At the 5’ end of the mRNA transcript, a special enzyme complex adds a freaky nucleotide called 7-methylguanate. This methylguanate is linked up to the transcript in a backwards fashion, forming the 5’ cap. The cap protects the new transcript from digestion by the cell, which doesn’t like mRNA to hang around too long. At the other end of the transcript, an enzyme called poly-A polymerase adds a long string of adenines to the transcript. This poly-A tail is a signal that says “I’m mRNA. I need to get out of the nucleus to be useful. Let me pass.”