Phenol/chloroform extraction

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General Information

Phenol/chloroform extraction is an easy way to remove proteins from your nucleic acid samples and can be carried out in a manner that is very close to quantitative. Nucleic acids remain in the aqueous phase and proteins separate into the organic phase or lie at the phase interface.

General Procedure

It is typically easiest to carry the extraction out in 1.7–2 mL eppendorf tubes.

  1. Dilute your nucleic acid sample to 100–700 µL or divide your samples into tubes such that you have no more than 700 µL per tube. It is difficult to do the extraction with volumes smaller than 100 µL. The sample can be concentrated again after precipitation.
  2. Add an equal volume of phenol to the tube, vortex vigorously to mix the phases.
  3. Spin in a microfuge at top speed for 1–2 min to separate the phases.
  4. Remove the aqueous phase to a new tube, being careful not to transfer any of the protein at the phase interface.
  5. Repeat the phenol extraction two more times.
  6. Extract the sample two times with an equal volume of chloroform:isoamyl alcohol to remove any trace phenol.
  7. Precipitate the nucleic acid. (nucleic acid precipitation)

Reagents

  • Phenol equilibrated to pH 7.5
  • Chloroform:isoamyl alcohol in a 24:1 ratio (CHIZAM! in Joyce lab lingo)

Notes

  • Equilibrated phenol can typically be purchased from commercial sources. Alternatively, you can equilibrate it yourself. Be advised that this is NOT a fun procedure to carry out.
  • Phenol and chloroform should be used in a hood if possible.
  • Phenol is a dangerous substance that will burn you if it gets on your skin. WEAR GLOVES and BE CAREFUL. Check out the MSDS to verify the precautions you should take. A solution of PEG 400 is recommended for first aid. Phenol is both a systemic and local toxic agent. You want to visit the medical center.
  • If you're in a hurry, you can shorten the protocol to two phenol extractions and one chloroform extraction.
  • There are also commercial sources of phenol and chloroform mixed together and equilibrated. These are also sufficient for extraction, and I would recommend doing at least two extractions if you decide to go this route. Phenol:Chloroform:Isoamyl alcohol usually has an upper layer of buffer saturated water in the bottle. Do not use this buffer layer -- you want the phenol/chloroform layer underneath.
  • Be careful to determine which layer is the phenol. The density of pure phenol (unlike phenol:chloroform:isoamyl alcohol) is almost 1.0. Small changes in the density of your water layer (excess salt, e.g.) can lead to layer inversion.
  • Removing the lower layer first can make it easier to recover the upper layer
  • Recovery can be improved by "washing" the upper layer by adding water, vortexing, recentrifuging, and recovering the water layer again.

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