title: Positioning of NORs and NOR-bearing chromosomes in relation to nucleoli.
abstract: It is widely accepted that chromosomes occupy more or less fixed positions in mammalian interphase nucleus. However, relation between large-scale order of chromosome positioning and gene activity remains unclear. We used the model of the human ribosomal genes to address specific aspects of this problem. Ribosomal genes are organized at particular chromosomal sites in clusters termed nucleolus organizer regions (NORs). Only some NORs, called competent are generally accepted to be transcriptionally active during interphase. Importantly in this respect, the regularities in distribution of competent, and non-competent NORs among the specific chromosomes were already established in two human-derived cell lines: transformed HeLa and primary LEP cells. In the present study, using FISH and immunocytochemistry, we found that in HeLa and LEP cells the large-scale positioning of the NOR-bearing chromosomes with regard to nucleoli is linked to the transcription activity of rDNA. Namely, the tendency of rDNA-bearing chromosomes to associate with nucleoli correlates with the number of transcriptionally competent NORs in the respective chromosome homologs. Regarding the position of NORs, we found that not only competent but also most of the non-competent NORs are included in the nucleoli. Some intranucleolar NORs (supposedly non-competent) are situated on elongated chromatin protrusions connecting nucleoli with respective chromosome territories spatially distanced from nucleoli.
authors: Institute of Cellular Biology and Pathology, First Faculty of Medicine, Charles University in Prague, Czech Republic. mfial@lf1.cuni.cz
authors: Markéta
authors: M
authors: Kalmárová
authors: Evgeny
authors: E
authors: Smirnov
authors: Martin
authors: M
authors: Masata
authors: Karel
authors: K
authors: Koberna
authors: Anna
authors: A
authors: Ligasová
authors: Alexey
authors: A
authors: Popov
authors: Ivan
authors: I
authors: Raska
mesh_terms: D002877:Chromosomes, Human; D015342:DNA Probes; D006367:HeLa Cells; D006801:Humans; D007399:Interphase; D009697:Nucleolus Organizer Region
chemical_list: D015342:DNA Probes
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