Prokaryotic and eukaryotic messenger RNAs (mRNAs) have different structures. Prokaryotic mRNAs are often polycistronic (that is, they carry the information for more than one protein) while eukaryotic mRNAs are monocistronic and almost always code for a single protein. Eukaryotic mRNAs also have structural features that prokaryotic ones do not. While prokaryotic mRNAs generally have only the common four bases A, C, G, and U, eukaryotic mRNAs contain a modification known as a cap at the 5′ end. The cap is a complex structure with an unusual 5′‐5′ arrangement phosphodiester linkage. In addition, some of the nucleosides in the cap are methylated: The most 5′ base is 7‐methylguanosine, and one or two other sugars are methylated at the 2′ oxygen as shown in Figure 1 .
Eukaryotic mRNAs often have long 3′ untranslated sequences—sequences that follow the stop codon for the protein they encode. These mRNAs generally conclude with a sequence of up to 200 adenosines, the polyadenylic acid ( polyA) sequence at the 3′ end. This sequence isn't coded by the DNA template for the gene; it is added post‐transcriptionally. Not all mRNAs are polyadenylated. For example, histone mRNAs lack polyA tails. Polyadenylation seems to play a role in regulating the stability of mRNAs. An early event in the breakdown of some mRNAs is the removal of their polyA tails.