The nucleolus is the prominent nuclear organelle in which the biogenesis of ribosomal RNA and ribosomes takes place. Understanding of the molecular processes in the nucleolus is rapidly expanding; however, opinions and results on the precise localization of active ribosomal genes - in either of two nucleolar subcompartments, fibrillar centers and dense fibrillar components - are still divided. This review discusses the difficulties in studying the nucleolar structure using microscopy, and provides an overview of the published data, critically examining their relevance to the controversy. Additionally, evidence showing that the dense fibrillar components encompass the Christmas tree structures is discussed and ways to reconcile the controversy are proposed.

Laboratory of Gene Expression 1st Faculty of Medicine Charles University Institute of Experimental Medicine Academy of Sciences of the Czech Republic Albertov 4 12800 Prague 2 Czech Republic

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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

Fine structure of the "PcG body" in human U-2 OS cells established by correlative light-electron microscopy

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

Pontin is localized in nucleolar fibrillar centers

Asymmetrical distribution of the transcriptionally competent NORs in mitosis

Positioning of the NOR-bearing chromosomes in relation to nucleoli in daughter cells after mitosis