Ribosomes are found in both prokaryotes and eukaryotes. They are complexes
of proteins and three (prokaryotes) or four (eukaryotes) rRNA molecules
called subunits. The main function of ribosomes is as the site of mRNA
translation; once the two subunits are joined by the mRNA from the nucleus,
the ribosome translates mRNA into a specific sequence of amino acids.
Ribosomes are also located in mitochondria and chloroplasts of eukaryotic cells; they are always smaller than cytoplasmic ribosomes. Prokaryotic and eukaryotic ribosomes perform the same functions; however, eukaryotic ribosomes are much larger than prokaryotic ones and most of their proteins are different.