Fig. 2.

Comparison of yeast and metazoan nucleoli. The eukaryotic nucleolus is defined by the Pol I-transcribed ribosomal cluster. In yeast, the ribosomal cluster is located on the linear map in one group on chromosome XII; consequently, the yeast nucleolus can be visualized by FISH microscopy as a single crescent-shaped structure, typically localized to one side of the nucleus. In yeast, the tRNA genes can be visualized by FISH microscopy as a single cluster localized to the nucleolus. Metazoans generally have multiple clusters of ribosomal genes; thus, metazoan nuclei usually have several nucleoli spread throughout the nucleus. The metazoan nucleus is generally several times larger than the yeast nucleus