SUMMARY
The nucleolus is a large nuclear domain generated by the act of building ribosomes. It illustrates the
compartmentation of the nuclear functions, since it is in the nucleolus that transcription of the
ribosomal genes, maturation and processing of the 47S ribosomal RNAs (rRNAs) into 18S, 5.8S and
28S rRNA, and almost complete assembly of the 40S and 60S ribosome subunits take place. The
shape, size and organization of the nucleoli vary with their activity. Nuleolar activity is a cell cycle
dependent-process. In electron microscopy, the nucleolus exhibits three main components: fibrillar
centers (FCs), a dense fibrillar component (DFC) and a granular component (GC), corresponding to
different steps of ribosome biogenesis. The steady state between transcription, processing and export
of ribosomal subunits engenders this organization. Conversely, inactivation or blockage of one of
these processes modifies the organization of the nucleolus and ultimately induces nucleolar
disassembly. The nucleolus is also a plurifunctional domain, a key partner of chromatin architecture
in the nucleus and it plays a crucial role in several cellular functions in addition to ribosome
production.
The nucleolus is assembled at the end of mitosis, is active during interphase, and disassembled in
prophase. The nucleolar transcription and processing machineries are inherited from parental to
daughter cells through mitosis. The polymerase I (pol I) transcription machinery is repressed during
mitosis although assembled with ribosomal genes. Repression of pol I transcription is achieved at
the end of prophase and is maintained during mitosis through phosphorylation of transcription
factors by the cyclin-dependent kinase (CDK) 1. The nucleolar processing machineries relocalize
from the nucleolus towards the periphery of all chromosomes until telophase and this chromosome
association depends on CDK1 activity. As a consequence of natural inhibition of CDK1 activity, pol
I transcription is restored in telophase. The processing machineries are recruited to the sites of
rDNA transcription after a temporary transit in foci known as prenucleolar bodies.
In conclusion, the behavior of the nucleolus illustrates the fact that the dynamics of nuclear
organization are integrated in a network of interactions and controls that is largely dependent on the
coordination of cell cycle controls.
KEY WORDS
nucleolus; cell cycle; dynamics; organization; nuclear domain; ribosome
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