Editing Microtubule (section)

=== 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>