Genetics and Molecular Biology of Circadian Clocks

Abstract

Oscillations characterize a state of temporal order in nonlinear dynamic systems far from equilibrium. The oscillatory state requires interactive structures (activating or inhibiting interactions) between the system components, energy dissipation and a set of specific conditions. Living systems demonstrate a wide spectrum of oscillations, ranging from action potentials to circadian and annual rhythms (Rensing and Jaeger 1985). An even wider spectrum of oscillations exists in nonliving, i.e., physical and chemical, systems, showing that the oscillatory state as such is not confined to living systems and does not require genetic information. Cells and organisms, however, have made use of the available oscillatory mechanisms in the course of evolution, for example, for signal transmission (action potentials, pulsatile hormone release, intracellular calcium waves), for locomotory or pumping mechanisms (cilia, leg, wing movements, heartbeat, breathing, peristaltic muscle contractions) and for “clock” functions (circadian, lunar and annual rhythms). The term “clock” has been introduced mainly as a metaphor for the basic mechanism of the latter rhythms because it directs a number of processes (“hands” of the clock) in a rhythmic fashion. These rhythms probably allow optimal adaptation of the organism to the periodic changes in the environment. They also establish a state of temporal order that may per se represent a functional advantage. In man, a multitude of circadian rhythms in almost every functional variable have been described, whose maxima and minima map to defined phases of sleep and wakefulness (or night and day, respectively).