(adapted from Bruce A. Roe, Department of Chemistry and Biochemistry, The University of Oklahoma, Norman, Oklahoma 73019 broe@ou.edu)
Typically, 2.5 - 3 volumes of an ethanol/acetate solution is added to the DNA sample in a microcentrifuge tube, which is placed in an ice-water bath for at least 10 minutes. Frequently, this precipitation is performed by incubation at -20C overnight (1). To recover the precipitated DNA, the tube is centrifuged, the supernatant discarded, and the DNA pellet is rinsed with a more dilute ethanol solution. After a second centrifugation, the supernatant again is discarded, and the DNA pellet is dried in a Speedy-Vac.
Protocol
1. Add 2.5-3 volumes of 95% ethanol/0.12 M sodium acetate to the DNA sample contained in a 1.5 ml microcentrifuge tube, invert to mix, and incubate in an ice-water bath for at least 10 minutes. It is possible to place the sample at -20°C overnight at this stage.
2. Centrifuge at 12,000 rpm in a microcentrifuge (Fisher) for 15 min at 4°C, decant the supernatant, and drain inverted on a paper towel.
3. Add 80% ethanol (corresponding to about two volume of the original sample), incubate at room temperature for 5-10 min and centrifuge again for 5 min, and decant and drain the tube, as above.
4. Place the tube in a Savant Speed-Vac and dry the DNA pellet for about 5-10 min, or until dry.
5. Always dissolve dried DNA in 10 mM Tris-HCl, pH 7.6-8.0, 0.1 mM EDTA (termed 10:0.1 TE buffer).
6. It is advisable to aliquot the DNA purified in large scale isolations (i.e. 100 ug or more) into several small (0.5 ml) microcentrifuge tubes for frozen storage because repeated freezing and thawing is not advisable.
Notes on precipitation of nucleic acids
A. General rules
Most nucleic acids may be precipitated by addition of monovalent cations and two to three volumes of cold 95% ethanol, followed by incubation at 0 to -70 °C. The DNA or RNA then may be pelleted by centrifugation at 10 - 13,000 x g for 15 min at 4°C. A subsequent wash with 70% ethanol, followed by brief centrifugation, removes residual salt and moisture.
