In eukaryotes, three kinds of RNA—rRNA, tRNA, and mRNA—are produced based on the activity of three distinct RNA polymerases, whereas, in prokaryotes, only one RNA polymerase exists to create all kinds of RNA molecules.[3] RNA polymerase II of eukaryotes transcribes the primary transcript, a transcript destined to be processed into mRNA, from the antisense DNA template in the 5' to 3' direction, and this newly synthesized primary transcript is complementary to the antisense strand of DNA.[2] RNA polymerase II constructs the primary transcript using a set of four specific ribonucleoside monophosphate residues (Adenosine monophosphate (AMP), Cytidine monophosphate (CMP), Guanosine monophosphate (GMP), and Uridine monophosphate (UMP)) that are added continuously to the 3' hydroxyl group on the 3' end of the growing mRNA.[1]