Editing Spindle checkpoint (section)

== Overview and importance ==

The beginning of metaphase is characterized by the connection of the microtubules to the kinetochores of the chromosomes, as well as the alignment of the chromosomes in the middle of the cell. Each chromatid has its own kinetochore, and all of the microtubules that are bound to kinetochores of sister chromatids radiate from opposite poles of the cell. These microtubules exert a pulling force on the chromosomes towards the opposite ends of the cells, while the cohesion between the sister chromatids opposes this force.

At the metaphase to anaphase transition, this cohesion between sister chromatids is dissolved, and the separated chromatids are pulled to opposite sides of the cell by the spindle microtubules. The chromatids are further separated by the physical movement of the spindle poles themselves. Premature dissociation of the chromatids can lead to chromosome missegregation and aneuploidy in the daughter cells. Thus, the job of the spindle checkpoint is to prevent this transition into anaphase until the chromosomes are properly attached, before the sister chromatids separate.

In order to preserve the cell's identity and proper function, it is necessary to maintain the appropriate number of [[chromosome]]s after each [[cell division]]. An error in generating daughter cells with fewer or greater number of chromosomes than expected (a situation termed [[aneuploidy]]), may lead in best case to cell death, or alternatively it may generate catastrophic [[phenotype|phenotypic]] results.<ref name= Sinha2019>{{citation | last1 = Sinha | first1 = D. | last2 = Duijf | first2 = P.H.G. | last3 = Khanna | first3 = K.K. | year = 2019 | title = Mitotic slippage: an old tale with a new twist | journal = Cell Cycle | volume = 18 | issue = 1 | pages = 7–15 | doi = 10.1080/15384101.2018.1559557 | pmid = 30601084| pmc = 6343733 }}</ref><ref>{{cite journal | vauthors = Santaguida S, Amon A | title = Short- and long-term effects of chromosome mis-segregation and aneuploidy | journal = Nature Reviews Molecular Cell Biology | volume = 16 | issue = 8 | pages = 473–85 | date = August 2015 | pmid = 26204159 | doi = 10.1038/nrm4025 | hdl = 1721.1/117201 | s2cid = 205495880 | hdl-access = free }}</ref> Examples include:
* In cancer cells, [[aneuploidy]] is a frequent event, indicating that these cells present a defect in the machinery involved in [[chromosome segregation]], as well as in the mechanism ensuring that segregation is correctly performed.
* In humans, [[Down syndrome]] appears in children carrying in their cells one extra copy of [[chromosome 21]], as a result of a defect in [[chromosome segregation]] during [[meiosis]] in one of the progenitors. This defect will generate a [[gamete]] (spermatozoide or oocyte) with an extra chromosome 21. After [[fertilisation]], this gamete will generate an [[embryo]] with three copies of chromosome 21.