Editing Genetics (section)

=== Gene regulation ===
{{Main|Regulation of gene expression}}

The genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. A gene is expressed when it is being transcribed into mRNA and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. [[Transcription factor]]s are regulatory proteins that bind to DNA, either promoting or inhibiting the transcription of a gene.<ref>{{cite journal | vauthors = Brivanlou AH, Darnell JE | title = Signal transduction and the control of gene expression | journal = Science | volume = 295 | issue = 5556 | pages = 813–818 | date = February 2002 | pmid = 11823631 | doi = 10.1126/science.1066355 | s2cid = 14954195 | citeseerx = 10.1.1.485.6042 | bibcode = 2002Sci...295..813B }}</ref> Within the genome of ''[[Escherichia coli]]'' bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid [[tryptophan]]. However, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. The presence of tryptophan directly affects the activity of the genes—tryptophan molecules bind to the [[Trp repressor|tryptophan repressor]] (a transcription factor), changing the repressor's structure such that the repressor binds to the genes. The tryptophan repressor blocks the transcription and expression of the genes, thereby creating [[negative feedback]] regulation of the tryptophan synthesis process.<ref>Alberts et al. (2002), [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.1269#1270 II.3. Control of Gene Expression – The Tryptophan Repressor is a Simple Switch That Turns Genes On and Off in Bacteria] {{webarchive|url=https://web.archive.org/web/20070629040218/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.1269 |date=29 June 2007 }}</ref>

[[File:Zinc finger DNA complex.png|thumb|upright|left|Transcription factors bind to DNA, influencing the transcription of associated genes.]]

Differences in gene expression are especially clear within [[multicellular organism]]s, where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes. All the cells in a multicellular organism derive from a single cell, differentiating into variant cell types in response to external and [[Cell signaling|intercellular signals]] and gradually establishing different patterns of gene expression to create different behaviors. As no single gene is responsible for the [[Ontogeny|development]] of structures within multicellular organisms, these patterns arise from the complex interactions between many cells.{{cn|date=October 2022}}

Within [[eukaryote]]s, there exist structural features of [[chromatin]] that influence the transcription of genes, often in the form of modifications to DNA and chromatin that are stably inherited by daughter cells.<ref>{{cite journal | vauthors = Jaenisch R, Bird A | title = Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals | journal = Nature Genetics | volume = 33 | issue = Suppl | pages = 245–254 | date = March 2003 | pmid = 12610534 | doi = 10.1038/ng1089 | s2cid = 17270515 }}</ref> These features are called "[[epigenetic]]" because they exist "on top" of the DNA sequence and retain inheritance from one cell generation to the next. Because of epigenetic features, different cell types [[cell culture|grown]] within the same medium can retain very different properties. Although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of [[paramutation]], have multigenerational inheritance and exist as rare exceptions to the general rule of DNA as the basis for inheritance.<ref>{{cite journal | vauthors = Chandler VL | title = Paramutation: from maize to mice | journal = Cell | volume = 128 | issue = 4 | pages = 641–645 | date = February 2007 | pmid = 17320501 | doi = 10.1016/j.cell.2007.02.007 | s2cid = 6928707 | doi-access = free }}</ref>
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