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Polymerases are enzymes that catalyze the synthesis of nucleic acids by adding nucleotides to a growing chain in a template-directed manner. They are essential for information transfer in living cells and occur as two broad classes: DNA polymerases, which replicate and repair genomes, and RNA polymerases, which transcribe DNA into RNA. Most polymerases require a primer and a template strand and operate with high fidelity, often aided by proofreading activities and accessory proteins.

DNA polymerases replicate genomes and repair DNA. In bacteria, the replicative enzyme is DNA polymerase III, while DNA polymerase I participates in processing Okazaki fragments and DNA repair. In eukaryotes, main replicative polymerases include Pol delta and Pol epsilon, with Pol alpha/primase initiating synthesis; Pol gamma replicates the mitochondrial genome. Many DNA polymerases possess 3'→5' exonuclease proofreading activity, improving fidelity; several specialized polymerases participate in repair or translesion synthesis, often with lower fidelity.

RNA polymerases transcribe RNA from a DNA template. In bacteria, a single RNA polymerase carries out transcription with sigma factors guiding initiation. In archaea and eukaryotes, multi-subunit RNA polymerases (notably RNA Pol II in eukaryotes) synthesize various RNA species, including mRNA precursors, rRNA, and tRNA. Unlike DNA polymerases, RNA polymerases do not require a primer and incorporate ribonucleotides, which influences processing and error handling.

Mechanism and regulation. Polymerases use metal ions (typically Mg2+) in a two-metal-ion catalytic mechanism to form phosphodiester bonds, guided by a DNA or RNA template and a nucleotide triphosphate substrate. Fidelity is maintained by proofreading domains or separate exonucleases in many DNA polymerases. Processivity is enhanced by accessory factors and clamp proteins, coordinating replication with cell cycle and repair pathways.

History and applications. Polymerases were foundational to molecular biology, with discovery attributed to Arthur Kornberg in 1956. Biotechnological use includes PCR with thermostable DNA polymerases, DNA sequencing, cloning, and various diagnostic assays, making polymerases central tools in research and medicine.