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) .
ORC3L, ORC4L and
ORC5L form core complex, its stability is regulated by
ORC1L , .
ORC complex recruits CDC18L
and Cdt1, which are both required for the subsequent loading
of the MCM complex .
CDC18L is capable of inducing the binding between
Cdt1 and MCM complex . 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 . It is shown that
MCM2 and MCM3/5 are regulatory
subunits. MCM2 and MCM3/5
inhibit the helicase activity of the MCM4/6/7  by converting its double trimer structure into a heterotetramer or
heteropentamer, respectively .
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 .
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 .
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 .
also inhibit by phoshorylation some eukaryotic DNA replication machinery
elements in late S-phase. There are
MCM4/6/7 helicase ,
CDC18L  and Cdt1.
Cyclin A are inhibited by ubiqitination with help of
APC/hCdh complex. Cdt1 is regulated by Geminin
and ubiqitination with help SCF complex.
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