Final answer:
The untranslated ends of prokaryotic and eukaryotic mRNA serve to regulate expression and enhance stability and translation efficiency. In prokaryotes, they also assist in the proper termination of transcription and initiation of degradation, while in eukaryotes, they are part of post-transcriptional processing which includes splicing, the addition of protective caps and tails, and regulation of export and translation.
Step-by-step explanation:
The ends of prokaryotic and eukaryotic mRNA are not translated into protein, and this lack of translation serves important purposes. In both types of cells, these sequences play roles in regulation and stability of the mRNA molecules. In prokaryotes, despite the coupling of transcription and translation due to the lack of a nuclear membrane, non-translated sequences can help in correctly terminating transcription and initiating the proper degradation of mRNA. In eukaryotes, mRNA processing includes the addition of a 5' cap and a 3' poly-A tail as well as the removal of introns. These modifications aid in the mRNA's export from the nucleus, its stability in the cytoplasm, and the efficiency of translation initiation by ribosomes. Furthermore, the presence of untranslated regions (UTRs) at both ends of mRNA molecules also prevents degradation and can include binding sites for regulatory proteins that influence the mRNA's translatability and lifespan.