70S ribosomes are composed of two subunits of unequal sizes. The larger (50S) subunit is composed of 5-S rRNA, 23-S rRNA, and about 34 proteins. The smaller (30S) subunit is composed of 16-S rNA and about 21 proteins. 70S ribosomes play a central role in synthesizing proteins in prokaryotes and a few eukaryotes.