Editing Nucleolus (section)

== Function and ribosome assembly ==
{{Main|Ribosome biogenesis}}

[[File:NucleolusNCc.jpg|thumb|Electron micrograph of part of a [[HeLa]] cell. The image is a screen capture from [https://upload.wikimedia.org/wikipedia/commons/1/11/Live-cell-immunogold-labelling-of-RNA-polymerase-II-srep08324-s2.ogv this movie], which shows a Z-stack of the cell.]]
In [[ribosome biogenesis]], two of the three eukaryotic [[RNA polymerase]]s ([[RNA polymerase I|Pol I]] and [[RNA polymerase III|Pol III]]) are required, and these function in a coordinated manner. In an initial stage, the [[Ribosomal RNA|rRNA]] genes are transcribed as a single unit within the nucleolus by [[RNA polymerase I]]. In order for this transcription to occur, several pol I-associated factors and DNA-specific trans-acting factors are required. In [[yeast]], the most important are: UAF ([[upstream activating factor]]), [[TATA Binding Protein|TBP]] (TATA-box binding protein), and [[core binding factor]] (CBF), which bind promoter elements and form the [[Transcription preinitiation complex|preinitiation complex]] (PIC), which is in turn recognized by RNA polymerase. In humans, a similar PIC is assembled with [[selective factor 1|SL1]], the promoter selectivity factor (composed of TBP and [[TBP-associated factor]]s, or TAFs), transcription initiation factors, and [[UBTF|UBF]] (upstream binding factor). RNA polymerase I transcribes most rRNA transcripts ([[28S ribosomal RNA|28S]], [[18S ribosomal RNA|18S]], and [[5.8S ribosomal RNA|5.8S]]), but the [[5S ribosomal RNA|5S]] rRNA subunit (component of the 60S ribosomal subunit) is transcribed by RNA polymerase III.<ref>{{cite book  |first1=Pamela C. |last1=Champe |first2=Richard A. |last2=Harvey |first3=Denise R. |last3=Ferrier | name-list-style = vanc |title=Lippincott's Illustrated Reviews: Biochemistry |url=https://books.google.com/books?id=M_YOW50cg9oC |year=2005 |publisher=Lippincott Williams & Wilkins |isbn=978-0-7817-2265-0}}</ref>

Transcription of rRNA yields a long precursor molecule ([[45S pre-rRNA]]), which still contains the [[internal transcribed spacer]] (ITS) and [[external transcribed spacer]] (ETS). Further processing is needed to generate the 18S RNA, 5.8S, and 28S RNA molecules. In eukaryotes, the RNA-modifying enzymes are brought to their respective [[recognition site]]s by interaction with guide RNAs, which bind these specific sequences.  These guide RNAs belong to the class of small nucleolar RNAs ([[snoRNA]]s), which are complexed with proteins and exist as small-nucleolar-[[ribonucleoprotein]]s ([[snoRNP]]s). Once the rRNA subunits are processed, they are ready to be assembled into larger ribosomal subunits. However, an additional rRNA molecule, the 5S rRNA, is also necessary. In yeast, the 5S rDNA sequence is localized in the intergenic spacer and is transcribed in the nucleolus by RNA polymerase.

In higher [[eukaryote]]s and plants, the situation is more complex, for the 5S DNA sequence lies outside the NOR and is transcribed by RNA Pol III in the [[nucleoplasm]], after which it finds its way into the nucleolus to participate in the ribosome assembly. This assembly not only involves the rRNA, but also [[ribosomal protein]]s. The genes encoding these r-proteins are transcribed by Pol II in the nucleoplasm by a "conventional" pathway of protein synthesis (transcription, pre-mRNA processing, nuclear export of mature mRNA, and [[translation (biology)|translation]] on cytoplasmic ribosomes). The mature r-proteins are then imported into the nucleus and, finally, the nucleolus. Association and maturation of rRNA and r-proteins result in the formation of the 40S (small) and 60S (large) subunits of the complete ribosome. These are exported through the [[nuclear pore complex]]es to the cytoplasm, where they remain free or become associated with the [[endoplasmic reticulum]], forming the [[rough endoplasmic reticulum]] (RER).<ref name="Alberts-2002">{{cite book |first1=Bruce |last1=Alberts |first2=Alexander |last2=Johnson |first3=Julian |last3=Lewis |first4=Martin |last4=Raff |first5=Keith |last5=Roberts |first6=Peter |last6=Walter | name-list-style = vanc |title=Molecular Biology of the Cell |publisher=Garland Science |location=New York |year=2002 |isbn=978-0-8153-3218-3 |pages=331–3 |url=https://www.ncbi.nlm.nih.gov/books/NBK21054/ |edition=4th}}</ref><ref name="Cooper-2007">{{cite book |first1=Geoffrey M. |last1=Cooper |first2=Robert E. |last2=Hausman | name-list-style = vanc |title=The Cell: A Molecular Approach |publisher=Sinauer Associates |year=2007 |isbn=978-0-87893-220-7 |pages=371–9 |edition=4th}}</ref>

In human [[Endometrium|endometrial cells]], a network of nucleolar channels is sometimes formed. The origin and function of this network have not yet been clearly identified.<ref>{{cite journal|last1=Wang|first1=Tzuneng|last2=Schneider|first2=J | name-list-style = vanc |title=Origin and fate of the nucleolar channel system of normal human endometrium|journal=Cell Research|date=1 July 1992|volume=2|issue=2|pages=97–102|doi=10.1038/cr.1992.10|doi-access=free}}</ref>