Ribosomes were rst described in the middle of the twentieth century using electron microscopy and biochemical methods. Not long thereafter, it was discovered that they sometimes were linked together like a beaded necklace in what were termed polysomes. Eventually, it was found that ribosomes attached to mRNA and, when several ribosomes were attached to the same mRNA, these were the polysomes. Over the next few decades, both bacterial and eukaryotic ribosomes were studied. All of them had a small subunit (SSU) and a large subunit (LSU) and all performed translation of the mRNA into polypeptides by attaching amino acids to the growing polypeptide chain one at a time according to a triplet nucleotide code that species the next amino acid to be added to that chain.