The 5′ end of eukaryotic mRNA is capped with a guanine nucleotide (which is methylated forming 7-methyguanosine). The cap (5′-G) is added to the mRNA after transcription. The addition of 5′ G is catalyzed by a nuclear enzyme, guanylyl transferase. The cap is linked to the 5′ terminus of the mRNA through an unusual 5′,5′-triphospahe linkage. The 5′ cap is formed by condensation of a molecule of GTP with the triphosphate at the 5′ end of the transcript. The guanine is subsequently methylated at N-7 to form 7-methylguanosine. Additional methyl groups are added to the 2′ hydroxyls (-OH) of the first and second nucleotides adjacent to the cap. The methyl groups are derived from S-adenosylmethionine.
At the 3′ end, most eukaryotic mRNAs have a string of 80-250 adenylate residues called the poly(A) tail.