  Yeast cells produce daughter cells through a DNA replication and mitosis
cycle associated with checkpoints and governed by the cell cycle regulatory
network. To ensure genome stability and genetic information inheritance, this
regulatory network must be dynamically robust against various fluctuations.
Here we construct a simplified cell cycle model for a budding yeast to
investigate the underlying mechanism that ensures robustness in this process
containing sequential tasks (DNA replication and mitosis). We first establish a
three-variable model and select a parameter set that qualitatively describes
the yeast cell cycle process. Then, through nonlinear dynamic analysis, we
demonstrate that the yeast cell cycle process is an excitable system driven by
a sequence of saddle-node bifurcations with ghost effects. We further show that
the yeast cell cycle trajectory is globally attractive with modularity in both
state and parameter space, while the convergent manifold provides a suitable
control state for cell cycle checkpoints. These results not only highlight a
regulatory mechanism for executing successive cell cycle processes, but also
provide a possible strategy for the synthetic network design of sequential-task
processes.
