Figure 2.

Radiation effects on initiation in the DHFR ori-β locus. CHOC 400 cells were synchronized as in Fig. 1, and were mock-irradiated or irradiated with 9 Gy either 50 min (Upper) or 90 min (Lower) after release from mimosine. Replication intermediates were prepared 30 or 90 min later for analysis by the neutral/neutral 2D gel replicon mapping method (19). D is a cartoon of the patterns traced by restriction fragments containing either replication bubbles or single forks (see text). (Lower) A map of the 120 kb region surrounding the DHFR and 2BE2121 genes and the intergenic region, which contains ori-β and ori-γ, as well as a matrix attachment region (M; ref. 22).