Addition of the 3′ poly(A) tail to the transcript. Polyadenylation factors are associated with Pol II during transcription. After transcription of the 5′-AAUAAA sequence, these proteins transfer to the transcript and help recruit cleavage factors and poly(A) binding protein (PABP) before dissociating. The poly(A) tail is synthesized by polyadenylate polymerase (PAP).