Large subunit Small subunit Figure 4-22 Structure of a Eukaryotic Ribosome. Each nbosome is made up of a large subunit and a small subunit that join together when they attach to a messenger RNA and begin to make a protein. The large subunit of a eukaryotic ribosome consists of about 45 different protein molecules and 3 molecules of RNA. The small subunit has about 33 proteins and 1 molecule of RNA. The fully assembled ribosome of a eukaryotic cell has a diameter of about 30 nm. The ribosomes and ribosomal subunits of prokaryotic cells are slightly smaller than those of eukaryotic cells and consist of their own distinctive protein and RNA molecules22