The nucleolus (also called nucleole) 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. The nucleolus ultrastructure can be visualized through an electron microscope, while the organization and dynamics can be studied through fluorescent protein tagging and fluorescent recovery after photobleaching (FRAP). Malfunction of nucleoli can be the cause for several human diseases.
Three major components of the nucleolus are recognized: the fibrillar centers (FC), the dense fibrillar component (DFC) and granular components (GC). 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.
Function and ribosome assembly
Nucleoli are formed around specific genetic loci called nucleolar organizing regions (NORs), first described by Barbara McClintock. Because of this non-random organization, the nucleolus is defined as a "genetically determined element." A NOR is composed of tandem repeats of rRNA genes, which can be found in several different chromosomes. The human genome, for example, contains more than 200 clustered copies of the rRNA genes on five different chromosomes (13, 14, 15, 21, 22). 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 biogenesis, two of the three eukaryotic RNA polymerases (pol I and III) are required which function in a coordinated manner. In an initial stage, the rRNA genes are transcribed as a single unit within the nucleolus by RNA pol I or III. In order for this transcription to occur, several pol I-associated factors and DNA-specific transacting factors are required. In yeast, the most important are: UAF (upstream activating factor), TBP (tata-box binding protein) and CF (core factor), which bind promoter elements and form the pre-initiation complex (PIC), which is in turn recognized by RNA pol. In humans, a similar PIC is assembled with SLI, the promoter selectivity factor (composed of TBP and TBP-associated factors, or TAFs), IFs (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.
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