The origin and consequences of noise in biochemical systems

Surprising things happen when we take the discreteness of molecule number seriously, abandoning the notion that chemical concentrations may be treated as continuous variables. Here we will show how ideas of discreteness force us into dealing with issues of noise and randomness. We will arrive at a probabilistic description of reaction kinetics which, in the limit of large numbers, will reproduce the familiar reactionrate description. We will show how probabilistic systems may be treated numerically using Monte Carlo simulations, and use such simulations to investigate how frequently large random fluctuations can change the state of a bistable system. This will help us understand the stability and spontaneous switching rates of actual biological switches such as those which underlie cell fate determination during development, or long-term potentiation in neurons.