Circadian clocks: What makes them tick?

In the not too distant past, it was common belief that rhythms in the physi cal environment were the driving force, to which organisms responded passiv ely, for the observed daily rhythms in measurable physiological and behavio ral variables. The demonstration that this was not the case, but that both plants and animals possess accurate endogenous time-measuring machinery (i. e., circadian clocks) contributed to heightening interest in the study of c ircadian biological rhythms. In the last few decades, flourishing studies h ave demonstrated that most organisms have at least one internal circadian t imekeeping device that oscillates with a period close to that of the astron omical day (i.e., 24h). To date, many of the physiological mechanisms under lying the control of circadian rhythmicity have been described, while the i mprovement of molecular biology techniques has permitted extraordinary adva ncements in our knowledge of the molecular components involved in the machi nery underlying the functioning of circadian clocks in many different organ isms, man included. In this review, we attempt to summarize our current und erstanding of the genetic and molecular biology of circadian clocks in cyan obacteria, fungi, insects, and mammals.