starts to replicate, elongating the primer with around 20 nucleotides. Synthesis of the leading and lagging strand is then taken over by Pol ε and Pol δ, respectively. Pol δ is encoded by the POLD1 gene and Pol ε by the POLE gene.
Reviewed by Sally Robertson, BSc