Editing Gene expression (section)

===Folding===
{{main|Protein folding}}
[[Image:Protein folding.png|thumb|right|300px|alt=Process of protein folding.|Protein before (left) and after (right) folding]]

Each [[protein]] exists as an unfolded [[polypeptide]] or random coil when translated from a sequence of [[mRNA]] into a linear chain of [[amino acid]]s. This polypeptide lacks any developed three-dimensional structure (the left hand side of the neighboring figure). The polypeptide then folds into its characteristic and functional [[protein structure|three-dimensional structure]] from a [[random coil]].<ref name=Alberts>{{cite book | vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walters P | title =Molecular Biology of the Cell | edition = Fourth | publisher =Garland Science| year =2002 | location =New York and London | chapter =The Shape and Structure of Proteins |chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26830/ | isbn =978-0-8153-3218-3 | author-link =Bruce Alberts}}</ref> Amino acids interact with each other to produce a well-defined three-dimensional structure, the folded protein (the right hand side of the figure) known as the [[native state]]. The resulting three-dimensional structure is determined by the amino acid sequence ([[Anfinsen's dogma]]).<ref name="Anfinsen">{{cite journal | vauthors = Anfinsen CB | title = The formation and stabilization of protein structure | journal = The Biochemical Journal | volume = 128 | issue = 4 | pages = 737–749 | date = July 1972 | pmid = 4565129 | pmc = 1173893 | doi = 10.1042/bj1280737 | author-link = Christian B. Anfinsen }}</ref>

The correct three-dimensional structure is essential to function, although some parts of functional proteins [[Intrinsically disordered proteins|may remain unfolded]].<ref>{{cite book|vauthors = Berg JM,Tymoczko JL, (([[Lubert Stryer]]; Web content by Neil D. Clarke))|title=Biochemistry|publisher=W. H. Freeman|location=San Francisco |year=2002 |isbn=978-0-7167-4684-3 |chapter=3. Protein Structure and Function |chapter-url= https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=stryer%5Bbook%5D+AND+215168%5Buid%5D&rid=stryer.chapter.280}}</ref> Failure to fold into the intended shape usually produces inactive proteins with different properties including toxic [[prion]]s. Several [[neurodegenerative]] and other [[disease]]s are believed to result from the accumulation of ''misfolded'' proteins.<ref name="Selkoe:03">{{cite journal | vauthors = Selkoe DJ | title = Folding proteins in fatal ways | journal = Nature | volume = 426 | issue = 6968 | pages = 900–904 | date = December 2003 | pmid = 14685251 | doi = 10.1038/nature02264 | s2cid = 6451881 | bibcode = 2003Natur.426..900S }}</ref> Many [[allergy|allergies]] are caused by the folding of the proteins, for the immune system does not produce antibodies for certain protein structures.<ref>{{cite book | vauthors = Alberts B, Bray D, Hopkin K, Johnson A, Lewis J, Raff M, Roberts K, Walter P | chapter = Protein Structure and Function | title = Essential Cell Biology | edition = 3rd | location = New York | publisher = Garland Science, Taylor and Francis Group, LLC | year = 2010 | pages = 120–170 |isbn=978-0-8153-4129-1}}</ref>

Enzymes called [[Chaperone (protein)|chaperone]]s assist the newly formed protein to attain ([[protein folding|fold]] into) the 3-dimensional structure it needs to function.<ref name="Hebert2007">{{cite journal | vauthors = Hebert DN, Molinari M | title = In and out of the ER: protein folding, quality control, degradation, and related human diseases | journal = Physiological Reviews | volume = 87 | issue = 4 | pages = 1377–1408 | date = October 2007 | pmid = 17928587 | doi = 10.1152/physrev.00050.2006 | url = https://works.bepress.com/cgi/viewcontent.cgi?article=1021&context=daniel_hebert }}</ref> Similarly, RNA chaperones help RNAs attain their functional shapes.<ref name="Russell2008">{{cite journal | vauthors = Russell R | title = RNA misfolding and the action of chaperones | journal = Frontiers in Bioscience | volume = 13 | issue = 13 | pages = 1–20 | date = January 2008 | pmid = 17981525 | pmc = 2610265 | doi = 10.2741/2557 }}</ref> Assisting protein folding is one of the main roles of the endoplasmic reticulum in eukaryotes.