Editing Spindle checkpoint (section)

==== Sister chromatid cohesion during mitosis ====

===== Cohesin: SMC proteins =====
Sister chromatids stay associated from S phase (when DNA is replicated to generate two identical copies, the two chromatids) until anaphase. At this point, the two sister chromatids separate and travel to opposite poles in the dividing cell. Genetic and biochemical studies in yeast and in egg's extracts in ''[[Xenopus laevis]]'' identified a polyprotein complex as an essential player in sister chromatids cohesion (see the review from Hirano in 2000<ref name="Hirano2000">{{cite journal | vauthors = Hirano T | title = Chromosome cohesion, condensation, and separation | journal = Annual Review of Biochemistry | volume = 69 | pages = 115–44 | year = 2000 | pmid = 10966455 | doi = 10.1146/annurev.biochem.69.1.115 }}</ref>). This complex is known as the [[cohesin]] complex and in ''[[Saccharomyces cerevisiae]]'' is composed of at least four subunits: Smc1p, Smc3p, Scc1p (or Mcd1p) and Scc3p. Both Smc1p and Smc3p belong to the family of proteins for the '''Structural Maintenance of Chromosomes''' (SMC), which constitute a group of chromosomic [[ATPase]]s highly conserved, and form an heterodimer (Smc1p/Smc3p). Scc1p is the homolog in ''S.cerevisiae'' of Rad21, first identified as a protein involved in [[DNA repair]] in ''S. pombe''. These four proteins are essential in yeast, and a mutation in any of them will produce premature sister chromatid separation. In yeast, cohesin binds to preferential sites along chromosome arms, and is very abundant close to the centromeres, as it was shown in a study using chromatin immunoprecipitation.<ref name="Tanaka2001">{{cite journal | vauthors = Tanaka K, Hao Z, Kai M, Okayama H | title = Establishment and maintenance of sister chromatid cohesion in fission yeast by a unique mechanism | journal = The EMBO Journal | volume = 20 | issue = 20 | pages = 5779–90 | date = October 2001 | pmid = 11598020 | pmc = 125673 | doi = 10.1093/emboj/20.20.5779 }}</ref>

===== The role of heterochromatin =====
Classical cytologic observations suggested that sister chromatids are more strongly attached at [[heterochromatin|heterochromatic]] regions,<ref name="Gonzalez91">{{cite journal | vauthors = Gonzalez C, Casal Jimenez J, Ripoll P, Sunkel CE | title = The spindle is required for the process of sister chromatid separation in Drosophila neuroblasts | journal = Experimental Cell Research | volume = 192 | issue = 1 | pages = 10–5 | date = January 1991 | pmid = 1898588 | doi = 10.1016/0014-4827(91)90150-S }}</ref> and this suggested that the special structure or composition of heterochromatin might favour cohesin recruitment.<ref name="Losada2001b">{{cite journal | vauthors = Losada A, Hirano T | title = Shaping the metaphase chromosome: coordination of cohesion and condensation | journal = BioEssays | volume = 23 | issue = 10 | pages = 924–35 | date = October 2001 | pmid = 11598959 | doi = 10.1002/bies.1133 | s2cid = 31210810 }}</ref> In fact, it has been shown that Swi6 (the homolog of HP-1 in ''S. pombe'') binds to methylated [[lysine|Lys]] 9 of [[histone]] H3 and promotes the binding of cohesin to the centromeric repeats in ''S. pombe''.<ref name="Bernard2001">{{cite journal | vauthors = Bernard P, Maure JF, Partridge JF, Genier S, Javerzat JP, Allshire RC | title = Requirement of heterochromatin for cohesion at centromeres | journal = Science | volume = 294 | issue = 5551 | pages = 2539–42 | date = December 2001 | pmid = 11598266 | doi = 10.1126/science.1064027 | bibcode = 2001Sci...294.2539B | s2cid = 31166180 }}</ref><ref name="Nonaka2002">{{cite journal | vauthors = Nonaka N, Kitajima T, Yokobayashi S, Xiao G, Yamamoto M, Grewal SI, Watanabe Y | title = Recruitment of cohesin to heterochromatic regions by Swi6/HP1 in fission yeast | journal = Nature Cell Biology | volume = 4 | issue = 1 | pages = 89–93 | date = January 2002 | pmid = 11780129 | doi = 10.1038/ncb739 | s2cid = 23036084 | doi-access = free }}</ref> More recent studies indicate that the [[RNAi]] machinery regulates heterochromatin establishment, which in turn recruits cohesin to this region, both in ''S. pombe''<ref name="Hall2003">{{cite journal | vauthors = Hall IM, Noma K, Grewal SI | title = RNA interference machinery regulates chromosome dynamics during mitosis and meiosis in fission yeast | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 1 | pages = 193–8 | date = January 2003 | pmid = 12509501 | pmc = 140924 | doi = 10.1073/pnas.232688099 | bibcode = 2003PNAS..100..193H | doi-access = free }}</ref> and in vertebrate cells.<ref name="Fukagawa2004">{{cite journal | vauthors = Fukagawa T, Nogami M, Yoshikawa M, Ikeno M, Okazaki T, Takami Y, Nakayama T, Oshimura M | title = Dicer is essential for formation of the heterochromatin structure in vertebrate cells | journal = Nature Cell Biology | volume = 6 | issue = 8 | pages = 784–91 | date = August 2004 | pmid = 15247924 | doi = 10.1038/ncb1155 | s2cid = 24798145 }}</ref>  However, there must be other mechanisms than heterochromatin to ensure an augmented cohesion at centromeres, because ''S. cerevisiae'' lacks heterochromatin next to centromeres, but the presence of a functional centromere induces an increase of cohesin association in a contiguous region, spanning 20-50kb.<ref name="Weber2004">{{cite journal | vauthors = Weber SA, Gerton JL, Polancic JE, DeRisi JL, Koshland D, Megee PC | title = The kinetochore is an enhancer of pericentric cohesin binding | journal = PLOS Biology | volume = 2 | issue = 9 | pages = E260 | date = September 2004 | pmid = 15309047 | pmc = 490027 | doi = 10.1371/journal.pbio.0020260 | doi-access = free }} {{open access}}</ref>

In this direction, '''Orc2''' (one protein included in the [[origin recognition complex]], ORC, implicated in the initiation of [[DNA replication]] during [[S phase]]) is also located on kinetochores during mitosis in human cells;<ref name=Prasanth2004>{{cite journal | vauthors = Prasanth SG, Prasanth KV, Siddiqui K, Spector DL, Stillman B | title = Human Orc2 localizes to centrosomes, centromeres and heterochromatin during chromosome inheritance | journal = The EMBO Journal | volume = 23 | issue = 13 | pages = 2651–63 | date = July 2004 | pmid = 15215892 | pmc = 449767 | doi = 10.1038/sj.emboj.7600255 }}</ref> in agreement with this localization, some observations indicate that Orc2 in yeast is implicated in sister chromatid cohesion, and its removal induces SAC activation.<ref name=Shimada2007>{{cite journal | vauthors = Shimada K, Gasser SM | title = The origin recognition complex functions in sister-chromatid cohesion in Saccharomyces cerevisiae | journal = Cell | volume = 128 | issue = 1 | pages = 85–99 | date = January 2007 | pmid = 17218257 | doi = 10.1016/j.cell.2006.11.045 | doi-access = free }}</ref> It has also been observed that other components of the ORC complex (such as orc5 in ''S. pombe'') are implicated in cohesion.<ref name=Kato2008>{{cite journal | vauthors = Kato H, Matsunaga F, Miyazaki S, Yin L, D'Urso G, Tanaka K, Murakami Y | title = Schizosaccharomyces pombe Orc5 plays multiple roles in the maintenance of genome stability throughout the cell cycle | journal = Cell Cycle | volume = 7 | issue = 8 | pages = 1085–96 | date = April 2008 | pmid = 18414064 | doi = 10.4161/cc.7.8.5710 | doi-access = free }}</ref> However, the molecular pathway involving the ORC proteins seems to be additive to the cohesins' pathway, and it is mostly unknown.

===== Function of cohesion and its dissolution =====
[[File:Chromosome cohesion - en.png|thumb|500px|Scheme showing sister chromatids cohesion, anchored to spindle [[microtubule]]s via their kinetochores]]
Centromeric cohesion resists the forces exerted by spindle microtubules towards the poles, which generate tension between sister kinetochores. In turn, this tension stabilizes the attachment microtubule-kinetochore, through a mechanism implicating the protein [[Aurora B kinase|Aurora B]] (a review about this issue : Hauf and Watanabe 2004<ref name="Hauf2004">{{cite journal | vauthors = Hauf S, Watanabe Y | title = Kinetochore orientation in mitosis and meiosis | journal = Cell | volume = 119 | issue = 3 | pages = 317–27 | date = October 2004 | pmid = 15507205 | doi = 10.1016/j.cell.2004.10.014 | doi-access = free }}</ref>).

Indeed, a decrease in the cellular levels of cohesin generates the premature separation of sister chromatids, as well as defects in chromosome congression at the metaphase plate and delocalization of the proteins in the '''chromosomal passenger complex''', which contains the protein Aurora B.<ref name="Sonoda2001">{{cite journal | vauthors = Sonoda E, Matsusaka T, Morrison C, Vagnarelli P, Hoshi O, Ushiki T, Nojima K, Fukagawa T, Waizenegger IC, Peters JM, Earnshaw WC, Takeda S | title = Scc1/Rad21/Mcd1 is required for sister chromatid cohesion and kinetochore function in vertebrate cells | journal = Developmental Cell | volume = 1 | issue = 6 | pages = 759–70 | date = December 2001 | pmid = 11740938 | doi = 10.1016/S1534-5807(01)00088-0 | doi-access = free }}</ref><ref name="Vass2003">{{cite journal | vauthors = Vass S, Cotterill S, Valdeolmillos AM, Barbero JL, Lin E, Warren WD, Heck MM | title = Depletion of Drad21/Scc1 in Drosophila cells leads to instability of the cohesin complex and disruption of mitotic progression | journal = Current Biology | volume = 13 | issue = 3 | pages = 208–18 | date = February 2003 | pmid = 12573216 | doi = 10.1016/S0960-9822(03)00047-2 | bibcode = 2003CBio...13..208V | url = https://www.pure.ed.ac.uk/ws/files/10691787/Depletion_of_Drad21_Scc1_in_Drosophila_cells_leads_to_instability_of_the_and_disruption_of_mitotic_cohesin_complex_progression.pdf | hdl = 20.500.11820/b75b5706-3f21-4cfe-85be-466268afc918 | s2cid = 16037196 | hdl-access = free }}</ref>
The proposed structure for the cohesin complex suggests that this complex connects directly both sister chromatids.<ref name="Haering2002">{{cite journal | vauthors = Haering CH, Löwe J, Hochwagen A, Nasmyth K | title = Molecular architecture of SMC proteins and the yeast cohesin complex | journal = Molecular Cell | volume = 9 | issue = 4 | pages = 773–88 | date = April 2002 | pmid = 11983169 | doi = 10.1016/S1097-2765(02)00515-4 | doi-access = free }}</ref> In this proposed structure, the SMC components of cohesin play a structural role, so that the SMC heterodimer may function as a DNA binding protein, whose conformation is regulated by [[adenosine triphosphate|ATP]].<ref name="Hirano1999">{{cite journal | vauthors = Hirano T | title = SMC-mediated chromosome mechanics: a conserved scheme from bacteria to vertebrates? | journal = Genes & Development | volume = 13 | issue = 1 | pages = 11–9 | date = January 1999 | pmid = 9887095 | doi = 10.1101/gad.13.1.11 | doi-access = free }}</ref> Scc1p and Scc3p, however, would play a regulatory role.<ref name="Hirano2000" />

In ''S. cerevisiae'', Pds1p (also known as [[securin]]) regulates sister chromatids cohesion, because it binds and inhibits the protease [[Esp1]]p ('''separin''' or '''separase'''). When anaphase onset is triggered, the [[anaphase-promoting complex]] ('''APC/C''' or Cyclosome) degrades securin. APC/C is a ring E3 ubiquitin ligase that recruits an E2 ubiquitin-conjugating enzyme loaded with ubiquitin. Securin is recognized only if Cdc20, the activator subunit, is bound to the APC/C core.  When securin, Cdc20, and E2 are all bound to APC/C E2 ubiquitinates securin and selectively degrades it. Securin degradation releases the protease Esp1p/separase, which degrades the cohesin rings that link the two sister chromatids, therefore promoting sister chromatids separation.<ref name="Ciosk1998">{{cite journal | vauthors = Ciosk R, Zachariae W, Michaelis C, Shevchenko A, Mann M, Nasmyth K | title = An ESP1/PDS1 complex regulates loss of sister chromatid cohesion at the metaphase to anaphase transition in yeast | journal = Cell | volume = 93 | issue = 6 | pages = 1067–76 | date = June 1998 | pmid = 9635435 | doi = 10.1016/S0092-8674(00)81211-8 | doi-access = free }}</ref> It has been also shown that Polo/Cdc5 [[kinase]] phosphorylates [[serine]] residues next to the cutting site for Scc1, and this phosphorylation would facilitate the cutting activity.<ref name="Alexandru2001">{{cite journal | vauthors = Alexandru G, Uhlmann F, Mechtler K, Poupart MA, Nasmyth K | title = Phosphorylation of the cohesin subunit Scc1 by Polo/Cdc5 kinase regulates sister chromatid separation in yeast | journal = Cell | volume = 105 | issue = 4 | pages = 459–72 | date = May 2001 | pmid = 11371343 | doi = 10.1016/S0092-8674(01)00362-2 | doi-access = free }}</ref>

Although this machinery is conserved through evolution,<ref name="Leismann2001">{{cite journal | vauthors = Leismann O, Herzig A, Heidmann S, Lehner CF | title = Degradation of Drosophila PIM regulates sister chromatid separation during mitosis | journal = Genes & Development | volume = 14 | issue = 17 | pages = 2192–205 | date = September 2000 | pmid = 10970883 | pmc = 316890 | doi = 10.1101/gad.176700 }}</ref><ref name="Zur2001">{{cite journal | vauthors = Zur A, Brandeis M | title = Securin degradation is mediated by fzy and fzr, and is required for complete chromatid separation but not for cytokinesis | journal = The EMBO Journal | volume = 20 | issue = 4 | pages = 792–801 | date = February 2001 | pmid = 11179223 | pmc = 145417 | doi = 10.1093/emboj/20.4.792 }}</ref> in vertebrates most cohesin molecules are released in prophase, independently of the presence of the APC/C, in a process dependent on Polo-like 1 ([[PLK1]]) and Aurora B.<ref name="Sumara2000">{{cite journal | vauthors = Sumara I, Vorlaufer E, Gieffers C, Peters BH, Peters JM | title = Characterization of vertebrate cohesin complexes and their regulation in prophase | journal = The Journal of Cell Biology | volume = 151 | issue = 4 | pages = 749–62 | date = November 2000 | pmid = 11076961 | pmc = 2169443 | doi = 10.1083/jcb.151.4.749 }}</ref> Yet it has been shown that a small quantity of Scc1 remains associated to centromeres in human cells until metaphase, and a similar amount is cut in anaphase, when it disappears from centromeres.<ref name="Losada2000">{{cite journal | vauthors = Losada A, Yokochi T, Kobayashi R, Hirano T | title = Identification and characterization of SA/Scc3p subunits in the Xenopus and human cohesin complexes | journal = The Journal of Cell Biology | volume = 150 | issue = 3 | pages = 405–16 | date = August 2000 | pmid = 10931856 | pmc = 2175199 | doi = 10.1083/jcb.150.3.405 }}</ref> On the other hand, some experiments show that sister chromatids cohesion in the arms is lost gradually after sister centromeres have separated, and sister chromatids move toward the opposite poles of the cell.<ref name="Gimenez2004">{{cite journal | vauthors = Giménez-Abián JF, Sumara I, Hirota T, Hauf S, Gerlich D, de la Torre C, Ellenberg J, Peters JM | title = Regulation of sister chromatid cohesion between chromosome arms | journal = Current Biology | volume = 14 | issue = 13 | pages = 1187–93 | date = July 2004 | pmid = 15242616 | doi = 10.1016/j.cub.2004.06.052 | doi-access = free | bibcode = 2004CBio...14.1187G }}</ref><ref name="Paliulis2004">{{cite journal | vauthors = Paliulis LV, Nicklas RB | title = Micromanipulation of chromosomes reveals that cohesion release during cell division is gradual and does not require tension | journal = Current Biology | volume = 14 | issue = 23 | pages = 2124–9 | date = December 2004 | pmid = 15589155 | doi = 10.1016/j.cub.2004.11.052 | doi-access = free | bibcode = 2004CBio...14.2124P }}</ref>

According to some observations, a fraction of cohesins in the chromosomal arms and the centromeric cohesins are protected by the protein '''Shugoshin''' (Sgo1), avoiding their release during prophase.<ref name="Nakajima2007">{{cite journal | vauthors = Nakajima M, Kumada K, Hatakeyama K, Noda T, Peters JM, Hirota T | title = The complete removal of cohesin from chromosome arms depends on separase | journal = Journal of Cell Science | volume = 120 | issue = Pt 23 | pages = 4188–96 | date = December 2007 | pmid = 18003702 | doi = 10.1242/jcs.011528 | doi-access = free }}</ref><ref name="McGuiness2005">{{cite journal | vauthors = McGuinness BE, Hirota T, Kudo NR, Peters JM, Nasmyth K | title = Shugoshin prevents dissociation of cohesin from centromeres during mitosis in vertebrate cells | journal = PLOS Biology | volume = 3 | issue = 3 | pages = e86 | date = March 2005 | pmid = 15737064 | pmc = 1054882 | doi = 10.1371/journal.pbio.0030086 | doi-access = free }} {{open access}}</ref> To be able to function as protector for the centromeric cohesion, Sgo1 must be inactivated at the beginning of anaphase, as well as Pds1p. In fact, both Pds1p and Sgo1 are substrates of APC/C in vertebrates.<ref name="Salic2004">{{cite journal | vauthors = Salic A, Waters JC, Mitchison TJ | title = Vertebrate shugoshin links sister centromere cohesion and kinetochore microtubule stability in mitosis | journal = Cell | volume = 118 | issue = 5 | pages = 567–78 | date = September 2004 | pmid = 15339662 | doi = 10.1016/j.cell.2004.08.016 | doi-access = free }}</ref>