Editing Cell nucleus (section)

===Nuclear envelope and pores===
{{Main|Nuclear envelope|Nuclear pore}}

[[Image:NuclearPore crop.svg|thumb|right|250px|A cross section of a [[nuclear pore]] on the surface of the [[nuclear envelope]] (1). Other diagram labels show (2) the outer ring, (3) spokes, (4) basket, and (5) filaments.]]

The [[nuclear envelope]] consists of two [[cell membrane|membranes]], an [[Inner nuclear membrane|inner]] and an [[outer nuclear membrane]], perforated by [[nuclear pore]]s.<ref name=Alberts2015>{{cite book|title=Molecular Biology of the Cell |edition=6 |vauthors=Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P |publisher=Garland Science |date=2015 |location=New York}}</ref>{{rp|649}} Together, these membranes serve to separate the cell's genetic material from the rest of the cell contents, and allow the nucleus to maintain an environment distinct from the rest of the cell. Despite their close apposition around much of the nucleus, the two membranes differ substantially in shape and contents. The inner membrane surrounds the nuclear content, providing its defining edge.<ref name="Lodish_2016"/>{{rp|14}} Embedded within the inner membrane, various proteins bind the intermediate filaments that give the nucleus its structure.<ref name=Alberts2015/>{{rp|649}} The outer membrane encloses the inner membrane, and is continuous with the adjacent [[endoplasmic reticulum]] membrane.<ref name=Alberts2015/>{{rp|649}} As part of the endoplasmic reticulum membrane, the outer nuclear membrane is studded with [[ribosome]]s that are actively translating proteins across membrane.<ref name=Alberts2015/>{{rp|649}} The space between the two membranes is called the perinuclear space, and is continuous with the endoplasmic reticulum [[Lumen (anatomy)|lumen]].<ref name=Alberts2015/>{{rp|649}}

In a mammalian nuclear envelope there are between 3000 and 4000 [[nuclear pore complex]]es (NPCs) perforating the envelope.<ref name=Alberts2015/>{{rp|650}} Each NPC contains an eightfold-symmetric ring-shaped structure at a position where the inner and outer membranes fuse.<ref name="Shulga">{{cite journal | vauthors = Shulga N, Mosammaparast N, Wozniak R, Goldfarb DS | title = Yeast nucleoporins involved in passive nuclear envelope permeability | journal = The Journal of Cell Biology | volume = 149 | issue = 5 | pages = 1027–38 | date = May 2000 | pmid = 10831607 | pmc = 2174828 | doi = 10.1083/jcb.149.5.1027 | department = Primary }}</ref> The number of NPCs can vary considerably across cell types; small [[glial cell]]s only have about a few hundred, with large [[Purkinje cell]]s having around 20,000.<ref name=Alberts2015/>{{rp|650}} The NPC provides selective transport of molecules between the [[nucleoplasm]] and the [[cytosol]].<ref name="Alberts2019">{{cite book |last1=Alberts |first1=Bruce |title=Essential cell biology |date=2019 |location=New York |isbn=9780393680393 |page=242 |edition=Fifth}}</ref> The nuclear pore complex is composed of approximately thirty different proteins known as [[nucleoporin]]s.<ref name=Alberts2015/>{{rp|649}} The pores are about 60–80 million [[atomic mass unit|daltons]] in [[molecular weight]] and consist of around 50 (in [[yeast]]) to several hundred proteins (in [[vertebrate]]s).<ref name = "Lodish_2016">{{cite book | vauthors = Lodish HF, Berk A, Kaiser C, Krieger M, Bretscher A, Ploegh H, Amon A, Martin KC, Darnell JE | display-authors = 6 | title = Molecular Cell Biology | date = 2016 | publisher = W.H. Freeman | location = New York | isbn = 978-1-4641-8339-3 | edition = Eighth }}</ref>{{rp|622–4}} The pores are 100&nbsp;nm in total diameter; however, the gap through which molecules freely diffuse is only about 9&nbsp;nm wide, due to the presence of regulatory systems within the center of the pore. This size selectively allows the passage of small water-soluble molecules while preventing larger molecules, such as [[nucleic acid]]s and larger proteins, from inappropriately entering or exiting the nucleus. These large molecules must be actively transported into the nucleus instead. Attached to the ring is a structure called the '''nuclear basket''' that extends into the nucleoplasm, and a series of filamentous extensions that reach into the cytoplasm. Both structures serve to mediate binding to nuclear transport proteins.<ref name="Lodish">{{cite book | vauthors = Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipursky SL, Darnell J | title = Molecular Cell Biology | publisher = WH Freeman | edition = 5th | year = 2004 | location = New York | isbn = 978-0-7167-2672-2 | url-access = registration | url = https://archive.org/details/studentcompanion0000unse_r7k2 }}</ref>{{rp|509–10}}

Most proteins, ribosomal subunits, and some RNAs are transported through the pore complexes in a process mediated by a family of transport factors known as [[karyopherin]]s. Those karyopherins that mediate movement into the nucleus are also called importins, whereas those that mediate movement out of the nucleus are called exportins. Most karyopherins interact directly with their cargo, although some use [[Signal transducing adaptor protein|adaptor proteins]].<ref name="Pemberton">{{cite journal | vauthors = Pemberton LF, Paschal BM | title = Mechanisms of receptor-mediated nuclear import and nuclear export | journal = Traffic | volume = 6 | issue = 3 | pages = 187–98 | date = March 2005 | pmid = 15702987 | doi = 10.1111/j.1600-0854.2005.00270.x | s2cid = 172279 | department = Review | doi-access = free }}</ref> [[Steroid hormone]]s such as [[cortisol]] and [[aldosterone]], as well as other small lipid-soluble molecules involved in intercellular [[cell signaling|signaling]], can diffuse through the cell membrane and into the cytoplasm, where they bind [[nuclear receptor]] proteins that are trafficked into the nucleus. There they serve as [[transcription factor]]s when bound to their [[Ligand (biochemistry)|ligand]]; in the absence of a ligand, many such receptors function as [[histone deacetylase]]s that repress gene expression.<ref name="Lodish"/>{{rp|488}}