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Abstract

The cell cycle is an ordered sequence of events culminating in the formation of two identical daughter cells. Ensuring the order of the events is essential for genomic integrity and cell proliferation. The sudden and synchronous splitting of chromosomes during the metaphase to anaphase transition is one of the visually most dramatic events of the cell cycle. The transition is driven by the activity of the anaphase promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase, which initiates the destruction of its two essential targets, cyclin B and securin. Cyclin B degradation inactivates the cyclin-dependent kinase 1 (CDK1) and triggers a multitude of processes during mitotic exit. Degradation of securin releases separase from its inhibition. Active separase subsequently triggers the highly synchronous separation of sister chromatids. The separation is irreversible and therefore needs to be highly accurate and tightly coordinated with mitotic exit. Yet, little is known about the molecular events that determine the timing of the single processes and coordinate the individual processes relative to each other. I have systematically studied the dynamics of the metaphase to anaphase transition in the fission yeast Schizosaccharomyces pombe using live cell imaging assays with high temporal resolution. My analysis shows that the synchronicity of sister chromatid separation directly depends on the degradation kinetic of its upstream regulator securin, which suggests the absence of additional feedback regulation. Stochastic processes dominate the order in which sister chromatids separate, but an intrinsic bias in chromosome segregation exists, which is enhanced by decreased separase activity or securin degradation rates. Sister chromatid separation has to be tightly coordinated with the cyclin B degradation-driven processes of mitotic exit. I find the temporal order of events during the metaphase to anaphase transition to be remarkably robust against changes in securin and cyclin B, even if the overall timing of the respective events is severely altered. Competition of securin and cyclin B for the shared degradation machinery as well as systematic variability in the protein thresholds at which certain events occur contribute to the observed temporal robustness. I further investigated the consequences of potential misregulation between securin and cyclin B degradation-dependent events and show that high CDK1 activity at anaphase results in untimely destabilization of chromosome attachment, activation of the mitotic checkpoint and inhibition of the APC/C. Yet, we find that inhibition of the APC/C occurs with slow kinetics, which might provide an additional buffer against the detrimental consequences of such a loss in coordination.