For several years, we have been investigating the hypothesis that some biological control systems, such as that of the cell cycle, can be modeled as asynchronous circuits.  Most digital systems are synchronous, meaning that there is a regular global clock that keeps everything in time.  Asynchronous circuts are an alternative style digital circuit where individual components are assumed to react after arbitrary delays.  It is unlikely that cells have a global clock to control timing, and so, if they are digital, they must be more like asynchronous than synchronous circuits.

However, we have recently observed discrepancies between our digital models and the actual behavior of bacterial cells in experiments.  In several cases, the model predicts that knocking out a key signal would halt the cell cycle at a particular stage, but, in experiments, the cell cycle is temporarily suspended and then resumes.  This behavior is interesting because it makes cells robust to inactivation or deletion of key genes.

One explanation for this behavior is that the digital abstraction breaks down because "false" is not quite false.  Transcription sometimes continues at low levels, leading to slow buildup of a transcription factor which, under normal circumstances, has no effect, but, when the cell cycle is abnormally halted, can result in resumption of a stalled process.  This effect can occur in both continuous and stochastic models.