After the mRNA is completed and cleaved off at the poly-A signal sequence, the left-over (residual) RNA strand remains bound to the DNA template and the RNA polymerase II unit, continuing to be transcribed. After this cleavage, a so-called exonuclease binds to the residual RNA strand and removes the freshly transcribed nucleotides one at a time (also called degrading the RNA), moving towards the bound RNA polymerase II. This exonuclease is XRN2 (5-3 Exoribonuclease 2) in humans. This model proposes that XRN2 proceeds to degrade the uncapped residual RNA from 5 to 3 until it reaches the RNA pol II unit. This causes the exonuclease to push off the RNA pol II unit as it moves past it, terminating the transcription while also cleaning up the residual RNA strand.
Similar to Rho-dependent termination, XRN2 triggers the dissociation of RNA polymerase II by either pushing the polymerase off of the DNA template or pulling the template out of the RNA polymerase. The mechanism by which this happens remains unclear, however, and has been challenged not to be the sole cause of the dissociation.
In order to protect the transcribed mRNA from degradation by the exonuclease, a 5 cap is added to the strand. This is a modified guanine added to the front of mRNA, which prevents the exonuclease from binding and degrading the RNA strand. A 3 poly(A) tail is added to the end of a mRNA strand for protection from other exonucleases as well.