LABELING FIXED CELLS : Labeling DNA Using DAPI
1. Prepare a stock solution at 10 mg/ml of DAPI (m.w. 350) in distilled water, protect from light, and store at 4 degree.
Prepare a  5000-fold dilution in PBS to be used for labeling.
2. Prepare a fresh 3.7% formaldehyde solution for fixation. Also prepare a 0.2% Triton X-100 solution for permeabilization.
3. Aspirate the cell medium.
4. Rinse cells three times with PBS(+).
5. Fix the cells for 10 minutes in 3.7% formaldehyde solution.
6. Aspirate and rinse the cells three times for 5 minutes each in PBS.
7. Permeabilize the cells by immersion in 0.2% Triton X-100 for 5 minutes.
8. Aspirate and rinse three times for 5 minutes each in PBS.
9. Incubate the cells at room temperature for 1-5 minutes in the DAPI labeling solution.
10. Aspirate off the labeling medium, rinse three times in PBS and mount.

LABELING LIVE CELLS : Labeling DNA with Hoechst 33342

1. Prepare a 10 ug/ml stock solution of Hoechst 33342 (m.w. 642) in distilled water, protect from light, and store at 4 degree.
Prepare a 1000-fold dilution in PBS(+) to be used for labeling.
2. Aspirate the cell medium.
3. Rinse cells three times with PBS(+).
4. Incubate the cells at room temperature for 10-30 minutes in the Hoechst labeling solution.
5. Aspirate the labeling medium and rinse three times in PBS(+) and mount.