A 1.0-kb DNA fragment from the end of the mouse gene described in the previous problem is examined by DNA footprint protection analysis (see Research Technique 8.1). Two samples are end-labeled with ³²P and one of the two is mixed with TFIIB, TFIID, and RNA polymerase II. The DNA exposed to these proteins is run in the right-hand lane of the gel shown below and the control DNA is run in the left-hand. Both DNA samples are treated with DNase I before running the samples on the electrophoresis gel. Explain the role of DNase I.