Editing Microtubule (section)

==Mitosis==

=== Centrosomes ===
[[File:Centriole3D.png|thumb|298x298px|A 3D diagram of a centriole. Each circle represents one microtubule. In total there are 27 microtubules organized into 9 bundles of 3.]]
The [[centrosome]] is the main MTOC ([[microtubule organizing center]]) of the cell during mitosis. Each centrosome is made up of two cylinders called [[centriole]]s, oriented at right angles to each other. The centriole is formed from 9 main microtubules, each having two partial microtubules attached to it. Each centriole is approximately 400&nbsp;nm long and around 200&nbsp;nm in circumference.<ref name="pmid10209087">{{Cite journal |vauthors=Marshall WF, Rosenbaum JL |date=March 1999 |title=Cell division: The renaissance of the centriole |journal=Current Biology |volume=9 |issue=6 |pages=R218–20 |bibcode=1999CBio....9.R218M |doi=10.1016/s0960-9822(99)80133-x |pmid=10209087 |s2cid=16951268 |doi-access=free}}</ref>

The centrosome is critical to mitosis as most microtubules involved in the process originate from the centrosome. The minus ends of each microtubule begin at the centrosome, while the plus ends radiate out in all directions. Thus the centrosome is also important in maintaining the polarity of microtubules during mitosis.<ref name="pmid9057082">{{Cite journal |vauthors=Pereira G, Schiebel E |date=February 1997 |title=Centrosome-microtubule nucleation |journal=Journal of Cell Science |volume=110 |issue=Pt 3 |pages=295–300 |doi=10.1242/jcs.110.3.295 |pmid=9057082}}</ref>

Most cells only have one centrosome for most of their cell cycle, however, right before mitosis, the centrosome duplicates, and the cell contains two centrosomes.<ref name="pmid11358861">{{Cite journal |vauthors=Hinchcliffe EH, Sluder G |date=May 2001 |title="It takes two to tango": understanding how centrosome duplication is regulated throughout the cell cycle |journal=Genes & Development |volume=15 |issue=10 |pages=1167–81 |doi=10.1101/gad.894001 |pmid=11358861 |doi-access=free}}</ref> Some of the microtubules that radiate from the centrosome grow directly away from the sister centrosome. These microtubules are called astral microtubules.  With the help of these astral microtubules the centrosomes move away from each other towards opposite sides of the cell. Once there, other types of microtubules necessary for mitosis, including interpolar microtubules and K-fibers can begin to form.<ref name="pmid28490474">{{Cite journal |vauthors=Forth S, Kapoor TM |date=June 2017 |title=The mechanics of microtubule networks in cell division |journal=The Journal of Cell Biology |volume=216 |issue=6 |pages=1525–1531 |doi=10.1083/jcb.201612064 |pmc=5461028 |pmid=28490474}}</ref>

A final important note about the centrosomes and microtubules during mitosis is that while the centrosome is the MTOC for the microtubules necessary for mitosis, research has shown that once the microtubules themselves are formed and in the correct place the centrosomes themselves are not needed for mitosis to occur.<ref>Khodjakov, A., Cole, R. W., Oakley, B. R. and Rieder, C. L. (2000). "Centrosome-independent mitotic spindle formation in vertebrates". ''Curr. Biol.'' 10, 59–67. doi:10.1016/S0960-9822(99)00276-6.</ref>

=== Microtubule subclasses ===
[[File:Spindle apparatus.svg|thumb|400 px|This diagram depicts the organization of a typical mitotic spindle found in animal cells.  Shown here are the three main types of microtubules during mitosis and how they are oriented in the cell and the mitotic spindle.]]'''Astral microtubules''' are a subclass of microtubules which only exist during and around mitosis. They originate from the centrosome, but do not interact with the chromosomes, kinetochores, or with the microtubules originating from the other centrosome.<ref name="pmid15738974">{{Cite journal |vauthors=Rosenblatt J |date=March 2005 |title=Spindle assembly: asters part their separate ways |journal=Nature Cell Biology |volume=7 |issue=3 |pages=219–22 |doi=10.1038/ncb0305-219 |pmid=15738974 |s2cid=8082479}}</ref> Instead their microtubules radiate towards the cell membrane. Once there they interact with specific motor proteins which create force that pull the microtubules, and thus the entire centrosome towards the cell membrane. As stated above, this helps the centrosomes orient themselves away from each other in the cell. However these astral microtubules do not interact with the mitotic spindle itself. Experiments have shown that without these astral microtubules, the mitotic spindle can form, however its orientation in the cell is not always correct and thus mitosis does not occur as effectively.<ref name="pmid21102610">{{Cite journal |vauthors=Knoblich JA |date=December 2010 |title=Asymmetric cell division: recent developments and their implications for tumour biology |journal=Nature Reviews. Molecular Cell Biology |volume=11 |issue=12 |pages=849–60 |doi=10.1038/nrm3010 |pmc=3941022 |pmid=21102610}}</ref> Another key function of the astral microtubules is to aid in cytokinesis. Astral microtubules interact with motor proteins at the cell membrane to pull the spindle and the entire cell apart once the chromosomes have been replicated.

'''Interpolar/Polar microtubules''' are a class of microtubules which also radiate out from the centrosome during mitosis. These microtubules radiate towards the mitotic spindle, unlike astral microtubules. Interpolar microtubules are both the most abundant and dynamic subclass of microtubules during mitosis. Around 95 percent of microtubules in the mitotic spindle can be characterized as interpolar. Furthermore, the half life of these microtubules is extremely short as it is less than one minute.<ref name="pmid7593192">{{Cite journal |vauthors=Zhai Y, Kronebusch PJ, Borisy GG |date=November 1995 |title=Kinetochore microtubule dynamics and the metaphase-anaphase transition |journal=The Journal of Cell Biology |volume=131 |issue=3 |pages=721–34 |doi=10.1083/jcb.131.3.721 |pmc=2120628 |pmid=7593192}}</ref>  Interpolar microtubules that do not attach to the kinetochores can aid in chromosome congregation through lateral interaction with the kinetochores.<ref name="pmid19525938">{{Cite journal |vauthors=Cai S, O'Connell CB, Khodjakov A, Walczak CE |date=July 2009 |title=Chromosome congression in the absence of kinetochore fibres |journal=Nature Cell Biology |volume=11 |issue=7 |pages=832–8 |doi=10.1038/ncb1890 |pmc=2895821 |pmid=19525938}}</ref>

'''K fibers/Kinetochore microtubules''' are the third important subclass of mitotic microtubules. These microtubules form direct connections with the kinetochores in the mitotic spindle. Each K fiber is composed of 20–40 parallel microtubules, forming a strong tube which is attached at one end to the centrosome and on the other to the kinetochore, located in the center of each chromosome. Since each centrosome has a K fiber connecting to each pair of chromosomes, the chromosomes become tethered in the middle of the mitotic spindle by the K fibers. K fibers have a much longer half life than interpolar microtubules, at between 4 and 8 minutes.<ref name="pmid19060894">{{Cite journal |vauthors=Bakhoum SF, Thompson SL, Manning AL, Compton DA |date=January 2009 |title=Genome stability is ensured by temporal control of kinetochore-microtubule dynamics |journal=Nature Cell Biology |volume=11 |issue=1 |pages=27–35 |doi=10.1038/ncb1809 |pmc=2614462 |pmid=19060894}}</ref> During the end of mitoses, the microtubules forming each K fiber begin to disassociate, thus shorting the K fibers. As the K fibers shorten the pair chromosomes are pulled apart right before cytokinesis. Previously, some researchers believed that K fibers form at their minus end originating from the centrosome just like other microtubules, however, new research has pointed to a different mechanism. In this new mechanism, the K fibers are initially stabilized at their plus end by the kinetochores  and grow out from there. The minus end of these K fibers eventually connect to an existing Interpolar microtubule and are eventually connected to the centrosome in this way.<ref name="pmid22736044">{{Cite journal |vauthors=Meunier S, Vernos I |date=June 2012 |title=Microtubule assembly during mitosis - from distinct origins to distinct functions? |journal=Journal of Cell Science |volume=125 |issue=Pt 12 |pages=2805–14 |doi=10.1242/jcs.092429 |pmid=22736044 |doi-access=free}}</ref>

=== Microtubule nuclear in the mitotic spindle ===
Most of the microtubules that form the mitotic spindle originate from the centrosome. Originally it was thought that all of these microtubules originated from the centrosome via a method called search and capture, described in more detail in a section above, however new research has shown that there are addition means of microtubule nucleation during mitosis. One of the most important of these additional means of microtubule nucleation is the RAN-GTP pathway. RAN-GTP associates with chromatin during mitosis to create a gradient that allows for local nucleation of microtubules near the chromosomes. Furthermore, a second pathway known as the augmin/HAUS complex (some organisms use the more studied augmin complex, while others such as humans use an analogous complex called HAUS) acts an additional means of microtubule nucleation in the mitotic spindle.<ref name="pmid22736044" />