Figure 7.

Exonuclease III detection of DNA nicks in plasmids treated with nuclear extract. Form II (nicked, relaxed) plasmid standards of pMII-MAR and pUC18 were prepared as described in Materials and Methods. Nuclear extract treated pMII-MAR (lanes 3 and 4) and pUC18 (lanes 7 and 8) plasmids were prepared separately as described in Figure 6 and were incubated for 0 (lanes 3 and 7) and 1 h (lanes 4 and 8) with 300 U exonuclease III at 37°C. Nicked standards of pMII-MAR (lanes 1 and 2) and pUC18 (lanes 5 and 6) were similarly treated for 0 (lanes 1 and 5) and 1 h (lanes 2 and 6) with 300 U exonuclease III. Aliquots (100 ng) were removed at the indicated times and reactions were terminated by the addition of EDTA to 100 mM, plasmids purified, and separated by agarose gel electrophoresis.