Product Code Database
The nucleolus (; : nucleoli ) is the largest structure in the cell nucleus of eukaryote cells. It is best known as the site of ribosome biogenesis. The nucleolus also participates in the formation of signal recognition particles and plays a role in the cell's response to stress. (2015). 9780470016176 ISBN 9780470016176 Nucleoli are made of , DNA and RNA, and form around specific chromosomal regions called nucleolar organizing regions. Malfunction of the nucleolus is the cause of several human conditions called "nucleolopathies" and the nucleolus is being investigated as a target for cancer chemotherapy.
Little was known about the function of the nucleolus until 1964, when a study of nucleoli by John Gurdon and Donald Brown in the African clawed frog Xenopus laevis generated increased interest in its function and detailed structure. They found that 25% of the frog eggs had no nucleolus, and that such eggs were not capable of life. Half of the eggs had one nucleolus and 25% had two. They concluded that the nucleolus had a function necessary for life. In 1966, Max L. Birnstiel and collaborators showed via nucleic acid hybridization experiments that DNA within nucleoli codes for ribosomal RNA.
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 . Reflecting the substantial increase in the DNA intergenic region, an original fibrillar component would have separated into the FC and the DFC. as PDF 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. Nucleoli of various plant species have been shown to have very high concentrations of iron 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 and fluorescent recovery after photobleaching (FRAP). Antibodies against the PAF49 protein can also be used as a marker for the nucleolus in immunofluorescence experiments. PAF49 antibody | GeneTex Inc. Genetex.com. Retrieved 2019-07-18.
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.
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 by interaction with guide RNAs, which bind these specific sequences. These guide RNAs belong to the class of small nucleolar RNAs (), which are complexed with proteins and exist as small-nucleolar-ribonucleoproteins (). 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 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 proteins. 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 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 complexes to the cytoplasm, where they remain free or become associated with the endoplasmic reticulum, forming the rough endoplasmic reticulum (RER). (2025). 9780878932207, Sinauer Associates. ISBN 9780878932207
In human Endometrium, a network of nucleolar channels is sometimes formed. The origin and function of this network have not yet been clearly identified.
|
|
