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The circadian clock, or circadian oscillator, in most living things makes it possible for organisms to coordinate their biology and behavior with daily environmental changes in the day-night cycle. The term circadian derives from the Latin ''circa'' (about) ''diem'' (a day), since when taken away from external cues (such as the day-night cycle), they do not run to exactly 24 hours. Clocks in humans in a lab in constant low light, for example, will average about 24.2 hours per day, rather than 24 hours exactly. Hence the term circadian. The normal body clock oscillates with a period of exactly 24 hours when it receives daily corrective signals from the environment, primarily daylight and darkness. Circadian clocks are the central mechanisms that drive circadian rhythms. They consist of three major components: # A central biochemical oscillator with a period of about 24 hours that keeps time # A series of input pathways to this central oscillator to allow entrainment of the clock # A series of output pathways tied to distinct phases of the oscillator that regulate overt rhythms in biochemistry, physiology, and behavior throughout an organism. The clock is reset as an organism senses environmental time cues of which the primary one is light. Circadian oscillators are ubiquitous in tissues of the body where they are synchronized by both endogenous and external signals to regulate transcriptional activity throughout the day in a tissue-specific manner. The circadian clock is intertwined with most cellular metabolic processes and it is affected by organism aging. The basic molecular mechanisms of the biological clock have been defined in vertebrate species, ''Drosophila melanogaster'', plants, fungi, bacteria, and presumably also in Archaea. == Transcriptional and non-transcriptional control == Evidence for a genetic basis of circadian rhythms in higher eukaryotes began with the discovery of the period (''per'') locus in ''Drosophila melanogaster'' from forward genetic screens completed by Ron Konopka and Seymour Benzer in 1971. Through the analysis of ''per'' circadian mutants and additional mutations on ''Drosophila'' clock genes, a model encompassing positive and negative autoregulatory feedback loops of transcription and translation has been proposed. Core circadian 'clock' genes are defined as genes whose protein products are necessary components for the generation and regulation of circadian rhythms. Similar models have been suggested in mammals and other organisms. Studies in cyanobacteria, however, changed our view of the clock mechanism, since it was found by Kondo and colleagues that these single-cell organisms could maintain accurate 24 hour timing in the absence of transcription, i.e. there was no requirement for a transcription-translation autoregulatory feedback loop for rhythms. Moreover, this clock was reconstructed in a test tube (i.e., in the absence of any cell components), proving that accurate 24 hour clocks can be formed without the need for genetic feedback circuits.〔 However, this mechanism was only applicable to cyanobacteria and not generic. In 2011, a major breakthrough in understanding came from the Reddy laboratory at the University of Cambridge. This group discovered circadian rhythms in redox proteins (peroxiredoxins) in cells that lacked a nucleus - human red blood cells. In these cells, there was no transcription or genetic circuits, and therefore no feedback loop. Similar observations were made in another system, and subsequently in mouse red blood cells. More importantly, redox oscillations as demonstrated by peroxiredoxin rhythms have now been seen in multiple distant kingdoms of life (eukaryotes, bacteria and archaea), covering the evolutionary tree.〔 Therefore, redox clocks look to be the grandfather clock, and genetic feedback circuits the major output mechanisms to control cell and tissue physiology and behavior. Therefore, the model of the clock has to be considered as a product of an interaction between both transcriptional circuits and non-transcriptional elements such as redox oscillations and protein phosphorylation cycles. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「circadian clock」の詳細全文を読む スポンサード リンク
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