Circadian oscillators of Drosophila and mammals.

Animals, plants, fungi and even some prokaryotic organisms display daily rhythms in behavior, physiology, metabolic activity and gene expression. These rhythms are not passively driven by environmental cycles (e.g. light and temperature) but are controlled by endogenous circadian clocks that keep time even in the absence of environmental time cues. Environmental cycles are nevertheless required to entrain these clocks so that they activate rhythmic processes at the appropriate time of day. In animals, circadian clocks reside in a variety of tissues, including the brain, sensory structures and a number of internal organs (Glossop and Hardin, 2002). Although all clocks drive rhythms in gene expression, some control tissue-autonomous rhythms in physiology and metabolism, whereas others form networks of clock tissues that control rhythms in behavior (BellPedersen et al., 2005; Chang, 2006).