Phenol extraction of DNA samples

(adapted from Bruce A. Roe, Department of Chemistry and Biochemistry, The University of Oklahoma, Norman, Oklahoma 73019 broe@ou.edu)

Phenol extraction is a common technique used to purify a DNA sample (1). Typically, an equal volume of TE-saturated phenol is added to an aqueous DNA sample in a microcentrifuge tube. The mixture is vigorously vortexed, and then centrifuged to enact phase separation. The upper, aqueous layer carefully is removed to a new tube, avoiding the phenol interface and then is subjected to two ether extractions to remove residual phenol. An equal volume of water-saturated ether is added to the tube, the mixture is vortexed, and the tube is centrifuged to allow phase separation. The upper, ether layer is removed and discarded, including phenol droplets at the interface. After this extraction is repeated, the DNA is concentrated by ethanol precipitation.

Protocol

1. Add an equal volume of TE-saturated phenol to the DNA sample contained in a 1.5 ml microcentrifuge tube and vortex for 15-30 seconds.

2. Centrifuge the sample for 5 minutes at room temperature to separate the phases.

3. Remove about 90% of the upper, aqueous layer to a clean tube, carefully avoiding proteins at the aqueous:phenol interface. At this stage the aqueous phase can be extracted a second time with an equal volume of 1:1 TE-saturated phenol:chloroform, centrifuged and removed to a clean tube as above but this additional extraction usually is not necessary if care is taken during the first phenol extraction.

4. Add an equal volume of water-saturated ether, vortex briefly, and centrifuge for 3 minutes at room temperature. Remove and discard the upper, ether layer, taking care to remove phenol droplets at the ether:aqueous interface. Repeat the ether extraction.

5. Ethanol precipitate the DNA by adding 2.5-3 volumes of ethanol-acetate.

Notes on phenol extraction of nucleic acids

The standard and preferred way to remove proteins from nucleic acid solutions is by extraction with neutralized phenol or phenol/chloroform. Generally, samples are extracted by addition of one-half volume of neutralized (with TE buffer, pH 7.5) phenol to the sample, followed by vigorous mixing for a few seconds to form an emulsion. Following centrifugation for a few minutes, the aqueous (top) phase containing the nucleic acid is recovered and transferred to a clean tube. Residual phenol then is removed by extraction with an equal volume of water-saturated diethyl ether. Following centrifugation to separate the phases, the ether (upper) phase is discarded and the nucleic acid is ethanol precipitated as described above.

A 1:1 mixture of phenol and chloroform also is useful for the removal of protein from nucleic acid samples. Following extraction with phenol/chloroform, the sample should be extracted once with an equal volume of chloroform, and ethanol precipitated as described above.