Pre-replication complex

The pre-RC is complete when DnaA occupies all of its binding sites within the bacterial origin of replication (oriC).

The particular sites on the oriC that DnaA binds to determines if the cell has a bORC (bacterial Origin Recognition Complex) or a pre-RC.

[5] In vivo, it has been observed that the DnaA binding to recognition sites occurs in the order: R1, R2, then R4, which forms the bORC.

The singular archaeal ORC protein recognizes the AT-rich tracts and binds DNA in an ATP-dependent fashion.

The mechanism of origin recognition in higher eukaryotes is not well understood but it is thought that the ORC1-6 proteins depend on unusual DNA topology for binding.

This timing and other regulatory mechanisms ensure that DNA replication will only occur once per cell cycle.

Assembly of the pre-RC relies on prior origin recognition, either by DnaA in prokaryotes or by ORC in archaea and eukaryotes.

[3] After the pre-RC is formed it must be activated and the replisome assembled in order for DNA replication to occur.

Single-strand binding proteins stabilize the newly formed replication bubble and interact with the DnaG primase.

Cdc45 then recruits key components of the replisome; the replicative DNA polymerase α and its primase.

[11] In S. cerevisiae, CDKs prevent formation of the replication complex during late G1, S, and G2 phases by excluding MCM2-7 and Cdt1 from the nucleus, targeting Cdc6 for degradation by the proteasome, and dissociating ORC1-6 from chromatin via phosphorylation.

[12] Prevention of re-replication in S. pombe is slightly different; Cdt1 is degraded by the proteasome instead of merely being excluded from the nucleus.

[13] Proteolytic regulation of Cdt1 is shared by higher eukaryotes including Caenorhabditis elegans, Drosophila melanogaster, X. laevis, and mammals.

Metazoans have a fourth mechanism to prevent re-replication; during S and G2 geminin binds to Cdt1 and inhibits Cdt1 from loading MCM2-7 onto the origin of replication.

[8] Defects in components of the eukaryotic replication complex are known to cause Meier-Gorlin syndrome, which is characterized by dwarfism, absent or hypoplastic patellae, small ears, impaired pre- and post-natal growth, and microcephaly.