Ribosome biogenesis is a highly dynamic, steady-state nucleolar process that involves synthesis and maturation of rRNA, its transient interactions with non-ribosomal proteins and RNPs and assembly with ribosomal proteins. In the few years of the 21st century, an exciting progress in the molecular understanding of rRNA and ribosome biogenesis has taken place. In this review, we discuss the recent results on the regulation of rRNA synthesis in relation to the functional organization of the nucleolus, and put an emphasis on the situation encountered in mammalian somatic cells.

DNA replication sites (RS) in synchronized HeLa cells have been studied at the electron microscopic level. Using an improved method for detection following the in vivo incorporation of biotin-16-deoxyuridine triphosphate, discrete RS, or foci are observed throughout the S-phase. In particular, the much larger RS or foci typically observed by fluorescence microscopic approaches in mid- and late-S-phase, are found to be composed of smaller discrete foci that are virtually identical in size to the RS observed in early-S-phase. Pulse-chase experiments demonstrate that the RS of early-S-phase are maintained when chased through S-phase and into the next cell generation. Stereologic analysis demonstrates that the relative number of smaller sized foci present at a given time remains constant from early through mid-S-phase with only a slight decrease in late-S-phase. 3-D reconstruction of serial sections reveals a network-like organization of the RS in early-S-phase and confirms that numerous smaller-sized replication foci comprise the larger RS characteristic of late-S-phase.