FIGURE 10-3 Synthesis of the 5′ cap on eukaryotic mRNAs. The 5′ end of a nascent RNA contains a 5′ triphosphate from the initiating rNTP. The γ phosphate is removed in the first step of capping, while the remaining α and β phosphates (orange) remain associated with the cap. The third phosphate of the 5′,5′ triphosphate bond is derived from the α phosphate of the GTP that donates the guanine. The methyl donor for methylation of the cap guanine and the first one or two riboses of the mRNA is S-adenosylmethionine (S-Ado-Met). See S. Venkatesan and B. Moss, 1982, Proc. Natl. Acad. Sci. USA 79:340.