<P> Cytoplasmic localization of mRNA is thought to be a function of the 3' UTR . Proteins that are needed in a particular region of the cell can also be translated there; in such a case, the 3' UTR may contain sequences that allow the transcript to be localized to this region for translation . </P> <P> Some of the elements contained in untranslated regions form a characteristic secondary structure when transcribed into RNA . These structural mRNA elements are involved in regulating the mRNA . Some, such as the SECIS element, are targets for proteins to bind . One class of mRNA element, the riboswitches, directly bind small molecules, changing their fold to modify levels of transcription or translation . In these cases, the mRNA regulates itself . </P> <P> The 3' poly (A) tail is a long sequence of adenine nucleotides (often several hundred) added to the 3' end of the pre-mRNA . This tail promotes export from the nucleus and translation, and protects the mRNA from degradation . </P> <P> An mRNA molecule is said to be monocistronic when it contains the genetic information to translate only a single protein chain (polypeptide). This is the case for most of the eukaryotic mRNAs . On the other hand, polycistronic mRNA carries several open reading frames (ORFs), each of which is translated into a polypeptide . These polypeptides usually have a related function (they often are the subunits composing a final complex protein) and their coding sequence is grouped and regulated together in a regulatory region, containing a promoter and an operator . Most of the mRNA found in bacteria and archaea is polycistronic, as is the human mitochondrial genome . Dicistronic or bicistronic mRNA encodes only two proteins . </P>

Mrna survives longer in eukaryotic cells than in bacterial cells and can be used repeatedly