Which of the following serves as the polyadenylation signal in eukaryotic mRNA? a) The presence of a stem/loop structure at the 3 ' end region of the RNA transcript followed by a series of uridine residues. b) The presence of a AAUAAA sequence at the 3 ' end region of the transcript. c) The presence of an AT rich sequence at the 3 ' end region of the transcript. d) The formation of a 5 '-3' lariat structure at the 3 end of the transcript..