2 3-tailing 3 -cleavage and polyadenylation 转录起始 elongation cap splIcing Poly(A)polymerase 5’cap= AAUAAA n 图13-mRNA3’ additiong of poly(A)tail转录起始 elongation 5’cap AAUAAA splicing Poly (A)polymerase 5’cap AAUAAA An 图 13- mRNA 3’additiong of Poly(A) tail 2 3’-tailing : 3’-cleavage and polyadenylation