Circadian oscillator

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.

The normal body clock oscillates with a period of exactly 24 hours, it entrains, when it receives sufficient 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:

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.

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.

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