Molecular Architecture of the Nucleolus

Abstract

The nucleolus was observed soon after the nucleus was recognized as being ubiquitous in eukaryotic cells. One of the first observations made when cells were examined with the electron microscope was that the nucleolus is the only prominent cellular organeile not delimited by a lipoprotein membrane. Yet, nucleoli are more or less sharply delineated from the nucleoplasm and usually possess a roughly spherical shape. In most cells, the nucleolus is partially surrounded by a layer of Condensed chromatin, designated nucleolus-associated chromatin. Only in some primitive eukaryotes does the nucleolus have a notably different structure, e.g., in Saccharomyces the nucleolus is seen as a “dense crescent”, a structure that occupies a large part of the nucleus (Molenaar et al. 1970, Sillevis-Smith et al. 1972). Nucleoli disappear during cell division and are reformed in telophase at the chromosomal nucleolus Organizer sites where the rRNA genes are located. In some cells, a large number of spherical nucleoli are formed in connection with amplified extrachromosomal rRNA genes (see II.2.3.).