<P> The 3 ′ - end of nascent messenger RNA is the site of post-transcriptional polyadenylation, which attaches a chain of 50 to 250 adenosine residues to produce mature messenger RNA . This chain helps in determining how long the messenger RNA lasts in the cell, influencing how much protein is produced from it . </P> <P> The 3 ′ - flanking region is a region of DNA that is not copied into the mature mRNA, but which is present adjacent to 3 ′ - end of the gene . It was originally thought that the 3 ′ - flanking DNA was not transcribed at all, but it was discovered to be transcribed into RNA and quickly removed during processing of the primary transcript to form the mature mRNA . The 3 ′ - flanking region often contains sequences that affect the formation of the 3 ′ - end of the message . It may also contain enhancers or other sites to which proteins may bind . </P> <P> The 3 ′ - untranslated region (3 ′ - UTR) is a region of the DNA which is transcribed into mRNA and becomes the 3 ′ - end of the message, but which does not contain protein coding sequence . Everything between the stop codon and the polyA tail is considered to be 3 ′ - untranslated . The 3 ′ - untranslated region may affect the translation efficiency of the mRNA or the stability of the mRNA . It also has sequences which are required for the addition of the poly (A) tail to the message, including the hexanucleotide AAUAAA . </P>

What part of nucleic acid structure is the basis for 5' and 3' directionality