replisome

Replisome is a multiprotein molecular machine that carries out DNA replication at the replication fork. It coordinates DNA unwinding, primer synthesis, primer extension, and lagging-strand processing to copy the genetic material with high fidelity. The exact composition varies among domains of life, but common core functions include a helicase to unwind the DNA duplex, primase to lay down RNA primers, DNA polymerases to synthesize new DNA, and a clamp and clamp-loader to ensure high processivity, aided by single-strand DNA binding proteins and topoisomerases to manage DNA supercoiling.

In bacteria, the core replisome centers on the DnaB helicase, DnaG primase, and DNA polymerase III holoenzyme. A clamp loader (gamma complex) puts the beta sliding clamp around DNA to keep the polymerase attached; two or more Pol III cores may work simultaneously on the leading and lagging strands. SSB stabilizes exposed single-stranded DNA. Topoisomerases relieve torsional strain ahead of the fork, and DNA gyrase helps introduce negative supercoils. On the lagging strand, RNA primers are removed and replaced, gaps filled by Pol I (in many bacteria), and DNA ligase seals nicks to complete Okazaki fragment processing.

In eukaryotes and archaea, the helicase is part of the CMG complex (Cdc45-MCM2-7-GINS). Priming is performed by DNA polymerase alpha-primase, followed by extension by Pol delta on the lagging strand and Pol epsilon on the leading strand. PCNA acts as the sliding clamp and RFC as the clamp loader. Okazaki fragment processing involves FEN1 and Dna2 nucleases and DNA ligase I. RPA stabilizes ssDNA; topoisomerases relieve supercoils during fork progression.