Editing Gene regulatory network (section)

{{Short description|Collection of molecular regulators}}
{{Use dmy dates|date=January 2020}}
[[Image:Gene Regulatory Network.jpg|thumb|right|360px|Structure of a gene regulatory network]]
[[Image:Gene Regulatory Network 2.jpg|thumb|right|360px|Control process of a gene regulatory network]]

A '''gene''' (or '''genetic''') '''regulatory network''' ('''GRN''') is a collection of molecular regulators that interact with each other and with other substances in the cell to govern the [[gene expression]] levels of [[mRNA]] and proteins which, in turn, determine the function of the cell. GRN also play a central role in [[morphogenesis]], the creation of body structures, which in turn is central to [[evolutionary developmental biology]] (evo-devo).

The regulator can be [[DNA]], [[RNA]], [[protein]] or any combination of two or more of these three that form a complex, such as a specific sequence of DNA and a [[transcription factor]] to activate that sequence. The interaction can be direct or indirect (through transcribed RNA or translated protein). In general, each mRNA molecule goes on to make a specific protein (or set of proteins). In some cases this protein will be [[Protein#Structural proteins|structural]], and will accumulate at the cell membrane or within the cell to give it particular structural properties. In other cases the protein will be an [[enzyme]], i.e., a micro-machine that catalyses a certain reaction, such as the breakdown of a food source or toxin. Some proteins though serve only to activate other genes, and these are the [[transcription factors]] that are the main players in regulatory networks or cascades. By binding to the [[promoter (biology)|promoter]] region at the start of other genes they turn them on, initiating the production of another protein, and so on. Some transcription factors are inhibitory.<ref name="pmid8930119">{{cite journal | vauthors = Latchman DS | title = Inhibitory transcription factors | journal = The International Journal of Biochemistry & Cell Biology | volume = 28 | issue = 9 | pages = 965–974 | date = September 1996 | pmid = 8930119 | doi = 10.1016/1357-2725(96)00039-8 }}</ref>

In single-celled organisms, regulatory networks respond to the external environment, optimising the cell at a given time for survival in this environment. Thus a yeast cell, finding itself in a sugar solution, will turn on genes to make enzymes that process the sugar to alcohol.<ref>{{cite journal | vauthors = Lee TI, Rinaldi NJ, Robert F, Odom DT, Bar-Joseph Z, Gerber GK, Hannett NM, Harbison CT, Thompson CM, Simon I, Zeitlinger J, Jennings EG, Murray HL, Gordon DB, Ren B, Wyrick JJ, Tagne JB, Volkert TL, Fraenkel E, Gifford DK, Young RA | display-authors = 6 | title = Transcriptional regulatory networks in Saccharomyces cerevisiae | journal = Science | volume = 298 | issue = 5594 | pages = 799–804 | date = October 2002 | pmid = 12399584 | doi = 10.1126/science.1075090 | publisher = Young Lab | s2cid = 4841222 | bibcode = 2002Sci...298..799L }}</ref> This process, which we associate with wine-making, is how the yeast cell makes its living, gaining energy to multiply, which under normal circumstances would enhance its survival prospects.

In multicellular animals the same principle has been put in the service of gene cascades that control body-shape.<ref>{{cite journal | vauthors = Davidson E, Levin M | title = Gene regulatory networks | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 102 | issue = 14 | pages = 4935 | date = April 2005 | pmid = 15809445 | pmc = 556010 | doi = 10.1073/pnas.0502024102 | doi-access = free | bibcode = 2005PNAS..102.4935D }}</ref> Each time a cell divides, two cells result which, although they contain the same genome in full, can differ in which genes are turned on and making proteins. Sometimes a 'self-sustaining feedback loop' ensures that a cell maintains its identity and passes it on. Less understood is the mechanism of [[epigenetics]] by which [[chromatin]] modification may provide cellular memory by blocking or allowing transcription. A major feature of multicellular animals is the use of [[morphogen]] gradients, which in effect provide a positioning system that tells a cell where in the body it is, and hence what sort of cell to become. A gene that is turned on in one cell may make a product that leaves the cell and [[diffusion|diffuses]] through adjacent cells, entering them and turning on genes only when it is present above a certain threshold level. These cells are thus induced into a new fate, and may even generate other [[morphogens]] that signal back to the original cell. Over longer distances morphogens may use the active process of [[signal transduction]]. Such signalling controls [[embryogenesis]], the building of a [[body plan]] from scratch through a series of sequential steps. They also control and maintain adult bodies through [[feedback]] processes, and the loss of such feedback because of a mutation can be responsible for the cell proliferation that is seen in [[cancer]]. In parallel with this process of building structure, the gene cascade turns on genes that make [[Protein#Structural proteins|structural protein]]s that give each cell the physical properties it needs.