Nucleoli and Formation of Ribosomes 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 accumulation of large amounts of RNA and proteins of the types found in ribosomes. The
nucleolus becomes considerably enlarged when the cell is actively synthesizing 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 RNAto 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 cytoplas mic proteins, as discussed more fully in Chapter 3.
