游离核糖体与附着核糖体 mRNA encoding a cytosolic protein free polyribosome remains free in cytosol in cytosol 5 -3'一5 统 common pool of ribosomal subunits in cytosol ER signal sequence mRNA encoding a protein targeted to ER remains polyribosome bound to ER membrano-bound membrane by multipla urowing polypeptide chains E月nembrane游离核糖体与附着核糖体