<P> Many protein - coding genes have more than one polyadenylation site, so a gene can code for several mRNAs that differ in their 3' end . Since alternative polyadenylation changes the length of the 3' untranslated region, it can change which binding sites for microRNAs the 3' untranslated region contains . MicroRNAs tend to repress translation and promote degradation of the mRNAs they bind to, although there are examples of microRNAs that stabilise transcripts . Alternative polyadenylation can also shorten the coding region, thus making the mRNA code for a different protein, but this is much less common than just shortening the 3' untranslated region . </P> <P> The choice of poly (A) site can be influenced by extracellular stimuli and depends on the expression of the proteins that take part in polyadenylation . For example, the expression of CstF - 64, a subunit of cleavage stimulatory factor (CstF), increases in macrophages in response to lipopolysaccharides (a group of bacterial compounds that trigger an immune response). This results in the selection of weak poly (A) sites and thus shorter transcripts . This removes regulatory elements in the 3' untranslated regions of mRNAs for defense - related products like lysozyme and TNF - α . These mRNAs then have longer half - lives and produce more of these proteins . RNA - binding proteins other than those in the polyadenylation machinery can also affect whether a polyadenyation site is used, as can DNA methylation near the polyadenylation signal . </P> <P> There is polyadenylation in the cytosol of some animal cell types, namely in the germ line, during early embryogenesis and in post-synaptic sites of nerve cells . This lengthens the poly (A) tail of an mRNA with a shortened poly (A) tail, so that the mRNA will be translated . These shortened poly (A) tails are often less than 20 nucleotides, and are lengthened to around 80--150 nucleotides . </P> <P> In the early mouse embryo, cytoplasmic polyadenylation of maternal RNAs from the egg cell allows the cell to survive and grow even though transcription does not start until the middle of the 2 - cell stage (4 - cell stage in human). In the brain, cytoplasmic polyadenylation is active during learning and could play a role in long - term potentiation, which is the strengthening of the signal transmission from a nerve cell to another in response to nerve impulses and is important for learning and memory formation . </P>

What happens if there is no poly a tail