Final answer:
The addition of a poly(A) tail to the 3' end of mRNA is facilitated by poly(A) polymerase after pre-mRNA is cleaved near an AAUAAA sequence, leading to the protection of the mRNA from degradation and assisting in its export to the cytoplasm.
Step-by-step explanation:
Polyadenylation of the 3' end of mRNA involves the addition of a poly(A) tail. This is carried out by an enzyme known as poly(A) polymerase. The process begins once pre-mRNA elongation is complete, and it involves cleavage of the pre-mRNA by an endonuclease that recognizes a specific AAUAAA consensus sequence. Following this cleavage, poly(A) polymerase adds about 200 adenosine monophosphate (AMP) residues to the newly formed 3' end. This addition occurs at the site recognized by poly(A) polymerase, which is usually marked by an AAUAA sequence. The poly(A) tail plays a critical role in protecting the mRNA from enzymatic degradation, as well as aiding in the export of the mRNA from the nucleus into the cytoplasm for protein synthesis.