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Concept
Télophase
Résumé
Telophase () is the final stage in both meiosis and mitosis in a eukaryotic cell. During telophase, the effects of prophase and prometaphase (the nucleolus and nuclear membrane disintegrating) are reversed. As chromosomes reach the cell poles, a nuclear envelope is re-assembled around each set of chromatids, the nucleoli reappear, and chromosomes begin to decondense back into the expanded chromatin that is present during interphase. The mitotic spindle is disassembled and remaining spindle microtubules are depolymerized. Telophase accounts for approximately 2% of the cell cycle's duration.
Cytokinesis typically begins before late telophase and, when complete, segregates the two daughter nuclei between a pair of separate daughter cells.
Telophase is primarily driven by the dephosphorylation of mitotic cyclin-dependent kinase (Cdk) substrates.
The phosphorylation of the protein targets of M-Cdks (Mitotic Cyclin-dependent Kinases) drives spindle assembly, chromosome condensation and nuclear envelope breakdown in early mitosis. The dephosphorylation of these same substrates drives spindle disassembly, chromosome decondensation and the reformation of daughter nuclei in telophase. Establishing a degree of dephosphorylation permissive to telophase events requires both the inactivation of Cdks and the activation of phosphatases.
Cdk inactivation is primarily the result of the destruction of its associated cyclin. Cyclins are targeted for proteolytic degradation by the anaphase promoting complex (APC), also known as the cyclosome, a ubiquitin-ligase. The active, CDC20-bound APC (APC/CCDC20) targets mitotic cyclins for degradation starting in anaphase. Experimental addition of non-degradable M-cyclin to cells induces cell cycle arrest in a post-anaphase/pre-telophase-like state with condensed chromosomes segregated to cell poles, an intact mitotic spindle, and no reformation of the nuclear envelope. This has been shown in frog (Xenopus) eggs, fruit flies (Drosophilla melanogaster), budding (Saccharomyces cerevisiae) and fission (Schizosaccharomyces pombe) yeast, and in multiple human cell lines.
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