FIGURE 5-14 Structure of the 5′ methylated cap. The distinguishing chemical features of the 5′ methylated cap on eukaryotic mRNA are (1) the 5′→5′ linkage of 7-methylguanylate to the initial nucleotide of the mRNA molecule and (2) the methyl group on the 2′ hydroxyl of the ribose of the first nucleotide (base 1). Both of these features occur in all animal cells and in cells of higher plants; yeasts lack the methyl group on nucleotide 1. The ribose of the second nucleotide (base 2) is also methylated in vertebrates. See A. J. Shatkin, 1976, Cell 9:645.