The mixture was incubated in a −20 ◦C freezer for 20 min, thawed, and centrifuged at 8000 × g at 4 ◦C for 15 min to remove proteins, humic substances, SDS and RNA. 500 l portions of the clear aqueous phase were collected into new 1.5 ml tubes and DNA was precipitated by adding 300 l of isopropanol and leaving the mixture to stand for 1 h at room temperature.