Figure 3.  Cartoon of the polymerization cycle.  At the top of the drawing, the incoming dNTP is bound at the N (or nucleoside-binding) site and the end of the primer is at the P (or priming) site.  The incoming dNTP is base-paired to the template, but incorporation has not occurred.  In the second diagram, the chemical step has just been completed, with the release of PPi.  In the third diagram, the nucleic acid has been translocated so that the end of the primer resides at the P site.  In the bottom diagram, the next incoming dNTP has been bound and polymerization can continue.  Pyrophosphorolysis is the reverse of polymerization, and can be represented as the top two diagrams happening in reverse order, with the incorporation of pyrophosphate and the regeneration of dNTP.  However, in the case of AZT excision, the pyro-phosphate donor is ATP, not PPi, so the excision product is a dinucleoside tetraphosphate (see Figure 5).