Nucleolus
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In cell biology, the nucleolus is, strictly speaking, a "suborganelle" of the nucleus, which is an organelle. It is roughly spherical-shaped surrounded by a layer of condensed chromatin. No membrane separates the nucleolus from the nucleoplasm. Nucleoli are made of protein and ribosomal DNA (rDNA), which rDNA serves as a fundamental composition. Most plant and animal cells have one or more nucleoli although some do not. It is known that they carry out the production and maturation of ribosomes, so that large amount of ribosomes are found inside the nucleolus. Other than ribosome biogenesis, nucleolus is believed to have other roles in cellular activity. The nucleolus disappears during cell reproduction, because there is no need for ribosomes. The nucleolus is reformed when the cells complete their formation and begin moving their chromosomes into NORs.
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Structure
Study of structure of nucleolus is mainly concentrated in its RNA synthesis related area. There are fibrillar centres (FC), dense fibrillar component(DFC), the granular component(GC) and rDNA. Other components are interstices and chromatin inclusions. To distinguish FC, DFC and GC, their form under microscope is observed.
Fibrillar centres (FC)
It is made up by a network of fine 4-5 nm thick fibrils. Their shape, typically, is roughly globular, with diameters ranging from about 50nm to 1 μm. Number and size of FCs per nucleolus is variable, and it changes with the cellular activity and the need for ribosome production. Cells with lower cellular activity usually have less FC than others. Research pointed out that FC are likely the site where the primary rRNA transcript is generated.
Dense Fibrillar Component(DFC)
It is also made up by very fine (3-5nm) and densely packed fibrils. DFC usually surrounds FCs when they are present and form a meshwork. As this is particularly true for activated states, the amount of DFC roughly reflects the nucleolar engagement in ribosome biogenesis. Sometimes this meshwork occupies large areas of the nucleolus, occasionally interspersed with small FCs. The term fibrillar complex is used to describe this occasion.
During S phase of cell cycle, the increase in Upstream Binding Factor(UBF) association may be due to the increase in its ability to compete with the histones for binding to the rDNA. The increment is done by localizes itself to the FC and DFC of the nucleolus.
Granular component(GC)
It appears to consist of small granules with a diameter of about 15nm. They typically form a mass surrounding the fibrillar complexes and embedds the FCs and DFC. Thus a transition zone between DFC and GC can be observed. Although nucleolus is non membrane-bound, due to the presence of GC, the border with the surrounding chromatin and nucleoplasm is usually distinct.
Ribosomal DNA(rDNA)
rDNA is a set of tandemly-repeated genes coding for preribosomal RNA serves as its foundation. Because they have the ability to initiate the formation of nucleoli during interphase, they are the templates for nucleolus formation, these segments of the chromosomes are called nucleolus organizer regions or NORs.
Functions
Traditionally, nucleolus main role is considered to be RNA biogenesis, since its fundamental component rRNA is coding for preribosomal RNA. Following synthesis, rRNA molecules are attached to proteins, forming ribosomal subunits, which leave for the cytosol through nuclear pores. These nuclear pores are known as the Nuclear Pore Complex (NPC).
Besides, recent research pointed out that nucleolus is also responsible for the trafficking of various prominent small RNA species. Nucleolus helps them during their maturation process and route to their final cellular destination. Moreover, although nucleoli become invisible every time during cell division, more recent studies found that they are involved in cell cycle regulation. Its non-traditional roles are: the interaction with viral components, the regulation of tumor suppressor and oncogene activities, the cell cycle regulation in yeast, the signal recognition particle assembly, the modification of small RNAs, the control of aging and modulating telomerase function.
See also
External links
- Nucleolus under electron microscope
- Nucleolus under electron microscope II
- Recent concerns about nucleolus
References
- Mark O.J. Olson(2004). The Nucleolus Georgetown, Texas : Landes Bioscience / Eurekah.Com. Kluwer Academic/Plenum Publishers. New York. ISBN 0-306-47873-0
- Khadzhiolov, Asen A.(1985). The nucleolus and ribosome biogenesis Wien : Springer-Verlag. ISBN 3-211-81790-5
- Marc Thiry, Guy Goessens(1996). The nucleolus during the cell cycle Hong Kong : Springer ; Austin, Tex. : R.G. Landes Company. New York. ISBN 3-540-61352-8
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