Polymerases

Polymerases are enzymes that catalyze the formation of polymers by linking monomer units. In biology, they most commonly synthesize nucleic acids by appending nucleotides to a growing chain. Most nucleic acid polymerases add nucleotides in the 5' to 3' direction and use an exposed template strand to guide incorporation; some RNA polymerases can initiate synthesis de novo without a primer, whereas many DNA polymerases require a primer to start.

DNA polymerases replicate and repair genomes. During replication, DNA polymerases extend a primer along the template with high fidelity, aided by proofreading exonuclease activity in many enzymes. In bacteria and eukaryotes, distinct polymerases participate in replication and repair, and accessory factors such as sliding clamps increase processivity. Common laboratory examples include Taq polymerase, a thermostable DNA polymerase used in PCR, and cellular polymerases such as DNA polymerase delta and epsilon in eukaryotes.

RNA polymerases transcribe RNA from a DNA template in cells. Eukaryotes have three main nuclear RNA polymerases (I, II, III) responsible for rRNA, mRNA, tRNA, and other RNAs, respectively. Bacterial RNA polymerase, with sigma factors, recognizes promoters to initiate transcription. Some RNA polymerases, such as RNA-dependent RNA polymerases in viruses, synthesize RNA from RNA templates and do not require a DNA template.

Additional polymerases include reverse transcriptases that synthesize DNA from RNA templates, commonly used in molecular biology, and various polymerases involved in specialized RNA, DNA repair, or metabolic pathways. The study of polymerases encompasses kinetics, fidelity, processivity, and regulation across organisms.