<P> Mrc1 / Claspin proteins couple leading - strand synthesis with the CMG complex helicase activity . Mrc1 interacts with polymerase ε as well as Mcm proteins . The importance of this direct link between the helicase and the leading - strand polymerase is underscored by results in cultured human cells, where Mrc1 / Claspin is required for efficient replication fork progression . These results suggest that efficient DNA replication also requires the coupling of helicases and leading - strand synthesis...</P> <P> DNA polymerases require additional factors to support DNA replication . DNA polymerases have a semiclosed' hand' structure, which allows the polymerase to load onto the DNA and begin translocating . This structure permits DNA polymerase to hold the single - stranded DNA template, incorporate dNTPs at the active site, and release the newly formed double - stranded DNA . However, the structure of DNA polymerases does not allow a continuous stable interaction with the template DNA . </P> <P> To strengthen the interaction between the polymerase and the template DNA, DNA sliding clamps associate with the polymerase to promote the processivity of the replicative polymerase . In eukaryotes, the sliding clamp is a homotrimer ring structure known as the proliferating cell nuclear antigen (PCNA). The PCNA ring has polarity with surfaces that interact with DNA polymerases and tethers them securely to the DNA template . PCNA - dependent stabilization of DNA polymerases has a significant effect on DNA replication because PCNAs are able to enhance the polymerase processivity up to 1,000-fold . PCNA is an essential cofactor and has the distinction of being one of the most common interaction platforms in the replisome to accommodate multiple processes at the replication fork, and so PCNA is also viewed as a regulatory cofactor for DNA polymerases . </P> <P> PCNA fully encircles the DNA template strand and must be loaded onto DNA at the replication fork . At the leading strand, loading of the PCNA is an infrequent process, because DNA replication on the leading strand is continuous until replication is terminated . However, at the lagging strand, DNA polymerase δ needs to be continually loaded at the start of each Okazaki fragment . This constant initiation of Okazaki fragment synthesis requires repeated PCNA loading for efficient DNA replication . </P>

When does dna replication happen in both prokaryotes and eukaryotes