Fig. 1. Multiphase structure of a nucleolus.

a A nucleolus is composed of multiple fibrillar centers (FC) microphases in the sea of the granular component (GC). There is a layer of dense fibrillar component (DFC) between each FC and GC. b RNA polymerase I (Pol I) molecules (white particles) are entrapped in FC microphases (light blue) and the active rDNA units (black line) are localized at the surfaces of microphases. Nascent pre-rRNAs (green particles) are thus at the surfaces of the microphase and form a DFC layer with RNA-binding proteins (magenta particles). The interface between FC and DFC is located at a distance $r_{\mathrm{in}}$ from the center and the interface between DFC and GC is located at a distance $r_{\mathrm{ex}}$.