The nuclei of most cells contain one or more highly staining structures called nucleoli. The nucleolus, unlike most other organelles discussed here, does not have a limiting membrane. Instead, it is simply an accu-mulation of large amounts of RNA and proteins of the types found in ribosomes. The nucleolus becomes con-siderably enlarged when the cell is actively synthesiz-ing proteins.
Formation of the nucleoli (and of the ribosomes in the cytoplasm outside the nucleus) begins in the nucleus. First, specific DNA genes in the chromosomes cause RNA to be synthesized. Some of this is stored in the nucleoli, but most of it is transported outward through the nuclear pores into cytoplasm. Here, it is used in conjunction with specific proteins to assemble “mature” ribosomes that play an essential role in forming cytoplasmic proteins.