Biology Reference
In-Depth Information
the necessary requirements are met. DNA is replicated completely and only once
during a single cell cycle (Fujita 1999; Diffley 2001; Lei and Tye 2001). During G
1
phase, prereplication complexes (pre-RC) assemble at the origins of DNA replica-
tion. The multisubunit origin recognition complex (ORC) is the first to bind to the
DNA and serves as a nucleation point for the recruitment of other factors. Cdc6 and
Cdt1 are recruited to the complex and facilitate the loading of the family of six
MCM proteins (Maiorano et al. 2000; Nishitani et al. 2000, 2001; Rialland et al.
2002). Cyclin E expression is induced, and the active cdk2/cyclin E kinase promotes
the transition into S phase. Cyclin E also facilitates the formation of pre-RC in a
kinase-independent fashion through physical interaction with Cdt1 and the MCM
proteins (Geng et al. 2003; Ekholm-Reed et al. 2004; Geng et al. 2007).
At the beginning of S phase, cyclin A accumulates and forms an active kinase
complex with cdk2. Regulation of cyclin A occurs at both the protein and mRNA
levels (Glotzer et al. 1991; Henglein et al. 1994; Desdouets et al. 1995; Schulze
et al. 1995; Zwicker et al. 1995; Zwicker and Muller 1997; Bottazzi et al. 2001;
Tessari et al. 2003). In S phase, cdk2/cyclin A and cdc7/Dbf4 complexes mediate
the firing of DNA origins of replication and promote DNA replication (for review
see Nishitani and Lygerou 2002; Machida et al. 2005). At the same time, Cdt1 is
released from the replication complex, and its binding to the ORC is prevented by
geminin, a protein that accumulates during S, G
2
, and M phases. This ensures that
DNA replication initiates at each origin only once and prevents polyploidy.
Following the complete replication of the parental chromosomes, cells enter G
2
and
proteins involved in mitosis (M phase) begin to accumulate. Cdk1 and cyclin B play a
major role at this point. Cyclin B first associates with cdk1 to form an inactive complex.
At the G
2
/ M transition, dephosphorylation of cdk1 by cdc25 phosphatase induces the
activation of the kinase, which in turn promotes the onset of mitosis (Millar and Russell
1992). Cdk1/cyclin B and cdk1/cyclin A complexes phosphorylate many substrates that
facilitate condensation of the chromosomes, disassembly of the nuclear envelope, and
modification of the cell's architecture to ensure that there is an ordered and even segre-
gation of the chromosomes to the daughter cells. Inactivation of the cdk1 complexes
occurs during mitosis by degradation of cyclins A and B through the ubiquitin-depend-
ent proteolytic pathway involving the anaphase-promoting complex (APC) E3 ubiquitin
ligase and the proteasome (Glotzer et al. 1991). The APC also ubiquitinates geminin
and targets it for proteasome degradation as the cells exit mitosis, thereby allowing
loading of the pre-RCs onto the chromatin of daughter cells during G
1
phase (McGarry
and Kirschner 1998). This degradation of the cyclins and geminin continues until the
onset of S phase (Brandeis and Hunt 1996).
Several checkpoints throughout the cell cycle ensure that progression will halt
if there is DNA damage or aberrant spindle formation (for review see Lukas et al.
2004), thus protecting the integrity of the genome. The tumor suppressors p53
and the Rb family of pocket proteins (Rb, p107, and p130) are the best studied of
these checkpoint sentinels. The Rb proteins, in their hypophosphorylated forms,
bind to the E2F family of transcription factors, which then function as transcrip-
tional repressors. Phosphorylation of the Rb proteins in late G
1
dissociates these
complexes, allowing the E2F factors to activate transcription of multiple genes,
many of which encode proteins required for DNA replication (Dyson 1998). p53
