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.