LAUSR

Circadian clocks are biological timing systems that synchronize physiology and behavior with the daily light/dark cycle. Circadian timekeeping is based on cell-autonomous feedback loops that oscillate with a specific endogenous period under constant conditions. Recurrent environmental stimuli such as light and temperature synchronize (entrain) cell-based circadian oscillators under physiological conditions. In mammals, light signals from the retina are transduced to the suprachiasmatic nucleus in the hypothalamus, which consists of a network of tightly coupled robustly oscillating clock neurons. These signals are in turn relayed to cellular clocks throughout the body. Most of the components of cellular circadian clocks are known, and the major interactions driving the feedback loops have been identified. However, the principles by which the clock components accurately measure time at the molecular level are only partially understood. In this review, we focus on the molecular mechanisms identified thus far that contribute to slow biochemical processes underlying the measurement of time on a 24-hour scale in the circadian clocks of the cyanobacterium Synechococcus elongatus , the filamentous fungus Neurospora crassa , and the metazoans Drosophila melanogaster , Mus musculus and humans.