Poly A tail

The addition of a poly(A) tail is a modification to pre-MRNA at the 3' end. Most eukaryotic mRNAs contain a poly(A) tail after transcription. Polyadenylation is the process by which the tail is added. In this process a specfic endonuclease cleaves the RNA just downstream from a sequence 'AAUAAA', then following this cleavage a poly(A) polymerase adds around 100- 250 adenylate residues forming the poly(A) tail, using ATP as the energy source. Once mRNA has this poly(A) tail it is deemed 'polyadenylated'. The function of the poly(A) tail has still not been fully discovered, but evidence suggests it improves translation efficiency and refines mRNA stability. The poly A tail enables the mRNA to be able to leave the nucleus via a nuclear pore and begin the process of translation.