  Gene regulatory dynamics is governed by molecular processes and therefore
exhibits an inherent stochasticity. However, for the survival of an organism it
is a strict necessity that this intrinsic noise does not prevent robust
functioning of the system. It is still an open question how dynamical stability
is achieved in biological systems despite the omnipresent fluctuations. In this
paper we investigate the cell-cycle of the budding yeast Saccharomyces
cerevisiae as an example of a well-studied organism.
  We study a genetic network model of eleven genes that coordinate the
cell-cycle dynamics using a modeling framework which generalizes the concept of
discrete threshold dynamics. By allowing for fluctuations in the
transcription/translation times, we introduce noise in the model, accounting
for the effects of biochemical stochasticity. We study the dynamical attractor
of the cell cycle and find a remarkable robustness against fluctuations of this
kind. We identify mechanisms that ensure reliability in spite of fluctuations:
'Catcher' states and persistence of activity levels contribute significantly to
the stability of the yeast cell cycle despite the inherent stochasticity.
