Precipitation of nudeic acids

Alcohol is by far the most important predpitation agent used with nucleic acids. Predpitation is usually carried out with 2-3 (v/v) of ethanol or 1 (v/v) of isopropanol in the presence of 0.1-0.5 M Na or K acetate at pH 5.0 and 0 °C salt concentrations higher than 1 M interfere with precipitation. Monovalent cations and ethanol produce a conformational change in the nudeic add which leads to precipitation. For quantitative predpitation, the mixture should be kept for 15 min at -70 °C or 30 min at -20 °C. This is particularly important for dilute solutions ( 10 pg ml-1) more concentrated solutions, above about 0.25 mg ml-1, are predpitated quantitatively even at room temperature. Sodium and potassium acetate salts in the mixture are partially predpitated with the nuddc acid, but they can be removed by washing 

For very dilute nucleic acid solutions, precipitation with polyethylene glycol (PEG) is preferred. PEG 6000 is added to a concentration of 10% (w/v), and the solution is allowed to stand on ice for 2 h. The precipitate is removed by centrifugation and washed with 70% (v/v) ethanol. PEG can be used for fractional precipitation, since high molecular weight DNA is precipitated at lower PEG concentrations than low molecular weight DNA. It should be emphasised that oligonucleotides of chain length less than 20 cannot be precipitated effectively with either alcohol or PEG. 

as a word of caution, it should be noted that nucleic acids precipitated by PEG can contain macromolecular impurities originating from the PEG these impurities are not detectable by the usual methods (e.g., electrophoresis or UV absorption spectroscopy) and they cannot be readily removed even by CsCl gradient centrifugation. 

Dialysis, ultrafiltration and lyophilisation are processes that are used to change the composition of the medium in which macromolecules are dissolved. The focus of our discussion is on proteins, and we consider methods that can be applied widely, from the processing of cell extracts to preparation of samples for experiments. 

According to Fick s law, the rate of diffusion v is given by the following expression  

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