10.18 Most eukaryotic mRNAs have a 5′ cap. The cap consists of a nucleotide with 7-methylguanine that is attached to the 5′ end of pre-mRNA by a unique 5′—5′ bond, as well as methyl groups added to the 2′ position of the sugars in the second and third nucleotides of the pre-mRNA, and sometimes a methyl group added to the base (N) on the initial nucleotide.