The nucleolus is the most obvious and clearly differentiated nuclear subcompartment. It is where ribosome biogenesis takes place and has been the subject of research over many decades. In recent years progress in our understanding of ribosome biogenesis has been rapid and is accelerating. This review discusses current understanding of how the biochemical processes of ribosome biosynthesis relate to an observable nucleolar structure. Emerging evidence is also described that points to other, unconventional roles for the nucleolus, particularly in the biogenesis of other RNA-containing cellular machinery, and in stress sensing and the control of cellular activity. Striking recent observations show that the nucleolus and its components are highly dynamic, and that the steady state structure observed by microscopical methods must be interpreted as the product of these dynamic processes. We still do not have detailed enough information to understand fully the organization and regulation of the various processes taking place in the nucleolus. However, the present power of light and electron microscopy (EM) techniques means that a description of nucleolar processes at the molecular level is now achievable, and the time is ripe for such an effort.

Institute of Cellular Biology and Pathology 1st Faculty of Medicine Charles University Prague Czech Republic

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Discontinuous transcription of ribosomal DNA in human cells

Visualization of the Nucleolus Using Ethynyl Uridine

Fluctuations of pol I and fibrillarin contents of the nucleoli

Reproduction of the FC/DFC units in nucleoli

Nucleolar DNA: the host and the guests

HP1β-dependent recruitment of UBF1 to irradiated chromatin occurs simultaneously with CPDs

Duration of the first steps of the human rRNA processing

Nucleologenesis in the Caenorhabditis elegans embryo

Heterogeneity in the kinetics of nuclear proteins and trajectories of substructures associated with heterochromatin

Structure and epigenetics of nucleoli in comparison with non-nucleolar compartments

Chromatin position in human HepG2 cells: although being non-random, significantly changed in daughter cells

Pontin is localized in nucleolar fibrillar centers

Asymmetrical distribution of the transcriptionally competent NORs in mitosis

Evolution of rDNA in Nicotiana allopolyploids: a potential link between rDNA homogenization and epigenetics

Positioning of NORs and NOR-bearing chromosomes in relation to nucleoli