Chapter category: RNA
Dynamics of Nucleolar Components
The Nucleolus
Edited by: Mark O.J. OlsonISBN: 0-306-47873-0
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Chapter authors:
Thierry Cheutin, Tom Misteli and Miroslav Dundr
The nucleolus is one of the best-characterized cellular organelles. Its large physical size facilitated its early discovery and led to its detailed morphological description.1-3 The essential role of the nucleolus in ribosome biogenesis has encouraged extensive functional studies and to date the nucleolus is the only nuclear sub-compartment with a well-defined function.4-6 Nucleoli are found in virtually all nucleated cells of most animals and tissues at most stages of development.7,8 The ubiquitous presence of nucleoli and the fact that nucleoli undergo little changes in shape or size when observed for extended periods of time in living cells by light microscopy have led to the intuitive assumption that nucleoli are stable, static structures. This view has recently been challenged and it has now become clear that the nucleolus is a highly dynamic intracellular organelle.2-6 Early indications that the nucleolus is not a static structure, but can be plastic, came from the observation that the morphological appearance of the nucleolus is often tissue-specific. In fact, clinicians have traditionally used the structural features of the nucleolus as an indicator for pathological changes of tissues. Furthermore, in most mammalian cell types the nucleolus increases in size as cells progress from G1 to G2, thus indicating that the nucleolus is structurally dynamic. The most dramatic manifestation of the dynamic nature of the nucleolus, however, occurs during mitosis, when it disassembles completely.9-11 As cells enter pro-metaphase ribosomal gene expression ceases. Concomitantly the nucleolus loses its structural integrity and its components disperse throughout the mitotic cell.5,11-13 Upon entry into telophase the nucleolus reforms around the transcriptionally reactivated ribosomal genes in the nucleolar organizing region (NOR). This correlation between transcriptional activity of ribosomal genes and structural integrity of the nucleolus is not limited to mitotic repression of rDNA transcription. In Xenopus oocytes nucleoli disassemble rapidly in the ovulated egg which is transcriptionally inactive.14 Similarly, treatment of cells with transcription inhibitors results in rapid reorganization of nucleolar structure.15-17 While these observations clearly demonstrate that the overall architecture of the nucleolus is dynamic, recent in vivo imaging approaches have revealed an additional, molecular, level of nucleolar dynamics. These experiments indicate that nucleolar resident proteins and RNA molecules are highly dynamic. They do not reside permanently in the nucleolus, but are steadily exchanged between the nucleolus and the surrounding nucleoplasm. Remarkably, the residence time of proteins and RNAs within the nucleus is on the order of tens of seconds and as a consequence the apparently stable global structure of the nucleolus reflects a highly dynamic steady-state of rRNA and proteins flowing through the nucleolus.18-20 The dynamic nature of nucleolar components at the molecular level has important consequences for the mechanisms which govern nucleolar architecture and function. This chapter aims to give an overview of the recent findings related to the newly discovered dynamic properties of nucleolar components and to discuss their implications.
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Introduction
Mark O.J. Olson
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Arlen W. Johnson
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Structure and Organization of Vertebrate Ribosomal DNA
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Genes within rDNA clusters from vertebrates are tandemly repeated in a head to tail fashion and exist at multiple chromosomal locations. Each gene is comprised of a coding region for 18S, 5.8S, ...
