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.
