Before revision:  Rev. E {\bf 83}, 042903 (2011)], a simple model for  the translation of messenger RNA  by ribosomes is provided, and the expression of translational ratio of protein is given.
After revision:  Rev. E {\bf 83}, 042903 (2011)], a simple model for describing the translation of messenger RNA by ribosomes is provided, and the expression of translational ratio of protein is given.