Editing Nucleolus (section)

== Structure ==
Three major components of the nucleolus are recognized: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC).<ref name="O'Sullivan-2013" /> Transcription of the [[ribosomal DNA|rDNA]] occurs in the FC.<ref name="Sirri-2008">{{cite journal | vauthors = Sirri V, Urcuqui-Inchima S, Roussel P, Hernandez-Verdun D | title = Nucleolus: the fascinating nuclear body | journal = Histochemistry and Cell Biology | volume = 129 | issue = 1 | pages = 13–31 | date = January 2008 | pmid = 18046571 | pmc = 2137947 | doi = 10.1007/s00418-007-0359-6 }}</ref> The DFC contains the protein [[fibrillarin]],<ref name="Sirri-2008" /> which is important in rRNA processing. The GC contains the protein [[NPM1|nucleophosmin]],<ref name="Sirri-2008" /> (B23 in the external image), which is also involved in [[ribosome biogenesis]].

However, it has been proposed that this particular organization is only observed in higher eukaryotes and that it evolved from a bipartite organization with the transition from [[anamniotes]] to [[amniote]]s. Reflecting the substantial increase in the DNA [[intergenic region]], an original fibrillar component would have separated into the FC and the DFC.<ref>{{cite journal | vauthors = Thiry M, Lafontaine DL | title = Birth of a nucleolus: the evolution of nucleolar compartments | journal = Trends in Cell Biology | volume = 15 | issue = 4 | pages = 194–9 | date = April 2005 | pmid = 15817375 | doi = 10.1016/j.tcb.2005.02.007 }} [http://www.lafontainelab.com/Suppl_data/Thiry_2005/S3.pdf as PDF] {{Webarchive|url=https://web.archive.org/web/20081217034953/http://www.lafontainelab.com/Suppl_data/Thiry_2005/S3.pdf |date=17 December 2008 }}</ref>
[[File:Widespread-Expression-of-BORISCTCFL-in-Normal-and-Cancer-Cells-pone.0022399.s011.ogv|thumb|Nucleus from a cell line. Fibrillarin in red. Transcription regulatory protein [[CTCFL]] in green. Nuclear DNA in blue.]]
Another structure identified within many nucleoli (particularly in plants) is a clear area in the center of the structure referred to as a nucleolar vacuole.<ref>{{cite journal | vauthors = Beven AF, Lee R, Razaz M, Leader DJ, Brown JW, Shaw PJ | title = The organization of ribosomal RNA processing correlates with the distribution of nucleolar snRNAs | journal = Journal of Cell Science | volume = 109 ( Pt 6) | issue = 6 | pages = 1241–51 | date = June 1996 | doi = 10.1242/jcs.109.6.1241 | pmid = 8799814 | url = http://jcs.biologists.org/cgi/content/short/109/6/1241 }}</ref>
Nucleoli of various plant species have been shown to have very high concentrations of iron<ref>{{cite journal | vauthors = Roschzttardtz H, Grillet L, Isaure MP, Conéjéro G, Ortega R, Curie C, Mari S | title = Plant cell nucleolus as a hot spot for iron | journal = The Journal of Biological Chemistry | volume = 286 | issue = 32 | pages = 27863–6 | date = August 2011 | pmid = 21719700 | pmc = 3151030 | doi = 10.1074/jbc.C111.269720 | doi-access = free }}</ref> in contrast to human and animal cell nucleoli.

The nucleolus [[ultrastructure]] can be seen through an [[electron microscope]], while the organization and dynamics can be studied through [[Fluorophore|fluorescent protein tagging]] and fluorescent recovery after [[photobleaching]] ([[Fluorescence recovery after photobleaching|FRAP]]). Antibodies against the PAF49 protein can also be used as a marker for the nucleolus in immunofluorescence experiments.<ref>[https://www.genetex.com/Product/Detail/PAF49-antibody/GTX102175 PAF49 antibody | GeneTex Inc]. Genetex.com. Retrieved 2019-07-18.</ref>

Although usually only one or two nucleoli can be seen, a diploid human cell has ten [[nucleolus organizer region]]s (NORs) and could have more nucleoli. Most often multiple NORs participate in each nucleolus.<ref>{{cite book | chapter = The Cell: Basic Structure and Function | title = Comprehensive Cytopathology | edition = third | vauthors = von Knebel Doeberitz M, Wentzensen N |date= 2008 }}</ref>