The untranslated region is seen in prokaryotes and eukaryotes, although the length and composition may vary. In prokaryotes, the 5 UTR is typically between 3 and 10 nucleotides long. In eukaryotes, the 5 UTR can be hundreds to thousands of nucleotides long. This is consistent with the higher complexity of the genomes of eukaryotes compared to prokaryotes. The 3 UTR  varies in length as well. The poly-A tail is essential for keeping the mRNA from being degraded. Although there is variation in lengths of both the 5 and 3 UTR, it has been seen that the 5 UTR length is more highly conserved in evolution than the 3' UTR length.