Ribosomes are small cellular organelles composed of ribonucleic acid (RNA) and proteins. They are found free in the cytoplasm or attached to the endoplasmic reticulum. Ribosomes are involved in the synthesis of proteins by attaching to messenger RNA (mrna) and moving down it one codon at a time and then stop until transfer RNA (trna) brings the required amino acid; when it reaches a stop codon it falls apart and releases the completed protein molecule for use by the cell.