Nucleoli
The nucleoli (little nucleus) are dark-staining, generally oval bodies located within the nucleus (see Fig. 2.13). They are not membrane-bound and are sites for ribosome synthesis and assembly.
The size and number of nucleoli vary between cells. Cells that are very actively synthesizing and secreting large amounts of protein are more likely to have a large nucleolus and/ or multiple structures. The nucleoli are closely linked to segments of chromatin (nucleolar organizer regions) that contain the genes that code for synthesis of rRNA. As the rRNA is synthesized, proteins previously made in the cytoplasm are combined in the nucleus to create one of the two subunits of the ribosomes. These subunits are transported out of the nucleus into the cytoplasm where they are combined to make mature ribosomes. The primary rRNA transcript has a sedimentation coefficient of 45 S (Svedberg units) that corresponds to 13,000 nucleic acid base pairs. From this precursor molecule, a 28 S (5,000 base pair) rRNA molecule is created and combined into the larger (60S) ribosomal subunit. A smaller 18S (2,000 base pair) rRNA moiety is also generated. It is incorporated into the smaller (40 S) ribosomal subunit. Two additional smaller rRNA molecules are also synthesized and incorporated in the larger ribosomal subunit in its final mature state. The four primary rRNA molecules needed to manufacture the complete ribosome are all formed from the same primary transcript. This ensures that all of the pieces necessary for ribosome synthesis will be available when the process is initiated.
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