Cell cycle - Start of DNA replication in early S phase

DNA replication and S phase progression

DNA replication is a key event of cell proliferation.

Replication begins with the ordered assembly of a multiprotein complex called the prereplicative complex (pre-RC). Pre-RC formation occurs during late M and early G1 phases of the cell cycle and licenses the DNA for replication during S phase. Transcription of many eukaryotic DNA replication machinery genes is activated by E2F transcriptional factors (e.g. E2F1/DP1 dimer).

Pre-RC consists of the origin recognition complex (ORC complex), the cell division cycle 6 homolog (CDC18L), Cdt1 and the minichromosome maintenance protein complex (MCM complex). Pre-RC components are recruited to the origins in a stepwise manner beginning with the ORC. Subunit 1 of ORC complex (ORC1L) may be recruited to origins by chromobox protein homolog 5 (HP1) [1]. Subunits ORC2L, ORC3L, ORC4L and ORC5L form core complex, its stability is regulated by ORC1L [2], [3].

ORC complex recruits CDC18L and Cdt1, which are both required for the subsequent loading of the MCM complex [4]. CDC18L is capable of inducing the binding between Cdt1 and MCM complex [5]. The MCM complex is a heterohexamer which plays a role of the replicative helicase in eukaryotic cells. Each of the six subunits is required for replication [6]. It is shown that MCM2 and MCM3/5 are regulatory subunits. MCM2 and MCM3/5 inhibit the helicase activity of the MCM4/6/7 [7] by converting its double trimer structure into a heterotetramer or heteropentamer, respectively [8].

Then cell division cycle 7-related protein kinase 2 (CDK2) and/or cell division cycle 7-related protein kinase (CDC7) and minichromosome maintenance deficient 10 protein (Mcm10) stimulate recruiting of CDC45-related protein (CDC45L) to the Pre-RC. Recruiting CDC45L and some other proteins promote maturation Pre-RC in pre-initiation complex (pre-IC). CDC45L recruits CDK2 to the replication foci, resulting in histon H1 phosphorylation, chromatin decondensation, and facilitation of fork progression [9].

In addition, replication proteins (RPA) are bound with the origin. There are three forms of RPA (RPA1, RPA2 and RPA3). Trimeric complex of these proteins is required for an association with DNA [10].

Serine/threonine protein phosphatase 2A (PP2A) activates by dephosphorylation DNA polymerase alpha/primase in late G1 phase. DNA polymerase alpha/primase is the only enzyme capable of initiating DNA synthesis de novo by first synthesizing an RNA primer and then extending the primer by polymerization to produce a short 30-nucleotide DNA extension. DNA polymerase alpha/primase forms complex with MCM2 in early S-phase. Then formation of new MCM2/ DNA polymerase alpha/primase complexes might be prevented by CDK2/CyclinA phosphorylation of DNA polymerase alpha/primase in late-S-phase [11].

CDK2/Cyclins also inhibit by phoshorylation some eukaryotic DNA replication machinery elements in late S-phase. There are MCM4/6/7 helicase [12], CDC18L [13] and Cdt1. ORC1L, CDC18L, Cyclin A are inhibited by ubiqitination with help of APC/hCdh complex. Cdt1 is regulated by Geminin and ubiqitination with help SCF complex.

References:

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   Intracranial arachnoidal cysts. Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery 1992 Mar;8(2):101-4
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