Ribosome

Ribosome, cell structure that uses genetic instructions transported in ribonucleic acid (RNA) to link a specific sequence of amino acids into chains to form proteins. Ribosomes, which measure about 0.00025 mm (0.00001 in), are dispersed in the cytoplasm (the cell contents outside the nucleus) of all prokaryotic cells— archaebacteria and bacteria. They are also found in the cytoplasm of all eukaryotic cells—cells of protists, fungi, plants, and animals—where they either float free in the cytoplasm or are bound to networks of membrane-enclosed tubules in the cytoplasm, called the endoplasmic reticulum. In eukaryotic cells, two types of cell structures called mitochondria and chloroplasts also contain ribosomes.