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Appropriate salt for DNA precipitation - (May/31/2006 )

I have precipitated DNA from several sources. For example from plasmids, phages - the usual stuff. Many times I am preparing a DNA insert (i.e. digesting it). After the digestion I usually do the following:

1) phenol:chloroform extractions
2) chloroform extraction
3) add 1/10 the volume of some salt source
4) add 2.5 vol cold ethanol
etc, etc

My question is how do you know what the appropriate salt source should be? I have used NaOAc, ammonium acetate and 10X STE. Is there some type of stead fast rule?

I have run across a protocol that I am changing. Which salt source should I use (this is a digested fragment of DNA from a plasmid) for ethanol precipitation of the DNA?

Thanks for anyone's help.


I dont know which is a more appropriate salt source as I always use Na Acetate.


Na acetate works well for most applications, but if your DNA of interest is small, say around 100 bp, or if you're concerned about heavy metals or detergents, you may have better success with ammonium acetate.
1/2 volume 7.5 M NH4 acetate pH7.5 plus 2 vol 95% EtOH




somewhere in this forum I read that ammonium acetate may cause problems with cloning, I think it was in the pinned "Tips on ethanol precipitation of nucleic acids and wash" thread.