Editing Spindle checkpoint (section)

=== Cell division: duplication of material and distribution to daughter cells ===
[[File:Three cell growth types.svg|thumb|350px|Three types of cell division: binary fission (taking place in [[prokaryote]]s), [[mitosis]] and [[meiosis]] (taking place in [[eukaryotic cell|eukaryote]]s).]]
When cells are ready to divide, because cell size is big enough or because they receive the appropriate stimulus,<ref name="Conlon99">{{cite journal | vauthors = Conlon I, Raff M | title = Size control in animal development | journal = Cell | volume = 96 | issue = 2 | pages = 235–44 | date = January 1999 | pmid = 9988218 | doi = 10.1016/S0092-8674(00)80563-2 | doi-access = free }}</ref> they activate the mechanism to enter into the cell cycle, and they duplicate most organelles during S (synthesis) phase, including their [[centrosome]]. Therefore, when the cell division process will end, each daughter cell will receive a complete set of organelles. At the same time, during S phase all cells must duplicate their [[DNA]] very precisely, a process termed [[DNA replication]]. Once DNA replication has finished, in eukaryotes the DNA molecule is compacted and condensed, to form the mitotic [[chromosome]]s, each one constituted by two sister [[chromatid]]s, which stay held together by the establishment of [[sister chromatid cohesion|cohesion]] between them; each chromatid is a complete DNA molecule, attached via [[microtubule]]s to one of the two centrosomes of the dividing cell, located at opposed poles of the cell. The structure formed by the centrosomes and the microtubules is named [[mitotic spindle]], due to its characteristic shape, holding the chromosomes between the two centrosomes. The sister chromatids stay together until [[anaphase]], when each travels toward the centrosome to which it is attached. In this way, when the two daughter cells separate at the end of the division process, each one will contain a complete set of chromatids. The mechanism responsible for the correct distribution of sister chromatids during cell division is named '''chromosome segregation'''.

To ensure that chromosome segregation takes place correctly, cells have developed a precise and complex mechanism. In the first place, cells must coordinate [[centrosome]] duplication with DNA replication, and a failure in this coordination will generate monopolar or multipolar mitotic spindles, which generally will produce abnormal chromosome segregation,<ref name="Meraldi99">{{cite journal | vauthors = Meraldi P, Lukas J, Fry AM, Bartek J, Nigg EA | title = Centrosome duplication in mammalian somatic cells requires E2F and Cdk2-cyclin A | journal = Nature Cell Biology | volume = 1 | issue = 2 | pages = 88–93 | date = June 1999 | pmid = 10559879 | doi = 10.1038/10054 | s2cid = 24795991 }}</ref> because in this case, chromosome distribution will not take place in a balanced way.

<!-- Deleted image removed: [[File:Chromosome bipolar attachments.jpg|thumb|left|Improper attachments of chromosomes to the mitotic spindle are correct to achieve bipolarity]] -->