Editing Cell nucleus (section)

{{short description|Eukaryotic membrane-bounded organelle containing DNA}}
{{pp-vandalism|expiry=indef|small=yes}}
{{Use dmy dates|date=August 2018}}
[[File:HeLa cells stained with Hoechst 33258.jpg|thumb|upright=1.35|right|[[HeLa|HeLa cells]] stained for [[nuclear DNA]] with the blue [[fluorescent]] [[Hoechst stain|Hoechst dye]]. The central and rightmost cells are in [[interphase]], thus their entire nuclei are labeled. On the left, a cell is going through [[mitosis]] and its DNA has condensed.]]
{{Organelle diagram}}

The '''cell nucleus''' ({{etymology|la|{{wikt-lang|la|nucleus}} or {{wikt-lang|la|nuculeus}}|kernel, seed}}; {{plural form}}: '''nuclei''') is a [[membrane-bound organelle]] found in [[eukaryote|eukaryotic]] [[cell (biology)|cells]]. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian [[red blood cell]]s, have [[#Anucleated_cells|no nuclei]], and a few others including [[osteoclast]]s have [[Multinucleate|many]]. The main structures making up the nucleus are the [[nuclear envelope]], a double membrane that encloses the entire organelle and isolates its contents from the cellular [[cytoplasm]]; and the [[nuclear matrix]], a network within the nucleus that adds mechanical support.

The cell nucleus contains nearly all of the cell's [[genome]]. [[Nuclear DNA]] is often organized into multiple [[chromosome]]s – long strands of [[DNA]] dotted with various [[protein]]s, such as [[histone]]s, that protect and organize the DNA. The [[gene]]s within these chromosomes are [[Nuclear organization|structured]] in such a way to promote cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating [[gene expression]].

Because the nuclear envelope is impermeable to [[Macromolecule|large molecules]], [[nuclear pore]]s are required to regulate [[nuclear transport]] of molecules across the envelope. The pores cross both nuclear membranes, providing a [[Ion channel|channel]] through which larger molecules must be [[Active transport|actively transported]] by carrier proteins while allowing free movement of small molecules and [[ion]]s. Movement of large molecules such as proteins and [[RNA]] through the pores is required for both gene expression and the maintenance of chromosomes. Although the interior of the nucleus does not contain any membrane-bound subcompartments, a number of [[nuclear bodies]] exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes. The best-known of these is the [[nucleolus]], involved in the assembly of [[ribosome]]s.