Editing Cell nucleus (section)

===Nucleolus===
{{Main|Nucleolus}}
{{Further|Nuclear bodies}}
[[Image:Micrograph of a cell nucleus.png|thumb|200px|An [[electron micrograph]] of a cell nucleus, showing the darkly stained [[nucleolus]]]]

The [[nucleolus]] is the largest of the discrete densely stained, membraneless structures known as [[nuclear bodies]] found in the nucleus. It forms around [[tandem repeat]]s of [[ribosomal DNA|rDNA]], DNA coding for [[ribosomal RNA]] (rRNA). These regions are called [[nucleolar organizer regions]] (NOR). The main roles of the nucleolus are to synthesize rRNA and [[Nucleolus#Function and ribosome assembly|assemble ribosomes]]. The structural cohesion of the nucleolus depends on its activity, as ribosomal assembly in the nucleolus results in the transient association of nucleolar components, facilitating further ribosomal assembly, and hence further association. This model is supported by observations that inactivation of rDNA results in intermingling of nucleolar structures.<ref name="Hernandez-Verdun">{{cite journal | vauthors = Hernandez-Verdun D | title = Nucleolus: from structure to dynamics | journal = Histochemistry and Cell Biology | volume = 125 | issue = 1–2 | pages = 127–37 | date = January 2006 | pmid = 16328431 | doi = 10.1007/s00418-005-0046-4 | url = https://hal.archives-ouvertes.fr/hal-00015455 | s2cid = 20769260 | department = Review }}</ref>

In the first step of ribosome assembly,  a protein called [[RNA polymerase I]] transcribes rDNA, which forms a large pre-rRNA precursor. This is cleaved into two [[LSU rRNA|large rRNA subunits]] – [[5.8S ribosomal RNA|5.8S]], and [[28S ribosomal RNA|28S]], and a [[SSU rRNA|small rRNA subunit]] [[18S ribosomal RNA|18S]].<ref name=Alberts2015/>{{rp|328}}<ref name="Lamond-Sleeman">{{cite journal | vauthors = Lamond AI, Sleeman JE | title = Nuclear substructure and dynamics | journal = Current Biology | volume = 13 | issue = 21 | pages = R825-8 | date = October 2003 | pmid = 14588256 | doi = 10.1016/j.cub.2003.10.012 | s2cid = 16865665 | department = Review | doi-access = free | bibcode = 2003CBio...13.R825L }}</ref> The transcription, post-transcriptional processing, and assembly of rRNA occurs in the nucleolus, aided by [[small nucleolar RNA]] (snoRNA) molecules, some of which are derived from spliced [[intron]]s from [[messenger RNA]]s encoding genes related to ribosomal function. The assembled ribosomal subunits are the largest structures passed through the [[nuclear pore]]s.<ref name="Lodish" />{{rp|526}}

When observed under the [[electron microscope]], the nucleolus can be seen to consist of three distinguishable regions: the innermost ''fibrillar centers'' (FCs), surrounded by the ''dense fibrillar component'' (DFC) (that contains [[fibrillarin]] and [[nucleolin]]), which in turn is bordered by the ''granular component'' (GC) (that contains the protein [[nucleophosmin]]). Transcription of the rDNA occurs either in the FC or at the FC-DFC boundary, and, therefore, when rDNA transcription in the cell is increased, more FCs are detected. Most of the cleavage and modification of rRNAs occurs in the DFC, while the latter steps involving protein assembly onto the ribosomal subunits occur in the GC.<ref name=Lamond-Sleeman />