For the assays in Figures 2 and 3C, the forked DNA was formed using the following two oligos (Table 1): 50duplex LEAD and 5’-32P-50duplex LAG. For the assays in Figure 3B, the forked DNA was formed using unlabeled 160mer duplex LEAD and 5’-32P-160mer duplex LAG. Oligos were annealed as described above.