Certain signal sites control where the proteins required for cleavage and polyadenylation will attach. The upstream poly(A) signal is a specific sequence of nucleotides.

The downstream poly(A) signal is not a specific sequence, rather it is a GU-rich region on the pre-mRNA.

After transcription has been completed, the first two cleavage factors attach to the up and downstream poly(A) signal sites.

These cleavage factors interact with each other to bring the signal sites into proximity with each other, forming a loop in the pre-mRNA.

Two additional cleavage factors then attach and help to stabilize the complex.

Poly(A) polymerase (PAP) then interacts with the complex and cleaves the strand of pre-mRNA at the poly(A) site.

Once cleavage has occured, most of the cleavage factors are released.