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Protoplasmic particles that are sites for the assembly of amino acids into the polypeptide chains of protein molecules in the order dictated by the genetic code of messenger RNA. During the process mRNA, formed in the nucleus, becomes attached to the ribosomes. The ribosomes of prokaryotic cells contain 60-65% ribosomal RNA and 35-40% protein while those of eukaryotic cells contain approximately equal quantities of each. In prokaryotic cells, e.g. Escherichia coli, they occur freely scattered throughout the protoplasm, but in eukaryotic cells most are associated with the membranes of the endoplasmic reticulum. They have also been identified in mitochondria and chloroplasts. Ribosomes consist of two parts or subunits, unequal in size and each containing RNA and a number of proteins. The ribosomes of prokaryotes are smaller than those in the cytoplasmic matrix of eukaryotes but similar in size to those in eukaryotic chloroplasts and mitochondria. During protein synthesis both the developing polypeptide chain and the ribosome are translocated along the mRNA molecule as the genetic code is translated. One of the proteins identified in the larger ribosomal subunit is peptidyl transferase, an enzyme that catalyses the formation of peptide bonds between amino acids.
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