Editing Cell nucleus (section)

==Evolution==
As the major defining characteristic of the eukaryotic cell, the nucleus's [[evolution]]ary origin has been the subject of much speculation. Four major hypotheses have been proposed to explain the existence of the nucleus, although none have yet earned widespread support.<ref name="Pennisi">{{cite journal | vauthors = Pennisi E | author-link = Elizabeth Pennisi | title = Evolutionary biology. The birth of the nucleus | journal = Science | volume = 305 | issue = 5685 | pages = 766–8 | date = August 2004 | pmid = 15297641 | doi = 10.1126/science.305.5685.766 | s2cid = 83769250 | department = News }}</ref><ref name="Devos_2014">{{cite journal | vauthors = Devos DP, Gräf R, Field MC | title = Evolution of the nucleus | journal = Current Opinion in Cell Biology | volume = 28 | pages = 8–15 | date = June 2014 | issue = 100 | pmid = 24508984 | pmc = 4071446 | doi = 10.1016/j.ceb.2014.01.004 | department = Review }}</ref><ref name="López-García_2015">{{cite journal | vauthors = López-García P, Moreira D | title = Open Questions on the Origin of Eukaryotes | journal = Trends in Ecology & Evolution | volume = 30 | issue = 11 | pages = 697–708 | date = November 2015 | pmid = 26455774 | pmc = 4640172 | doi = 10.1016/j.tree.2015.09.005 | bibcode = 2015TEcoE..30..697L | department = Review }}</ref>

The first model known as the "syntrophic model" proposes that a [[Symbiosis|symbiotic]] relationship between the [[archaea]] and [[bacteria]] created the nucleus-containing eukaryotic cell. (Organisms of the Archaeal and Bacterial domains have no cell nucleus.<ref>{{cite book | vauthors = Hogan CM | chapter = Archaea | title = Encyclopedia of Earth | veditors = Monosson E, Cleveland C | publisher = National Council for Science and the Environment | location = Washington, DC. | date = 2010 | archive-url = https://web.archive.org/web/20110511133400/http://www.eoearth.org/article/Archaea?topic=49496 | archive-date = 11 May 2011 | chapter-url = http://www.eoearth.org/article/Archaea?topic=49496 }}</ref>) It is hypothesized that the symbiosis originated when ancient archaea similar to modern [[Methanogenesis|methanogenic]] archaea, invaded and lived within bacteria similar to modern [[myxobacteria]], eventually forming the early nucleus. This theory is analogous to the accepted theory for the origin of eukaryotic mitochondria and [[chloroplast]]s, which are thought to have developed from a similar endosymbiotic relationship between proto-eukaryotes and aerobic bacteria.<ref name="Margulis">{{cite book | last= Margulis | first= Lynn | author-link= Lynn Margulis |name-list-style= vanc | year= 1981 | title= Symbiosis in Cell Evolution | pages= [https://archive.org/details/symbiosisincelle00marg/page/206 206–227] | publisher= W. H. Freeman and Company | location= San Francisco | isbn= 978-0-7167-1256-5 | url= https://archive.org/details/symbiosisincelle00marg/page/206 }}</ref> One possibility is that the nuclear membrane arose as a new membrane system following the origin of [[mitochondrion|mitochondria]] in an [[Archaea|archaebacterial]] host.<ref name="Martin">{{cite journal |vauthors=Martin W |title=Archaebacteria (Archaea) and the origin of the eukaryotic nucleus |journal=Curr Opin Microbiol |volume=8 |issue=6 |pages=630–7 |date=December 2005 |pmid=16242992 |doi=10.1016/j.mib.2005.10.004 |url=}}</ref> The nuclear membrane may have served to protect the genome from damaging [[reactive oxygen species]] produced by the protomitochondria.<ref>Bernstein, H., Bernstein, C. (2017). Sexual Communication in Archaea, the Precursor to Eukaryotic Meiosis. In: Witzany, G. (eds) Biocommunication of Archaea. Springer, Cham. https://doi.org/10.1007/978-3-319-65536-9_7</ref> The archaeal origin of the nucleus is supported by observations that archaea and eukarya have similar genes for certain proteins, including [[histone]]s. Observations that myxobacteria are motile, can form multicellular complexes, and possess [[kinase]]s and [[G protein]]s similar to eukarya, support a bacterial origin for the eukaryotic cell.<ref name="Lopez-Garcia">{{cite journal | vauthors = López-García P, Moreira D | title = Selective forces for the origin of the eukaryotic nucleus | journal = BioEssays | volume = 28 | issue = 5 | pages = 525–33 | date = May 2006 | pmid = 16615090 | doi = 10.1002/bies.20413 | department = Review }}</ref>

A second model proposes that proto-eukaryotic cells evolved from bacteria without an endosymbiotic stage. This model is based on the existence of modern [[Planctomycetota]] bacteria that possess a nuclear structure with primitive pores and other compartmentalized membrane structures.<ref name="Fuerst">{{cite journal | vauthors = Fuerst JA | title = Intracellular compartmentation in planctomycetes | journal = Annual Review of Microbiology | volume = 59 | pages = 299–328 | year = 2005 | pmid = 15910279 | doi = 10.1146/annurev.micro.59.030804.121258 | department = Review }}</ref>  A similar proposal states that a eukaryote-like cell, the [[Eukaryote#External phylogeny: relationship to Archaea and Bacteria|chronocyte]], evolved first and [[phagocytosed]] archaea and bacteria to generate the nucleus and the eukaryotic cell.<ref name="Hartman">{{cite journal | vauthors = Hartman H, Fedorov A | title = The origin of the eukaryotic cell: a genomic investigation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 99 | issue = 3 | pages = 1420–5 | date = February 2002 | pmid = 11805300 | pmc = 122206 | doi = 10.1073/pnas.032658599 | department = Primary | bibcode = 2002PNAS...99.1420H | doi-access = free }}</ref>

The most controversial model, known as ''[[viral eukaryogenesis]]'', posits that the membrane-bound nucleus, along with other eukaryotic features, originated from the infection of a prokaryote by a virus. The suggestion is based on similarities between eukaryotes and viruses such as linear DNA strands, mRNA capping, and tight binding to proteins (analogizing histones to [[viral envelope]]s). One version of the proposal suggests that the nucleus evolved in concert with [[phagocytosis]] to form an early cellular "[[predator]]".<ref name="Bell">{{cite journal | vauthors = Bell PJ | title = Viral eukaryogenesis: was the ancestor of the nucleus a complex DNA virus? | journal = Journal of Molecular Evolution | volume = 53 | issue = 3 | pages = 251–6 | date = September 2001 | pmid = 11523012 | doi = 10.1007/s002390010215 | s2cid = 20542871 | department = Comment | bibcode = 2001JMolE..53..251L | doi-access = free }}</ref> Another variant proposes that eukaryotes originated from early archaea infected by [[poxvirus]]es, on the basis of observed similarity between the [[DNA polymerase]]s in modern poxviruses and eukaryotes.<ref name="Takemura">{{cite journal | vauthors = Takemura M | title = Poxviruses and the origin of the eukaryotic nucleus | journal = Journal of Molecular Evolution | volume = 52 | issue = 5 | pages = 419–25 | date = May 2001 | pmid = 11443345 | doi = 10.1007/s002390010171 | s2cid = 21200827 | department = Primary | bibcode = 2001JMolE..52..419T }}</ref><ref name="Villareal">{{cite journal | vauthors = Villarreal LP, DeFilippis VR | title = A hypothesis for DNA viruses as the origin of eukaryotic replication proteins | journal = Journal of Virology | volume = 74 | issue = 15 | pages = 7079–84 | date = August 2000 | pmid = 10888648 | pmc = 112226 | doi = 10.1128/JVI.74.15.7079-7084.2000 | department = Primary }}</ref> It has been suggested that the unresolved question of the [[evolution of sex]] could be related to the viral eukaryogenesis hypothesis.<ref name="Bell2">{{cite journal | vauthors = Bell PJ | title = Sex and the eukaryotic cell cycle is consistent with a viral ancestry for the eukaryotic nucleus | journal = Journal of Theoretical Biology | volume = 243 | issue = 1 | pages = 54–63 | date = November 2006 | pmid = 16846615 | doi = 10.1016/j.jtbi.2006.05.015 | bibcode = 2006JThBi.243...54B | department = Primary <!-- but secondary to the original eukaryogenesis hypothesis --> }}</ref>

A more recent proposal, the ''exomembrane hypothesis'', suggests that the nucleus instead originated from a single ancestral cell that evolved a second exterior cell membrane; the interior membrane enclosing the original cell then became the nuclear membrane and evolved increasingly elaborate pore structures for passage of internally synthesized cellular components such as ribosomal subunits.<ref name="deRoos">{{cite journal | vauthors = de Roos AD | title = The origin of the eukaryotic cell based on conservation of existing interfaces | journal = Artificial Life | volume = 12 | issue = 4 | pages = 513–23 | year = 2006 | pmid = 16953783 | doi = 10.1162/artl.2006.12.4.513 | s2cid = 5963228 | department = Primary }}</ref>