A Wheel within a Wheel: Temperature Compensation of the Circadian Clock
نویسنده
چکیده
Circadian clocks influence numerous physiological and developmental functions in most if not all living organisms. Questions of how biological clocks operate have long intriguedbiologists.Howdocircadianclocks keep accurate time in the midst of constantly changing internal and external conditions, such as seasonally varying light/ dark cycles, large diurnal and seasonal temperature shifts, and a myriad of physiological and biochemical changes during development? Identification and analysis of core components of the clock in diverse organisms, including animals, plants, and fungi, have shown that the clock mechanism likely consists of interlocking negative and positive feedback loops. However, there is very little conservation among the core clock components between these groups, suggesting that numerous clocks have evolved independently and that circadian rhythmicity is an important adaptive feature of most organisms. Defining characteristics of circadian rhythms are that they are free-running, maintaining a periodicity of ;24 h under constant external conditions; they can be entrained by external cues, principally light and temperature cycles; and they exhibit temperature compensation, maintaining relatively constant periodicity over a broad range of physiological temperatures. Most biochemical processes speed up as the temperature rises; this would wreak havoc with the timing of daily rhythms, and circadian clocks have evolved to avoid this problem. Circadian clocks are often modeled as consisting of three main components: input pathways, the central oscillator or core clock mechanism, and output pathways resulting in circadian rhythms. Although it has been useful to view circadian systems in this fashion, it has become clear that these are likely arbitrary groupings with substantial overlap and complex interconnections (McClung et al., 2002). Newer models of clocks may begin to resemble complex mechanical clocks with their multiple interconnecting wheels and wheels within wheels (see figure panel A). If we imagine additional inputs and outputs feeding in and out of each of these wheels, we may begin to approximate the workings of a biological clock (see figure panel B).
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تاریخ انتشار 2006