A critical property of circadian oscillators is temperature compensation, or the stability of the period of the clock over a wide range of temperatures in the physiological range. The ‘time-givers’ (or z eitgebers) are generally the light/dark and temperature cycles, although rhythms in nutrient availability may also act as a resetting signal in some organisms. In order to remain synchronized with the environment, circadian clocks are reset or entrained by specific cues that relay information about the external time. The clock maintains an internal estimate of the passage of time and schedules physiological processes to occur at appropriate points in the day. The clocks in different kingdoms are composed of a network of transcription factors arranged in interlocking negative-feedback loops. Ĭircadian rhythms are governed by an internal timekeeper referred to as the circadian clock. Circadian outputs range from the relatively subtle, such as rhythms in photosynthesis, to the comparatively overt, such as rhythms in animal activity, plant leaf movement and floral opening. Circadian rhythms have since been observed in most eukaryotes and in many prokaryotes. Circadian rhythms were first measured in 1729 by the French biologist de Mairan, who noted that a heliotrope plant (probably Mimosa) sustained rhythms in leaf movement in continuous darkness. Persistent rhythms that have periodicities matching that of the Earth's rotation on its axis (approx. 24 h) are termed circadian. Some of these fluctuations occur solely in response to environmental factors, such as light/dark cycles, but a subset persist under constant conditions. Many aspects of the behaviour and physiology of plants and animals fluctuate over the course of each day.
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