Editing G2 phase (section)

{{short description|Second growth phase in the eukaryotic cell cycle, prior to mitosis}}
{{DISPLAYTITLE:G<sub>2</sub> phase}}
[[File:Animal cell cycle-en.svg|right|thumb|350px|Mitosis in an [[animal cell]] (phases ordered counter-clockwise), with G<sub>2</sub> labeled at bottom.]]
[[File:Human karyotype with bands and sub-bands.png|thumb|250px|Schematic [[karyogram]] of the human chromosomes, showing their usual state in the G<sub>0</sub> and G<sub>1</sub> phase of the cell cycle. At top center it also shows the chromosome 3 pair after having undergone [[DNA synthesis]], occurring in the [[S phase]] (annotated as S) of the cell cycle. This interval includes the G<sub>2</sub> phase and [[metaphase]] (annotated as "Meta.").<br>{{further|Karyotype}}]]

'''G<sub>2</sub> phase''', '''Gap 2 phase''', or '''Growth 2 phase''', is the third subphase of [[interphase]] in the [[cell cycle]] directly preceding [[mitosis]]. It follows the successful completion of [[S phase]], during which the cell’s [[DNA]] is [[DNA replication|replicated]]. G<sub>2</sub> phase ends with the onset of [[prophase]], the first phase of mitosis in which the cell’s [[chromatin]] condenses into [[chromosome]]s.

G<sub>2</sub> phase is a period of rapid cell growth and [[Protein biosynthesis|protein synthesis]] during which the cell prepares itself for mitosis. Curiously, G<sub>2</sub> phase is not a necessary part of the cell cycle, as some cell types (particularly young ''[[Xenopus]]'' embryos<ref name = "Alberts 2004">{{cite book | vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P | chapter = An Overview of the Cell Cycle | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26869/| title = Molecular Biology of the Cell | location = New York | publisher = Garland Science | edition = 4th | year = 2002 | isbn = 978-0-8153-3218-3 }}</ref> and some [[cancer]]s<ref name="Liskay 1977">{{cite journal | vauthors = Liskay RM | title = Absence of a measurable G2 phase in two Chinese hamster cell lines | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 74 | issue = 4 | pages = 1622–5 | date = April 1977 | pmid = 266201 | pmc = 430843 | doi = 10.1073/pnas.74.4.1622 | bibcode = 1977PNAS...74.1622L | doi-access = free }})</ref>) proceed directly from DNA replication to mitosis. Though much is known about the [[genetic network]] which regulates G2 phase and subsequent entry into mitosis, there is still much to be discovered concerning its significance and regulation, particularly in regards to cancer. One hypothesis is that the growth in G<sub>2</sub> phase is regulated as a method of cell size control. Fission yeast (''[[Schizosaccharomyces pombe]]'') has been previously shown to employ such a mechanism, via [[Cdr2 (S. pombe)|Cdr2]]-mediated spatial regulation of [[Wee1]] activity.<ref name="Nurse 2009">{{cite journal | vauthors = Moseley JB, Mayeux A, Paoletti A, Nurse P | title = A spatial gradient coordinates cell size and mitotic entry in fission yeast | journal = Nature | volume = 459 | issue = 7248 | pages = 857–60 | date = June 2009 | pmid = 19474789 | doi = 10.1038/nature08074 | bibcode = 2009Natur.459..857M | s2cid = 4330336 }}</ref> Though Wee1 is a fairly conserved negative regulator of mitotic entry, no general mechanism of cell size control in G2 has yet been elucidated.

Biochemically, the end of G<sub>2</sub> phase occurs when a threshold level of active [[cyclin B1]]/[[CDK1]] complex, also known as [[Maturation promoting factor]] (MPF) has been reached.<ref name="Sible 2003">{{cite journal | vauthors = Sha W, Moore J, Chen K, Lassaletta AD, Yi CS, Tyson JJ, Sible JC | title = Hysteresis drives cell-cycle transitions in Xenopus laevis egg extracts | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 3 | pages = 975–80 | date = February 2003 | pmid = 12509509 | pmc = 298711 | doi = 10.1073/pnas.0235349100 | bibcode = 2003PNAS..100..975S | author6-link = John J. Tyson | doi-access = free }}</ref> The activity of this complex is tightly regulated during G<sub>2</sub>. In particular, the G<sub>2</sub> checkpoint arrests cells in G<sub>2</sub> in response to DNA damage through inhibitory regulation of CDK1.