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- A gene regulatory network or genetic regulatory network (GRN) is a collection of DNA segments in a cell which interact with each other (indirectly through their RNA and protein expression products) and with other substances in the cell, thereby governing the rates at which genes in the network are transcribed into mRNA. In general, each mRNA molecule goes on to make a specific protein (or set of proteins). In some cases this protein will be structural, and will accumulate at the cell-wall or within the cell to give it particular structural properties. In other cases the protein will be an enzyme; 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 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. 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. 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. 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 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 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 structural proteins that give each cell the physical properties it needs.
- Et gennettverk (engelsk gene regulatory network) er en rekke gener som enten virker på hverandre eller danner en kaskade, og ulike gennettverk styrer genuttrykkingen i hver celle. Genene kan kode for proteiner som påvirker strukturen og formen til cellen, og som endrer cellens fysiske egenskaper. De kan også kode for enzymer som hjelper til med å bryte ned energikilder eller giftstoffer; eller de kan kode for transkripsjonsfaktorer, proteiner som har som oppgave å styre hvorvidt andre gener er skrudd av eller på. Disse transkripsjonsfaktorene er nøkkelfigurene i de regulatoriske gennettverkene. De binder til promotorregionen ved starten til de genene de slår på, eller i enhancerregionene til disse genene, og leder dermed transkripsjonsmaskineriet til å uttrykke disse genene. Sekvensene en transkripsjonsfaktor binder til kan finnes mange steder i genomet, og hver enkelt av transkripsjonsfaktorene kan dermed styre uttrykkingen av mange gener. I encellende organismer vil de regulatoriske nettverkene reagere på miljøet, slik at cellen til enhver tid kan vokse optimalt i det miljøet den befinner seg i. For eksempel vil en gjærcelle som er i et miljø med sukker skru på genene som brukes til å bryte ned sukkeret og danne alkohol. Og under miljømessig stress - som matmangel, eller annet - vil Gram-positive bakterier skru på genene som gjør at de danner en endospore, en struktur som kan tåle uttørking, varme, nedbrytende enzymer, og stråling langt bedre enn bakterien ville kunne gjøre i en vanlig dvaletilstand. I flercellede dyr brukes det samme prinsippet i genkaskader som bestemmer kroppsform. Hver gang en celle deler seg har begge de to dattercellene det samme genomet, men ulike gener kan være uttrykt i dem. Via epigenetiske mekanismer som endrer kromatinstrukturen til DNA kan en celles identitet - som altså avhenger av hvilke gener den uttrykker - bevares, selv i videre celledelinger, slik at de samme genene uttrykkes.
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