An in vitro system is used to study replication of a viral DNA genome that is double-stranded, closed circle. The location of the sites for the restriction MboI on this DNA and the letter names for each MboI fragment are shown in Figure 1. Replication reactions are carried out using viral DNA as a template, extracts of infected coils as a source of enzymes, and dNTPs, all of which are labeled with 32 P. The reaction products are digested with MboI, analyzed by agarose gel. The same reaction is carried out in the presence of increasing concentrations of radioactive 2', 3' dideoxyGTP (ddGTP), and the results are shown in lanes 2-4 in Figure 2. The origin of replication of this viral DNA is in which MboI fragment?