Cell cycle regulation
All somatic eukaryotic cells proceed through four phases in cell cycle: G1 (gap phase
1) in which the cell prepares for the upcoming events of S-phase; S (synthesis phase), in
which DNA is replicated; G2 (gap phase 2) in which the cell prepares for the upcoming
events of M-phase; and M (mitosis), in which chromosomes are separated over two new
nuclei. Progression through the cell cycle determines the rate of proliferation. Growth,
repair and maintenance of organisms all rely on the regulation of the cycle.
Progress in the eukaryotic cell cycle is driven by oscillations in the activities of
cyclin-dependent kinases (CDKs).
CDK activity is controlled by periodic synthesis and
degradation of positive regulatory subunits, Cyclins, as
well as by fluctuations in levels of negative regulators, by CDK inhibitors (INK4 and/or
KIP/CIP families), and by reversible phosphorylation .
In mammalian cells, different Cyclin/CDK complexes are
involved in regulating different cell cycle transitions: Cyclin D/CDK4
(or CDK6) for G1 progression,
Cyclin E/CDK2 for the G1-S
transition , Cyclin
A/CDK2 for S-phase progression , and Cyclin A/CDK1
 and Cyclin
for entry into M-phase.
Biochemically, D-type cyclins (Cyclin D) act, in part, as
regulatory subunits of CDK4 and
CDK6. Cyclin D/CDK4 (or
CDK6) complexes, together with Cyclin
E/CDK2 phosphorylate of the retinoblastoma
family of tumor suppressor proteins (Rb family) (Rb protein,
p107 and p130), thereby liberating the E2F transcription
factors (for example, E2F1 or
E2F4) . These transcription factors are
associated with DP1 and together they drive expression of
Cyclin E, Cyclin
A, CDK1 and products that are necessary for
the replication of DNA and beginning of the S phase. Cyclin E
and Cyclin A are positive-regulatory partners
of cyclin-dependent kinase 2 (CDK2). It is remarkable that
both Cyclin D/CDK4
(or CDK6) and
Cyclin E/CDK2 are necessary for
the induction of expression of Cyclin
A , .
The ability of Cyclin D to interact with and activate
their CDK partners is antagonized by CDK inhibitors  (see map 544. Regulation of G1/S checkpoint).
Participators of G1/S transition (see map On G1 to S phase
transition) and DNA-damages-induced G1/S checkpoint arrest (see map
426. ATM/ATR regulation of G1/S checkpoint) pathways take
part in G1-phase regulation too.
In addition, E2F1 is phosphorylated by
Cyclin A/Cdk2 directly in the S phase and its
phosphorylation by Cyclin A/Cdk2 may modulate its activity
Progression from G2- to M-phase is driven by activation of the
CDK1/Cyclin B complex within
the nucleus. Accordingly, the activity of
CDK1/Cyclin B complex is
regulated in a spatiotemporary pattern at several levels, namely (1) at the level of
transcription of Cyclin B (to a lower extent on that of
CDK1); (2) at the level of regulatory
CDK1 phosphorylations; (3) at the level of the subcellular
distribution of Cyclin B (see maps Start of
the mitosis and 441. ATM/ATR regulation of G1/S
Events controlling cell division are governed by the degradation of different
regulatory proteins by the ubiquitin-dependent pathway. Anaphase-promoting complex (APC)
is a one of ubiquitin ligases, which play a very important role in the cell cycle (see
map 472. Role APC in cell cycle regulation) , .
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