Editing G0 phase (section)

==Diversity of G<sub>0</sub> states==
[[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 [[G2 phase|G<sub>2</sub> phase]] and [[metaphase]] (annotated as "Meta.").<br>{{further|Karyotype}}]]
Three G<sub>0</sub> states exist and can be categorized as either reversible ([[wikt:quiescent|quiescent]]) or irreversible ([[Cellular senescence|senescent]] and [[Cellular differentiation|differentiated]]). Each of these three states can be entered from the G<sub>1</sub> phase before the cell commits to the next round of the cell cycle. Quiescence refers to a reversible G<sub>0</sub> state where subpopulations of cells reside in a 'quiescent' state before entering the cell cycle after activation in response to extrinsic signals. Quiescent cells are often identified by low [[RNA]] content, lack of cell proliferation markers, and increased label retention indicating low cell turnover.<ref name="Hüttmann2001">{{cite journal|last1=Hüttmann|first1=A|title=Functional heterogeneity within rhodamine123lo Hoechst33342lo/sp primitive hemopoietic stem cells revealed by pyronin Y|journal=Experimental Hematology|volume=29|issue=9|year=2001|pages=1109–1116|issn=0301-472X|doi=10.1016/S0301-472X(01)00684-1|pmid=11532352|doi-access=free}}</ref><ref name="FukadaUezumi2007">{{cite journal | vauthors = Fukada S, Uezumi A, Ikemoto M, Masuda S, Segawa M, Tanimura N, Yamamoto H, Miyagoe-Suzuki Y, Takeda S | title = Molecular signature of quiescent satellite cells in adult skeletal muscle | journal = Stem Cells | volume = 25 | issue = 10 | pages = 2448–59 | date = October 2007 | pmid = 17600112 | doi = 10.1634/stemcells.2007-0019 | doi-access = free }}</ref> [[Senescence]] is distinct from quiescence because senescence is an irreversible state that cells enter in response to DNA damage or degradation that would make a cell's progeny nonviable. Such [[DNA damage (naturally occurring)|DNA damage]] can occur from [[Telomere#Shortening|telomere shortening]] over many cell divisions as well as reactive oxygen species (ROS) exposure, oncogene activation, and cell-cell fusion. While senescent cells can no longer replicate, they remain able to perform many normal cellular functions.<ref>{{cite journal | vauthors = Hayflick L, Moorhead PS | title = The serial cultivation of human diploid cell strains | journal = Experimental Cell Research | volume = 25 | issue = 3 | pages = 585–621 | date = December 1961 | pmid = 13905658 | doi = 10.1016/0014-4827(61)90192-6 }}</ref><ref>{{cite journal | vauthors = Campisi J | title = Aging, cellular senescence, and cancer | journal = Annual Review of Physiology | volume = 75 | pages = 685–705 | date = February 2013 | pmid = 23140366 | pmc = 4166529 | doi = 10.1146/annurev-physiol-030212-183653 }}</ref><ref>{{cite journal | vauthors = Rodier F, Campisi J | title = Four faces of cellular senescence | journal = The Journal of Cell Biology | volume = 192 | issue = 4 | pages = 547–56 | date = February 2011 | pmid = 21321098 | pmc = 3044123 | doi = 10.1083/jcb.201009094 }}</ref><ref>{{cite journal | vauthors = Burton DG, Krizhanovsky V | title = Physiological and pathological consequences of cellular senescence | journal = Cellular and Molecular Life Sciences | volume = 71 | issue = 22 | pages = 4373–86 | date = November 2014 | pmid = 25080110 | pmc = 4207941 | doi = 10.1007/s00018-014-1691-3 }}</ref> Senescence is often a biochemical alternative to the self-destruction of such a damaged cell by [[apoptosis]]. In contrast to cellular senescence, quiescence is not a reactive event but part of the core programming of several different cell types. Finally, differentiated cells are stem cells that have progressed through a differentiation program to reach a mature – terminally differentiated – state. Differentiated cells continue to stay in G<sub>0</sub> and perform their main functions indefinitely.