Nucleolus

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For the solution concept in cooperative game theory, see Nucleolus (game theory).

Nucleolus contained within the cell nucleus
Animal cell diagram
Components of a typical animal cell:
  1. Nucleolus
  2. Nucleus
  3. Ribosome (dots as part of 5)
  4. Vesicle
  5. Rough endoplasmic reticulum
  6. Golgi apparatus (or, Golgi body)
  7. Cytoskeleton
  8. Smooth endoplasmic reticulum
  9. Mitochondrion
  10. Vacuole
  11. Cytosol (fluid that contains organelles; with which, comprises cytoplasm)
  12. Lysosome
  13. Centrosome
  14. Cell membrane

The nucleolus (

nucleolar organizing regions. Malfunction of nucleoli can be the cause of several human conditions called "nucleolopathies"[3][4] and the nucleolus is being investigated as a target for cancer chemotherapy.[5][6]

History

The nucleolus was identified by bright-field microscopy during the 1830s.[7] Theodor Schwann in his 1939 treatise describes that Schleiden had identified small corpuscleus in nuclei, and names the structure "Kernkörperchen". In a 1947 translation of the work to English, the structure receives the name of "nucleolus".[8][9]

In addition to these peculiarities of the cytoblast, already made known by Brown and Meyen, Schleiden has discovered in its interior a small corpuscle (see plate I, fig. 1, 4,) which, in the fully-developed cytoblast, looks like a thick ring, or a thick-walled hollow globule. It appears, however, to present a different appearance in different cytoblasts. Sometimes only the external sharply-defined circle of this ring can be distinguished, with a dark point in the centre,—occasionally, and indeed most frequently, only a sharply circumscribed spot. In other instances this spot is very small, and sometimes cannot be recognized at all. As it will frequently be necessary to speak of this body in the following treatise, I will for brevity’s sake name it the “nucleolus,” (Kernkorperchen, ‘nucleus-corpuscle.”)

— Theodor Schwann, translated by Henry Smith, Microscopical Researches Into the Accordance in the Structure and Growth of Animals and Plants, page 3

Little was known about the function of the nucleolus until 1964, when a study

Max L. Birnstiel and collaborators showed via nucleic acid hybridization experiments that DNA within nucleoli codes for ribosomal RNA.[11][12]

Structure

Three major components of the nucleolus are recognized: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC).[1] Transcription of the rDNA occurs in the FC.[13] The DFC contains the protein fibrillarin,[13] which is important in rRNA processing. The GC contains the protein nucleophosmin,[13] (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 amniotes. Reflecting the substantial increase in the DNA intergenic region, an original fibrillar component would have separated into the FC and the DFC.[14]

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.[15] Nucleoli of various plant species have been shown to have very high concentrations of iron[16] 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 fluorescent protein tagging and fluorescent recovery after photobleaching (FRAP). Antibodies against the PAF49 protein can also be used as a marker for the nucleolus in immunofluorescence experiments.[17]

Although usually only one or two nucleoli can be seen, a diploid human cell has ten nucleolus organizer regions (NORs) and could have more nucleoli. Most often multiple NORs participate in each nucleolus.[18]

Function and ribosome assembly

Main article: Ribosome biogenesis
Electron micrograph of part of a HeLa cell. The image is a screen capture from this movie, which shows a Z-stack of the cell.

In

TBP (TATA-box binding protein), and core binding factor (CBF), which bind promoter elements and form the preinitiation complex (PIC), which is in turn recognized by RNA polymerase. In humans, a similar PIC is assembled with SL1, the promoter selectivity factor (composed of TBP and TBP-associated factors, or TAFs), transcription initiation factors, and UBF (upstream binding factor). RNA polymerase I transcribes most rRNA transcripts (28S, 18S, and 5.8S), but the 5S rRNA subunit (component of the 60S ribosomal subunit) is transcribed by RNA polymerase III.[19]

Transcription of rRNA yields a long precursor molecule (45S pre-rRNA), which still contains the ITS and 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

snoRNPs
). 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

rough endoplasmic reticulum (RER).[20][21]

In human endometrial cells, a network of nucleolar channels is sometimes formed. The origin and function of this network have not yet been clearly identified.[22]

Sequestration of proteins

In addition to its role in ribosomal biogenesis, the nucleolus is known to capture and immobilize proteins, a process known as nucleolar detention. Proteins that are

long noncoding RNAs originating from intergenic regions of the nucleolus are responsible for this phenomenon.[23]

See also

References

  1. ^
    PMID 25436580
    .
  2. ISBN 978-0-470-01617-6
    .
  3. PMID 24631655
    .
  4. PMID 35096082
    .
  5. PMID 24389329
    .
  6. PMID 25464032
    .
  7. PMID 21106648
    .
  8. ^ "Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen / Von Th. Schwann. Mit vier Kupfertafeln". Wellcome Collection. Retrieved 12 March 2024.
  9. ^ Schwann, Theodor; Schwann, Theodor; Smith, Henry; Schleiden, M. J. (1847). Microscopical researches into the accordance in the structure and growth of animals and plants. London: Sydenham Society. p. 3.
  10. PMID 14106673
    .
  11. PMID 5963987
    .
  12. PMID 5943882
    .
  13. ^
    PMID 18046571
    .
  14. PMID 15817375. as PDF Archived 17 December 2008 at the Wayback Machine
  15. PMID 8799814
    .
  16. PMID 21719700
    .
  17. ^ PAF49 antibody | GeneTex Inc. Genetex.com. Retrieved 2019-07-18.
  18. ^ von Knebel Doeberitz M, Wentzensen N (2008). "The Cell: Basic Structure and Function". Comprehensive Cytopathology (third ed.).
  19. ISBN 978-0-7817-2265-0
    .
  20. ISBN 978-0-8153-3218-3
    .
  21. ISBN 978-0-87893-220-7
    .
  22. doi:10.1038/cr.1992.10
    .
  23. PMID 22284675
    .

Further reading

External links

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