Introduction

One cannot help but be impressed by the engineering, by evolution, of the cellular machinery. The cellular program that governs cell cycle and cell development does so robustly in the face of a fluctuating environment and energy sources. It integrates numerous signals, chemical and otherwise, each of which contains, perhaps, incomplete information of events that the cell must track in order to determine which biochemical subroutines to bring on- and off-line, or slow down and speed up. These signals, which are derived from internal processes, other cells and changes in the extracellular medium, arrive asynchronously and are multi-valued; that is, they are not merely ‘on’ or ‘off’ but have many values of meaning to the cellular apparatus. The cellular program also has a memory of signals that it has received in the past, and of its own particular history as written in the complement and concentrations of chemicals contained in the cell at any instant. These characteristics of robust, integrative, asynchronous, sequential and analog control are the hallmark of cellular control systems. Below, arguments will be made that there is also another characteristic of such control systems: there is often an irreducible nondeterminism in their function that, besides leading to differences in timing of cellular events across an otherwise genetically identical (isogenic) cell population, can also lead to profound differences in cell fate.