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Emerging concepts of nucleolar assembly

CNRS, Université Paris VI, Université Paris VII, Institut Jacques Monod, 2 place Jussieu, 75251 Paris Cedex 05, France

View larger version (26K): [in a new window]   Fig. 1. Nucleolus cycle in human cells. The nucleolus during interphase is organized in three main components: the fibrillar centers in yellow containing the rDNA; the dense fibrillar component in blue corresponding to sites of transcription and early rRNA processing; and the granular component in green corresponding to late rRNA processing. At the G2/M transition, the rRNA processing machinery (green) leaves the nucleolus and during prophase becomes partially distributed over the surface of all the condensed chromosomes. The rDNA present on some chromosomes are still active (yellow and blue). At metaphase, pol I transcription is repressed (red spots for inactive rDNA). The reactivation of pol I transcription in telophase is concomitant with the gathering of the rRNA processing machinery into PNBs (green spots) at the chromosome periphery. At the end of mitosis (M/G1), nucleolar domains start to reform around the active NORs and give rise to a complete nucleolus after association of several NORs in early G1.

View larger version (25K): [in a new window]   Fig. 2. Nucleolar assembly in cycling cells. During mitosis the RNA pol I machinery is associated with the rDNA in NORs (yellow bars) and the rRNA processing machinery (brown gradient) is in the cytoplasm mostly around all chromosomes. Pol I transcription is maintained repressed (red spots) during mitosis by the activity of CDK1—cyclin-B. At the end of mitosis (M/G1), the inhibition of CDK1—cyclin-B activity releases the mitotic silencing of pol I transcription (green spots) and induces the formation of PNBs corresponding to different rRNA processing complexes (orange and brown bodies). Recruitment (dashed orange arrow) of the early processing machinery is contemporary with activation of transcription (rRNA in blue), but the late processing machinery (dashed brown arrow) is recruited later by a controlled pathway (early G1). Finally in G1, association of several NORs completes nucleolar formation.