Editing Mitosis (section)

===Mitosis===
[[File:Stages of early mitosis in a vertebrate cell with micrographs of chromatids.svg|thumb|right|Stages of early mitosis in a vertebrate cell with [[micrograph]]s of [[chromatid]]s]]

====Preprophase (plant cells)====
{{main|Preprophase}}
In plant cells only, prophase is preceded by a [[preprophase]] stage. In highly [[vacuole|vacuolated]] plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of a [[phragmosome]], a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, preprophase is characterized by the formation of a ring of microtubules and [[actin]] filaments (called [[preprophase band]]) underneath the plasma membrane around the equatorial plane of the future mitotic [[spindle apparatus|spindle]]. This band marks the position where the cell will eventually divide. The cells of higher plants (such as the [[flowering plant]]s) lack [[centrioles]]; instead, microtubules form a spindle on the surface of the nucleus and are then organized into a spindle by the chromosomes themselves, after the nuclear envelope breaks down.<ref name="Lloyd-2006"/> The preprophase band disappears during nuclear envelope breakdown and spindle formation in prometaphase.<ref name="Raven-2005">{{cite book | vauthors = Raven PH, Evert RF, Eichhorn SE |title= Biology of Plants |url= https://archive.org/details/biologyofplants00rave_0 |url-access= registration |edition= 7th |publisher= [[W. H. Freeman and Company|W. H. Freeman and Co.]] |year= 2005 |location= New York |isbn= 978-0716710073}}</ref>{{rp|58–67}}

====Prophase====
{{main|Prophase}}
[[File:CONDENSING CHROMOSOMES 2.jpg|thumb|left|Interphase nucleus (left), condensing chromosomes (middle) and condensed chromosomes (right)]]

[[File:Prophase diagram.svg|thumb|[[Prophase]] during mitosis]]

During prophase, which occurs after G<sub>2</sub> interphase, the cell prepares to divide by tightly condensing its chromosomes and initiating mitotic spindle formation. During interphase, the genetic material in the nucleus consists of loosely packed [[chromatin]]. At the onset of prophase, chromatin fibers condense into discrete chromosomes that are typically visible at high magnification through a [[light microscope]]. In this stage, chromosomes are long, thin, and thread-like. Each chromosome has two chromatids. The two chromatids are joined at the centromere.

[[Gene transcription]] ceases during prophase and does not resume until late anaphase to early G<sub>1</sub> phase.<ref>{{cite journal | vauthors = Prasanth KV, Sacco-Bubulya PA, Prasanth SG, Spector DL | title = Sequential entry of components of the gene expression machinery into daughter nuclei | journal = Molecular Biology of the Cell | volume = 14 | issue = 3 | pages = 1043–57 | date = March 2003 | pmid = 12631722 | pmc = 151578 | doi = 10.1091/mbc.E02-10-0669 }}</ref><ref>
{{cite journal | vauthors = Kadauke S, Blobel GA | title = Mitotic bookmarking by transcription factors | journal = Epigenetics & Chromatin | volume = 6 | issue = 1 | pages = 6 | date = April 2013 | pmid = 23547918 | pmc = 3621617 | doi = 10.1186/1756-8935-6-6 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Prescott DM, Bender MA | title = Synthesis of RNA and protein during mitosis in mammalian tissue culture cells | journal = Experimental Cell Research | volume = 26 | issue = 2 | pages = 260–8 | date = March 1962 | pmid = 14488623 | doi = 10.1016/0014-4827(62)90176-3 }}</ref> The [[nucleolus]] also disappears during early prophase.<ref>{{cite book | vauthors = Olson MO | date=2011 |title=The Nucleolus |volume=15 of Protein Reviews |location=Berlin |publisher=Springer Science & Business Media |page=15 |isbn=9781461405146 }}</ref>

Close to the nucleus of an animal cell are structures called [[centrosome]]s, consisting of a pair of [[centriole]]s surrounded by a [[Pericentriolar material|loose collection of proteins]]. The centrosome is the coordinating center for the cell's [[microtubule]]s. A cell inherits a single centrosome at cell division, which is [[Centrosome cycle|duplicated by the cell]] before a new round of mitosis begins, giving a pair of centrosomes. The two centrosomes polymerize [[tubulin]] to help form a [[spindle apparatus|microtubule spindle apparatus]]. [[Motor proteins]] then push the centrosomes along these microtubules to opposite sides of the cell. Although centrosomes help organize microtubule assembly, they are not essential for the formation of the spindle apparatus, since they are absent from plants,<ref name="Lloyd-2006"/> and are not absolutely required for animal cell mitosis.<ref name="Basto-2006">{{cite journal | vauthors = Basto R, Lau J, Vinogradova T, Gardiol A, Woods CG, Khodjakov A, Raff JW | title = Flies without centrioles | journal = Cell | volume = 125 | issue = 7 | pages = 1375–86 | date = June 2006 | pmid = 16814722 | doi = 10.1016/j.cell.2006.05.025 | doi-access = free }}</ref>

====Prometaphase====
{{main|Prometaphase}}
At the beginning of prometaphase in animal cells, phosphorylation of [[lamin|nuclear lamins]] causes the [[nuclear envelope]] to disintegrate into small membrane [[Vesicle (biology and chemistry)|vesicles]]. As this happens, microtubules invade the nuclear space. This is called ''open mitosis'', and it occurs in some multicellular organisms. Fungi and some [[protist]]s, such as [[algae]] or [[trichomonad]]s, undergo a variation called ''closed mitosis'' where the spindle forms inside the nucleus, or the microtubules penetrate the intact nuclear envelope.<ref name="Heywood-1978"/><ref name="Ribeiro-2002"/>

In late prometaphase, ''kinetochore microtubules'' begin to search for and attach to chromosomal [[kinetochores]].<ref name="Chan-2005"/> A ''kinetochore'' is a [[protein]]aceous microtubule-binding structure that forms on the chromosomal centromere during late prophase.<ref name="Chan-2005"/><ref>{{cite journal | vauthors = Cheeseman IM, Desai A | title = Molecular architecture of the kinetochore-microtubule interface | journal = Nature Reviews. Molecular Cell Biology | volume = 9 | issue = 1 | pages = 33–46 | date = January 2008 | pmid = 18097444 | doi = 10.1038/nrm2310 }}</ref> A number of ''polar microtubules'' find and interact with corresponding polar microtubules from the opposite centrosome to form the mitotic spindle.<ref name="Winey-1995"/> Although the kinetochore structure and function are not fully understood, it is known that it contains some form of [[List of gene families#Motor proteins|molecular motor]].<ref name="Maiato-2004"/> When a microtubule connects with the kinetochore, the motor activates, using energy from [[Adenosine triphosphate|ATP]] to "crawl" up the tube toward the originating centrosome. This motor activity, coupled with polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome's two chromatids.<ref name="Maiato-2004"/>

====Metaphase====
[[File:Mitosis-fluorescent.jpg|thumb|right|A cell in late [[metaphase]]. All chromosomes (blue) but one have arrived at the [[metaphase plate]].]] 
{{main|Metaphase}}

[[File:Metaphase during Mitosis.svg|thumb|[[Metaphase]] during mitosis]]

After the microtubules have located and attached to the kinetochores in prometaphase, the two centrosomes begin pulling the chromosomes towards opposite ends of the cell. The resulting tension causes the chromosomes to align along the [[metaphase plate]] at the equatorial plane, an imaginary line that is centrally located between the two centrosomes (at approximately the midline of the cell).<ref name="Winey-1995"/> To ensure equitable distribution of chromosomes at the end of mitosis, the ''[[spindle checkpoint|metaphase checkpoint]]'' guarantees that kinetochores are properly attached to the mitotic spindle and that the chromosomes are aligned along the metaphase plate.<ref name="Chan-2003"/> If the cell successfully passes through the metaphase checkpoint, it proceeds to anaphase.

====Anaphase====
{{main|Anaphase}}

[[File:Anaphase during Mitosis.svg|thumb|[[Anaphase]] during mitosis]]

During ''anaphase A'', the [[cohesin]]s that bind sister chromatids together are cleaved, forming two identical daughter chromosomes.<ref name="FitzHarris-2012"/> Shortening of the kinetochore microtubules pulls the newly formed daughter chromosomes to opposite ends of the cell. During ''anaphase B'', polar microtubules push against each other, causing the cell to elongate.<ref name="Miller-2000"/> In late anaphase, [[chromosome]]s also reach their overall maximal condensation level, to help [[chromosome segregation]] and the re-formation of the nucleus.<ref>{{cite press release |title=Researchers Shed Light On Shrinking Of Chromosomes |url=https://www.sciencedaily.com/releases/2007/06/070611122252.htm |work=ScienceDaily |publisher=European Molecular Biology Laboratory |date=12 June 2007 }}</ref> In most animal cells, anaphase A precedes anaphase B, but some vertebrate egg cells demonstrate the opposite order of events.<ref name="FitzHarris-2012">{{cite journal | vauthors = FitzHarris G | title = Anaphase B precedes anaphase A in the mouse egg | journal = Current Biology | volume = 22 | issue = 5 | pages = 437–44 | date = March 2012 | pmid = 22342753 | doi = 10.1016/j.cub.2012.01.041 | doi-access = free | bibcode = 2012CBio...22..437F }}</ref>

====Telophase====
{{main|Telophase}}

[[File:Telophase during Mitosis.svg|thumb|[[Telophase]] during mitosis]]

Telophase (from the [[Greek language|Greek]] word ''τελος'' meaning "end") is a reversal of prophase and prometaphase events. At telophase, the polar microtubules continue to lengthen, elongating the cell even more. If the nuclear envelope has broken down, a new nuclear envelope forms using the membrane vesicles of the parent cell's old nuclear envelope. The new envelope forms around each set of separated daughter chromosomes (though the membrane does not enclose the centrosomes) and the nucleolus reappears. Both sets of chromosomes, now surrounded by new nuclear membrane, begin to "relax" or decondense. Mitosis is complete. Each daughter nucleus has an identical set of chromosomes. Cell division may or may not occur at this time depending on the organism.