Precipitation of DNA

A)Ethanol precipitation
1) Add to the DNA solution 1/10 volume of 3M NaOAC and 3 volume ethanol.
2) Leave in -70 degC freezer for 20 minutes.
3) Spin in microcentrifuge for 10 minutes to pellet DNA.
4) Wash with 70% ethanol and spin for 10 minutes.
5) Air-dry the DNA and resuspend in water, Tris buffer or TE buffer.

B) Isopropanol precipitation
1) Add to the DNA solution 1/2 vol. of 7.5M NH4OAc and 2 vol. isopropanol (1 vol. isopropanol is sufficient to ppt the DNA).
2) Incubate at room temperature for 10 minutes.
3) Spin for 10 minutes. Wash with 80% ethanol.
4) Air-dry DNA and resuspend in water or buffer of choice.

Note:
1) Advantage of isopropanol over ethanol - less volume, fewer salts co-precipitated, done at room temperature, and may be more effective in separating primers (which is not precipitated) from PCR products than ethanol.
2) For improved recovery of DNA from dilute solutions (<10 ng/ml), overnight incubation and extended (30 min) centrifugation is recommended.
3) In general, the length of time of centrifugation is more important for precipitation DNA than chilling the solution in -20 or -70 freezer.
4) Addition of ammonium acetate to 2.5 M (without ethanol) has been shown to be effective in precipitating proteins while leaving the DNA in solution.
5) NaOAc precipitation can precipitate large amount of protein together with the DNA.

C) Precipitation with glycogen
Useful for precipitation of low concentration of DNA
Use 1 ul of a 20 mg ml-1 stock to DNA solutions up to a volume of 1 ml.
More discussion in TIBS article Using carrier for precipitation nucleic acid

Ethanol Wash
Ethanol washes are performed after salt/EtOH precipitations to remove any residual salt from the nucleic acid pellet. The wash employs 70-80% EtOH which will solubilize salts but not nucleic acids.

Do
Add 70 - 80% EtOH to the nucleic acid pellet. The volume should be sufficient to at least cover the pellet and wet the sides of the tube when vortexed (there is no volume too large). Vortex the sample for 1 minute; the pellet should come loose from the tube and be broken up in the EtOH. Centrifuge the sample 10 - 30 minutes, to recollect the pellet. Aspirate off the EtOH.

Don't
Don't just add the EtOH and immediately decant. The pellet should be vortexed so that the EtOH can penetrate the sample and solubilize salt.
Don't forget to respin! The pellet must be firmly reaffixed to the tube so that it is not lost during aspiration.