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Circadian clock

A circadian clock, or circadian oscillator, also known as one’s internal alarm clock is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time.

Such a clock's in vivo period is necessarily almost exactly 24 hours (the earth's current solar day). In most living organisms, internally synchronized circadian clocks make it possible for the organism to anticipate daily environmental changes corresponding with the day–night cycle and adjust its biology and behavior accordingly.

The term circadian derives from the Latin circa (about) dies (a day), since when taken away from external cues (such as environmental light), 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.[1]

The normal body clock oscillates with an endogenous 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:

  • 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.[2] The circadian clock is intertwined with most cellular metabolic processes and it is affected by organism aging.[3] The basic molecular mechanisms of the biological clock have been defined in vertebrate species, Drosophila melanogaster, plants, fungi, bacteria,[4][5] and presumably also in Archaea.[6][7][8]

In 2017, the Nobel Prize in Physiology or Medicine was awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young "for their discoveries of molecular mechanisms controlling the circadian rhythm" in fruit flies.[9]

  1. ^ Cromie W (1999-07-15). "Human Biological Clock Set Back an Hour". Harvard Gazette. Retrieved 2015-07-29.
  2. ^ Ueda HR, Hayashi S, Chen W, Sano M, Machida M, Shigeyoshi Y, et al. (February 2005). "System-level identification of transcriptional circuits underlying mammalian circadian clocks". Nature Genetics. 37 (2): 187–192. doi:10.1038/ng1504. PMID 15665827. S2CID 18112337.
  3. ^ Tevy MF, Giebultowicz J, Pincus Z, Mazzoccoli G, Vinciguerra M (May 2013). "Aging signaling pathways and circadian clock-dependent metabolic derangements". Trends in Endocrinology and Metabolism. 24 (5): 229–237. doi:10.1016/j.tem.2012.12.002. PMC 3624052. PMID 23299029.
  4. ^ Harmer SL, Panda S, Kay SA (2001). "Molecular bases of circadian rhythms". Annual Review of Cell and Developmental Biology. 17: 215–253. doi:10.1146/annurev.cellbio.17.1.215. PMID 11687489.
  5. ^ Lowrey PL, Takahashi JS (2004). "Mammalian circadian biology: elucidating genome-wide levels of temporal organization". Annual Review of Genomics and Human Genetics. 5: 407–441. doi:10.1146/annurev.genom.5.061903.175925. PMC 3770722. PMID 15485355.
  6. ^ Edgar RS, Green EW, Zhao Y, van Ooijen G, Olmedo M, Qin X, et al. (May 2012). "Peroxiredoxins are conserved markers of circadian rhythms". Nature. 485 (7399): 459–464. Bibcode:2012Natur.485..459E. doi:10.1038/nature11088. PMC 3398137. PMID 22622569.
  7. ^ Dvornyk V, Vinogradova O, Nevo E (March 2003). "Origin and evolution of circadian clock genes in prokaryotes". Proceedings of the National Academy of Sciences of the United States of America. 100 (5): 2495–2500. Bibcode:2003PNAS..100.2495D. doi:10.1073/pnas.0130099100. PMC 151369. PMID 12604787.
  8. ^ Whitehead K, Pan M, Masumura K, Bonneau R, Baliga NS (2009). "Diurnally entrained anticipatory behavior in archaea". PLOS ONE. 4 (5): e5485. Bibcode:2009PLoSO...4.5485W. doi:10.1371/journal.pone.0005485. PMC 2675056. PMID 19424498.
  9. ^ "The Nobel Prize in Physiology or Medicine 2017". www.nobelprize.org. Retrieved 2017-10-06.
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