metaphase

{{short description|Stage of cell division}}

{{for|the Product Lifecycle Management software|SDRC#Products}}

File:Spindle chromosomes-en.png

File:Metaphase chromosomes.jpg

File:Stages of early mitosis in a vertebrate cell with micrographs of chromatids.svg

Metaphase ({{etymology|grc|μετα- (meta-) beyond, above, transcending}} and {{etymology|grc|φάσις (phásis)|appearance}}) is a stage of mitosis in the eukaryotic cell cycle in which chromosomes are at their second-most condensed and coiled stage (they are at their most condensed in anaphase).{{cite web |title=Chromosome condensation through mitosis |url=https://www.sciencedaily.com/releases/2007/06/070611122252.htm |website=ScienceDaily |access-date=12 June 2007}} These chromosomes, carrying genetic information, align in the equator of the cell between the spindle poles at the metaphase plate, before being separated into each of the two daughter nuclei. This alignment marks the beginning of metaphase.{{cite book |last1=Alberts |first1=Bruce |last2=Hopkin |first2=Karen |last3=Johnson |first3=Alexander |last4=Morgan |first4=David |last5=Raff |first5=Martin |last6=Roberts |first6=Keith |last7=Walter |first7=Peter |title=Essential cell biology |date=2019 |publisher=W. W. Norton & Company |location=New York London |isbn=9780393680393 |pages=632–633 |edition=Fifth}} Metaphase accounts for approximately 4% of the cell cycle's duration.{{citation needed|date=December 2012}}

In metaphase, microtubules from both duplicated centrosomes on opposite poles of the cell have completed attachment to kinetochores on condensed chromosomes. The centromeres of the chromosomes convene themselves on the metaphase plate, an imaginary line that is equidistant from the two spindle poles.{{cite web|title=Metaphase plate|url=http://www.biology-online.org/dictionary/Metaphase_plate|work=Biology Dictionary|publisher=Biology Online|access-date=9 December 2012}} This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochore microtubules,{{cite web|title=Metaphase|url=http://www.nature.com/scitable/definition/metaphase-249|publisher=Nature Education|access-date=9 December 2012}} analogous to a tug-of-war between two people of equal strength, ending with the destruction of B cyclin.{{cite web|title=The Cell Cycle|url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellCycle.html|publisher=Kimball's Biology Pages|access-date=9 December 2012|archive-url=https://web.archive.org/web/20121119134405/http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellCycle.html|archive-date=19 November 2012|url-status=dead}}

In order to prevent deleterious nondisjunction events, a key cell cycle checkpoint, the spindle checkpoint, verifies this evenly balanced alignment and ensures that every kinetochore is properly attached to a bundle of microtubules and is under balanced bipolar tension. Sister chromatids require active separase to hydrolyze the cohesin that bind them together prior to progression to anaphase. Any unattached or improperly attached kinetochores generate signals that prevent the activation of the anaphase promoting complex (cyclosome or APC/C), a ubiquitin ligase which targets securin and cyclin B for degradation via the proteosome. As long as securin and cyclin B remain active, separase remains inactive, preventing premature progression to anaphase.