Structure and Function of Nucleolus

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The nucleolus (plural nucleoli) is a non-membrane bound structure. Composed of proteins and nucleic acids found within the nucleus. Ribosomal RNA (rRNA) is transcribed and assembled within the nucleolus. Three major components of the nucleolus are recognized: the fibrillar centers (FC), the dense fibrillar components (DFC), and granular components (GC). The DFC consists of newly transcribed rRNA bound to ribosomal proteins, while the GC, contains rRNA bound to ribosomal proteins that are beginning to assemble into ribosomes. 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. 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 are formed around specific genetic loci called nucleolar organizing regions (NORs). A NOR is composed of tandem repeats of ribosomal RNA genes, which can be found in several different chromosomes, the human genome, contains more than 200 clustered copies of the rRNA genes on five different chromosomes 13, 14, 15, 21, 22. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. During the G1 stage of the cellular cycle, all chromosomes must replicate themselves ready for mitosis. During the early stages of G1 each chromosome bearing rDNA repeat forms its very own NOR. The NOR have the ability to fuse together and form a single large nucleolus. In a typical eukaryote, a rRNA gene consists of a promoter, internal and external transcribed spacers (ITS/ETS), rRNA coding sequences (18S, 5.8S, 28S) and an external non-transcribed spacer. In ribosome synthesis, RNA polymerases I and III are required,...
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