III.

Synthesis

Ribosomes are required for protein synthesis; when cells need large numbers of proteins, they must first build numerous—sometimes hundreds of thousands—of ribosomes. To ensure rapid production of the RNA portion of ribosomes, certain eukaryotic cells contain thousands of copies of genes that code for ribosomal RNA. The locations of all the genes that code for the protein component of ribosomes are yet to be determined.

In eukaryotic cells, three of the four ribosomal RNA strands are synthesized in the nucleolus, a dense, granular structure in the nucleus. The fourth ribosomal RNA strand is synthesized outside of the nucleolus and then transported into the nucleolus for ribosome assembly.

Ribosomal proteins enter the nucleolus and combine with the four ribosomal RNA strands to create two ribosomal structures: the large and small subunits. These two beadlike subunits leave the nucleus separately through special openings called nuclear pores. The two subunits unite outside of the nucleus just before the ribosome begins to manufacture proteins. Prokaryotic cells lack a nucleus or nucleolus, and ribosomal synthesis takes place in the cytoplasm. In these cells, only a few copies of genes coding for ribosomal RNA are present.