Mixtures were centrifuged at 13000 rpm for 2 minutes at room temperature. The upper aqueous solution was transferred to another sterile 1.5 ml microcentrifuge tube. An equal volume of chloroform was added and centrifuged at 13,000 rpm for 5 minutes. Once again, the aqueous solution was transferred to a new 1.5 mL microcentrifuge tube and 1/10 volume of 3M sodium acetate solution was added. Then 2.5 volumes of cold absolute ethanol were added to precipitate the DNA. The mixture was incubated at -20°C overnight or at -80°C for 1-2