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Concept
S phase
Résumé
S phase (Synthesis Phase) is the phase of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during S-phase are tightly regulated and widely conserved.
G1/S transition
Entry into S-phase is controlled by the G1 restriction point (R), which commits cells to the remainder of the cell-cycle if there is adequate nutrients and growth signaling. This transition is essentially irreversible; after passing the restriction point, the cell will progress through S-phase even if environmental conditions become unfavorable.
Accordingly, entry into S-phase is controlled by molecular pathways that facilitate a rapid, unidirectional shift in cell state. In yeast, for instance, cell growth induces accumulation of Cln3 cyclin, which complexes with the cyclin dependent kinase CDK2. The Cln3-CDK2 complex promotes transcription of S-phase genes by inactivating the transcriptional repressor Whi5. Since upregulation of S-phase genes drive further suppression of Whi5, this pathway creates a positive feedback loop that fully commits cells to S-phase gene expression.
A remarkably similar regulatory scheme exists in mammalian cells. Mitogenic signals received throughout G1-phase cause gradual accumulation of cyclin D, which complexes with CDK4/6. Active cyclin D-CDK4/6 complex induces release of E2F transcription factor, which in turn initiates expression of S-phase genes. Several E2F target genes promote further release of E2F, creating a positive feedback loop similar to the one found in yeast.
DNA replication
Throughout M phase and G1 phase, cells assemble inactive pre-replication complexes (pre-RC) on replication origins distributed throughout the genome. During S-phase, the cell converts pre-RCs into active replication forks to initiate DNA replication. This process depends on the kinase activity of Cdc7 and various S-phase CDKs, both of which are upregulated upon S-phase entry.
Source officielle
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Concepts associés (18)
G2 phase
DISPLAYTITLE:G2 phase G2 phase, Gap 2 phase, or Growth 2 phase, is the third subphase of interphase in the cell cycle directly preceding mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated. G2 phase ends with the onset of prophase, the first phase of mitosis in which the cell’s chromatin condenses into chromosomes. G2 phase is a period of rapid cell growth and protein synthesis during which the cell prepares itself for mitosis.
G1 phase
DISPLAYTITLE:G1 phase The G1 phase, gap 1 phase, or growth 1 phase, is the first of four phases of the cell cycle that takes place in eukaryotic cell division. In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis. G1 phase ends when the cell moves into the S phase of interphase. Around 30 to 40 percent of cell cycle time is spent in the G1 phase.
Kinase dépendante des cyclines
vignette|350px|Schéma du cycle cellulaire. Cercle extérieur: I=Interphase, M=Mitose; cercle intérieur: M=Mitose et cytocinèse; G1=Phase G1; S=Phase S; G2=Phase G2. La durée de la mitose par rapport aux autres phases du cycle cellulaire est exagérée dans ce diagramme Les kinases dépendantes des cyclines (en anglais, cyclin-dependent kinase ou CDK) sont une famille de protéines kinases qui jouent un rôle majeur dans la régulation du cycle cellulaire.