Editing Circadian rhythm (section)

==Origin==
{{missing information|section|independently evolved four times [PMID 11533719]|date=September 2021}}
Circadian rhythms allow organisms to anticipate and prepare for precise and regular environmental changes. They thus enable organisms to make better use of environmental resources (e.g. light and food) compared to those that cannot predict such availability. It has therefore been suggested that circadian rhythms put organisms at a selective advantage in evolutionary terms. However, rhythmicity appears to be as important in regulating and coordinating ''internal'' metabolic processes, as in coordinating with the ''environment''.<ref>{{cite journal | vauthors = Sharma VK | title = Adaptive significance of circadian clocks | journal = Chronobiology International | volume = 20 | issue = 6 | pages = 901–919 | date = November 2003 | pmid = 14680135 | doi = 10.1081/CBI-120026099 | s2cid = 10899279 }}</ref> This is suggested by the maintenance (heritability) of circadian rhythms in fruit flies after several hundred generations in constant laboratory conditions,<ref>{{primary source inline|date=November 2013}} {{cite journal | vauthors = Sheeba V, Sharma VK, Chandrashekaran MK, Joshi A | title = Persistence of eclosion rhythm in Drosophila melanogaster after 600 generations in an aperiodic environment | journal = Die Naturwissenschaften | volume = 86 | issue = 9 | pages = 448–449 | date = September 1999 | pmid = 10501695 | doi = 10.1007/s001140050651 | s2cid = 13401297 | bibcode = 1999NW.....86..448S }}</ref> as well as in creatures in constant darkness in the wild, and by the experimental elimination of behavioral—but not physiological—circadian rhythms in [[quail]].<ref>{{primary source inline|date=November 2013}} {{cite journal | vauthors = Guyomarc'h C, Lumineau S, Richard JP | title = Circadian rhythm of activity in Japanese quail in constant darkness: variability of clarity and possibility of selection | journal = Chronobiology International | volume = 15 | issue = 3 | pages = 219–230 | date = May 1998 | pmid = 9653576 | doi = 10.3109/07420529808998685 }}</ref><ref>{{primary source inline|date=November 2013}} {{cite journal | vauthors = Zivkovic BD, Underwood H, Steele CT, Edmonds K | title = Formal properties of the circadian and photoperiodic systems of Japanese quail: phase response curve and effects of T-cycles | journal = Journal of Biological Rhythms | volume = 14 | issue = 5 | pages = 378–390 | date = October 1999 | pmid = 10511005 | doi = 10.1177/074873099129000786 | s2cid = 13390422 | doi-access = free }}</ref>

What drove circadian rhythms to evolve has been an enigmatic question. Previous hypotheses emphasized that photosensitive proteins and circadian rhythms may have originated together in the earliest cells, with the purpose of protecting replicating DNA from high levels of damaging [[ultraviolet]] radiation during the daytime. As a result, replication was relegated to the dark. However, evidence for this is lacking: in fact the simplest organisms with a circadian rhythm, the cyanobacteria, do the opposite of this: they divide more in the daytime.<ref>{{cite journal | vauthors = Mori T, Johnson CH | title = Independence of circadian timing from cell division in cyanobacteria | journal = Journal of Bacteriology | volume = 183 | issue = 8 | pages = 2439–2444 | date = April 2001 | pmid = 11274102 | pmc = 95159 | doi = 10.1128/JB.183.8.2439-2444.2001 }}</ref> Recent studies instead highlight the importance of co-evolution of [[redox]] proteins with circadian oscillators in all three domains of life following the [[Great Oxidation Event]] approximately 2.3 billion years ago.<ref name="Edgar 459–464"/><ref name="Bass 348–356"/> The current view is that circadian changes in environmental oxygen levels and the production of [[reactive oxygen species]] (ROS) in the presence of daylight are likely to have driven a need to evolve circadian rhythms to preempt, and therefore counteract, damaging [[Redox|redox reactions]] on a daily basis.

The simplest known [[circadian clock]]s are [[bacterial circadian rhythms]], exemplified by the prokaryote [[cyanobacteria]]. Recent research has demonstrated that the circadian clock of ''[[Synechococcus elongatus]]'' can be reconstituted ''in vitro'' with just the three proteins ([[KaiA]], [[KaiB]], [[KaiC]])<ref>{{cite journal | vauthors = Hut RA, Beersma DG | title = Evolution of time-keeping mechanisms: early emergence and adaptation to photoperiod | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 366 | issue = 1574 | pages = 2141–2154 | date = July 2011 | pmid = 21690131 | pmc = 3130368 | doi = 10.1098/rstb.2010.0409 }}</ref> of their central oscillator. This clock has been shown to sustain a 22-hour rhythm over several days upon the addition of [[Adenosine triphosphate|ATP]]. Previous explanations of the [[prokaryotic]] circadian timekeeper were dependent upon a DNA transcription/translation feedback mechanism.{{citation needed|date=November 2013}}

A defect in the human homologue of the ''[[Drosophila]]'' "[[period (gene)|period]]" gene was identified as a cause of the sleep disorder FASPS ([[Familial advanced sleep phase syndrome]]), underscoring the conserved nature of the molecular circadian clock through evolution. Many more genetic components of the biological clock are now known. Their interactions result in an interlocked feedback loop of gene products resulting in periodic fluctuations that the cells of the body interpret as a specific time of the day.<ref>{{cite journal | vauthors = Dubowy C, Sehgal A | title = Circadian Rhythms and Sleep in ''Drosophila melanogaster'' | journal = Genetics | volume = 205 | issue = 4 | pages = 1373–1397 | date = April 2017 | pmid = 28360128 | pmc = 5378101 | doi = 10.1534/genetics.115.185157 }}</ref>

It is now known that the molecular circadian clock can function within a single cell. That is, it is cell-autonomous.<ref>{{MEDRS|date=November 2013}} {{cite journal | vauthors = Nagoshi E, Saini C, Bauer C, Laroche T, Naef F, Schibler U | title = Circadian gene expression in individual fibroblasts: cell-autonomous and self-sustained oscillators pass time to daughter cells | journal = Cell | volume = 119 | issue = 5 | pages = 693–705 | date = November 2004 | pmid = 15550250 | doi = 10.1016/j.cell.2004.11.015 | s2cid = 15633902 | doi-access = free }}</ref> This was shown by [[Gene Block]] in isolated mollusk basal retinal neurons (BRNs).<ref>{{primary source inline|date=November 2013}} {{cite journal | vauthors = Michel S, Geusz ME, Zaritsky JJ, Block GD | title = Circadian rhythm in membrane conductance expressed in isolated neurons | journal = Science | volume = 259 | issue = 5092 | pages = 239–241 | date = January 1993 | pmid = 8421785 | doi = 10.1126/science.8421785 | bibcode = 1993Sci...259..239M | url = https://zenodo.org/record/1231259 }}</ref> At the same time, different cells may communicate with each other resulting in a synchronized output of electrical signaling. These may interface with [[endocrine gland]]s of the brain to result in periodic release of hormones. The receptors for these hormones may be located far across the body and synchronize the peripheral clocks of various organs. Thus, the information of the time of the day as relayed by the [[Human eye|eye]]s travels to the clock in the brain, and, through that, clocks in the rest of the body may be synchronized. This is how the timing of, for example, sleep/wake, body temperature, thirst, and appetite are coordinately controlled by the biological clock.<ref>{{cite journal | vauthors = Refinetti R | title = The circadian rhythm of body temperature | journal = Frontiers in Bioscience | volume = 15 | issue = 2 | pages = 564–594 | date = January 2010 | pmid = 20036834 | doi = 10.2741/3634 | s2cid = 36170900 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Scheer FA, Morris CJ, Shea SA | title = The internal circadian clock increases hunger and appetite in the evening independent of food intake and other behaviors | journal = Obesity | volume = 21 | issue = 3 | pages = 421–423 | date = March 2013 | pmid = 23456944 | pmc = 3655529 | doi = 10.1002/oby.20351 }}</ref>