The majority of eukaryotic mRNAs contain a tract of A residues at the end. These polyA-tails are not encoded in the DNA. Rather, they are added to the pre-mRNA "post-transcriptionally" (after transcription). The end of pre-mRNA is cut by a specific enzyme, and a polyA-tail is added to the mRNA strand.
A signal sequence (yellow) in the 3' UTR (blue) guides the addition of the polyA-tail.