ribosome

an organelle in the cytoplasm of a living cell; ribosomes attach to mRNA and move down it one codon at a time and stop until tRNA brings the required amino acid; when a ribosome reaches a stop codon it falls apart and releases the completed protein molecule for use by the cell; "the ribosome is the site of protein synthesis"

• organelle, cell organ — a specialized part of a cell; analogous to an organ