The eukaryotic cytoplasmic ribosome, which has a molecular mass of about 4 x 106 Da and a sedimentation coefficient of 80S, consists of one small and one large subunit. Small or 40S subunits, which are fairy constant in size (approximately 1.5 X 106 Da) in ribosomes from all eukaryotes, contain an 18S rRNA (approximately 1900 nucleotides) and about 33 different polypeptides. Large or 60S subunits vary in size from one species to another. For instance, large subunits in plants and mammals have molecular masses of 2.5 x 106 Da and 3.0 X 106 Da, respectively. Part of this size variation is due to variations in the size of the largest of the three rRNA molecules in the large subunit. This rRNA is about 5000 nucleotides long (sedimentation coefficient 28S) in mammals but only 3400 nucleotides long (sedimentation coefficient 25S) in yeast. The two other rRNA components in the large subunit are the 5.8S rRNA (approximately 160 nucieotides) and the 5S rRNA (approximately 120 nucleotides). Large subunits from mammals also have about 50 polypeptide subunits, whereas those from lower eukaryotes have somewhat fewer polypeptides. The 5.8S, 18S, and 25S/28S rRNAs are synthesized by RNA polymerase I, whereas the 5s rRNA is synthesized by RNA polymerase I, whereas the 5S rRNA is synthesized by RNA polymerase III.