Collective behavior in gene regulation: Metabolic clocks and cross-talking

Biological functions governed by the circadian clock are the evident result of the entrainment operated by the earth's day and night cycle on living organisms. However, the circadian clock is not unique, and cells and organisms possess many other cyclic activities. These activities are difficult to observe if carried out by single cells and the cells are not coordinated but, if they can be detected, cell-to-cell cross-talk and synchronization among cells must exist. Some of these cycles are metabolic and cell synchronization is due to small molecules acting as metabolic messengers. We propose a short survey of cellular cycles, paying special attention to metabolic cycles and cellular cross-talking, particularly when the synchronization of metabolism or, more generally, cellular functions are concerned. Questions arising from the observation of phenomena based on cell communication and from basic cellular cycles are also proposed.