tokens: After electroporation and centrifugation , the mitochondrial pellet was resuspended in 100 micro l of buffer [ 10 mM Tris - HCl ( pH 7 . 5 ) , 2 mM magnesium acetate and 0 . 33 M sucrose ] containing 60 U of DNase I ( Gibco - BRL ) and incubated for 1 h at room temperature .