To visualize DNA, the cells were stained with propidium iodide.  The cells were incubated with 1 mg/ml RNase for 45 min at 30℃, then with 30 µg/ml propidium iodide for 30 min at room temperature.  After washing 3 times, the cells were mounted over 1 drop of each buffer containing 50% glycerol (Mikroskopie, Merck, Germany) and the anti-fading reagent p-phenylenediamine at 1 mg/ml.  The cells were observed with a confocal microscope (LSM510, Zeiss, Germany).