1. Collect culture media, add 1 ml of trypsin to cell mono1ayer on 100-mmol/L dishes, scrape the cells, harvest cells (culture media and cell monolayer) by centrifugation (2,500 rpm, 5 min), and wash cell pellets with 1* PBS

2. Add 100 μl of lysis buffer (1% NP-40 in 20 mmol/L EDTA, 50 mmol/L Tris-HCl, pH 7.5) for 10 sec. [“Lysis of cells”]

* Preparation of lysis buffer (1 ml) : 10% NP-40 100 μl+ 200 mmol/L EDTA 100 μl + 0.2 M Tris-HCl (pH 7.5) 250 μl + D.W. 550 μl

3. Centrifugation (3,000 rpm, 5 min) and obtain supernatant

4. Add 10 μlof 10% SDS solution to pooled supernatant (final : 1% SDS), treat with 10 μl of 50 mg/ml RNase A (final 5 μg/μl) and incubate for 2 h at 56 ℃.

5. Add 10  μl of 25 mg/ml Proteinase K (final 2.5 μg/μl) and incubate for 2 h at 37 ℃

6. Add 1/2 vol. (65 μl) of 10 M ammol/Lonium acetate

7. Add 2.5 vol. (500 μl) of ice-cold ethanol and mix thoroughly

8. Stand for 1 h in – 80 C freezer (“ethanol precipitation”)

9. Centrifuge for 20 min at 12,000 rpm, wash the white pellet with 200  μl 80% ice-cold ethanol and air-dry for 10 min at room temperature

10. Dissolve the pellet with 50 μl of TE buffer

11. Determination of DNA concentration (Abs 260) and 2% agarose gel electrophoresis of the same concentration of DNA (about4 μg)

Reference:  http://www.nature.com/bjc/journal/v98/n4/full/6604193a.html