THE NUCLEOLUS

1. THE NUCLEOLUS The nucleolus (nucleoli) non-membrane bound structure composed of proteins and nucleic acids , transcriberibosomal RNA (rRNA) and assemble it within the cell. takes up to about 25% of the nuclear volume. Three major components of the nucleolus are recognized: • the fibrillar centers (FC), • The DFC or pars fibrosaconsists of newly transcribed rRNA bound to ribosomal proteins • granular component (GC) pars granulosa, contains RNA bound to ribosomal proteins that are beginning to assemble into ribosome. These different regions are thought to represent the sites of progressive stages of rRNAtranscription, processing, and ribosome assembly.

2. The rRNA genes are located in the fibrillar centers, with transcription occurring primarily at the boundary of the fibrillar centers and dense fibrillar component. Processing of the pre-rRNA is initiated in the dense fibrillar component and continues in the granular component, where the rRNA is assembled with ribosomal proteins to form nearly completed preribosomal subunits, ready for export to the cytoplasm. • 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.

3. ORGANIZATION Nucleoli are formed around specific genetic loci called nucleolar organizing regions (NORs), NOR is composed of tandem repeats of rRNA genes. The human genome contains more than 200 clustered copies of the rRNA genes on five different chromosomes (13, 14, 15, 21, 22). • In ribosome biogenesis, two of the three eukaryotic RNA polymerases (pol I and III) are required, and these function in a coordinated manner.

4. RIBOSOMAL BIOGENESIS cleavage within the external transcribed spacer near the 5′ end of the (45S) pre-rRNA, which takes place during the early stages of transcription.requires the U3 small nucleolar RNP Once transcription is complete, the external transcribed spacer at the 3′ end of the molecule is removed. cleavage at the 5′ end of the 5.8S region, yielding separate precursors to the 18S and 5.8S + 28S rRNAs. Additional cleavages then result in formation of the mature rRNAs. Processing follows a similar pattern in other species, although there are differences in the order of some of the cleavages. U3, nucleolar RNP

5. Nucleoli contain a large number (about 200) of small nucleolar RNAs (snoRNAs) that function in pre-rRNA processing. Like the spliceosomalsnRNAs, the snoRNAs are complexed with proteins, forming snoRNPs. Individual snoRNPs consist of single snoRNAs associated with eight to ten proteins. The snoRNPs then assemble on the pre-rRNA to form processing complexes ……U3 U5 U22 etc The snoRNAs contain short sequences complementary to rRNA. Base pairing between snoRNAs and pre-rRNA targets the enzymes that catalyze base modification (e.g., methylation, splicing etc) to the appropriate sites on pre-rRNA

6. Ribosome Assembly 1- ribosomal proteins : RNA polymerase II, 2-5S rRNA : RNA polymerase III

7. NUCLEOLAR SEQUESTRATION The nucleolus is known to capture and immobilize proteins, then are unable to diffuse and to interact with their binding partners. Targets of this post-translational regulatory mechanism include hTERT. It is now known that noncoding RNAs originating from intergenic regions of the nucleolus are responsible for this phenomenon.

8. SUB NUCLEAR BODIES Cajal bodies, Gemini of coiled bodies, Polymorphic interphasekaryosomal association (pika) Promyelocytic leukaemia (PML) bodies, Paraspeckles, Splicing speckles. as part of abnormal disease processes. NEMALINE MYOPATHY: the presence of small intranuclear rods has been reported , mutations in actin, and the rods themselves consist of mutant actin as well as other cytoskeletal proteins.