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Structural and functional organization of ribosomal genes within the mammalian cell nucleolus Derenzini M; Pasquinelli G; O'Donohue MF; Ploton D; Thiry MJ Histochem Cytochem 2006[Feb]; 54 (2): 131-45Data on the in situ structural-functional organization of ribosomal genes in the mammalian cell nucleolus are reviewed here. Major findings on chromatin structure in situ come from investigations carried out using the Feulgen-like osmium ammine reaction as a highly specific electron-opaque DNA tracer. Intranucleolar chromatin shows three different levels of organization: compact clumps, fibers ranging from 11 to 30 nm, and loose agglomerates of extended DNA filaments. Both clumps and fibers of chromatin exhibit a nucleosomal organization that is lacking in the loose agglomerates of extended DNA filaments. In fact, these filaments constantly show a thickness of 2-3 nm, the same as a DNA double-helix molecule. The loose agglomerates of DNA filaments are located in the fibrillar centers, the interphase counterpart of metaphase NORs, therefore being constituted by ribosomal DNA. The extended, non-nucleosomal configuration of this rDNA has been shown to be independent of transcriptional activity and characterizes ribosome genes that are either transcribed or transcriptionally silent. Data reviewed are consistent with a model of control for ribosome gene activity that is not mediated by changes in chromatin structure. The presence of rDNA in mammalian cells always structurally ready for transcription might facilitate a more rapid adjustment of the ribosome production in response to the metabolic needs of the cell.|Animals[MESH]|Cell Nucleolus/*genetics/metabolism/ultrastructure[MESH]|Chromatin/genetics/ultrastructure[MESH]|DNA, Ribosomal/chemistry[MESH]|Humans[MESH]|Mammals[MESH]|Microscopy, Electron[MESH]|Nucleic Acid Conformation[MESH]|RNA Precursors/biosynthesis[MESH]|RNA, Ribosomal/biosynthesis/genetics[MESH]|Ribosomes/*genetics[MESH]|Transcription, Genetic[MESH] |
