In addition to splicing eukaryotic mRNAs are further processed before they become mature mRNAs ready to be translated into polypeptides. They get a 5-prime cap and a 3-prime polyetail. The 5-prime cap is a methylated guanosine nucleotide. This cap helps move mRNA out of the nucleus and into the cytoplasm. Near the 5-prime end of the mRNA is a sequence that recognizes an enzyme called poly-A polymerase. That was the blue molecule shown here, which then binds and catalyzes the addition of several hundred adenosine nucleotides to the 3-prime end of the transcript. This polyetail seems to govern how many times this message will be used to translate a polypeptide. In vitro translation experiments, show that the polyetail gets shorter and shorter the longer a solution of mRNAs is allowed to translate proteins. Here are some capping details. A capping enzyme brings a methylated guanosine nucleotide and an mRNA molecule head-to-head, as shown here. The check marks being the nucleotides. The enzyme then catalyzes a condensation reaction to form this phosphotryester linkage. In this illustration, the cap and polyetail recruit binding proteins from the nucleus. One, shown here as a brown crescent, is a cap binding protein that binds to the 5-prime end of the messenger RNA. The cap protein, together with an RNA export factor shown as the red square here, guide the messenger RNA out of the nucleus through a nuclear pore complex. Once the mRNA is out, the export factor comes off and can return to the nucleus, while a translation initiation factor displaces the cap binding protein. The cap binding protein probably goes back into the nucleus to be recycled as well, although that's not shown here. A polyet binding protein, the green oval in this slide, binds to the 3-prime end of the message while it's in the nucleus and then remains bound in the cytoplasm.