ribosome /rī″bəsōm/ [ribose + Gk, soma, body] , an organelle composed of RNA and protein that functions in the synthesis of protein. Ribosomes interact with messenger RNA and transfer RNA to link amino acid into a polypeptide chain in a sequence determined by the sequence of nucleotides in the messenger RNA. Ribosomes may exist singly, in clusters as polysomes, or attached to the “rough” endoplasmic reticulum. See also translation.