Ethanol Precipitating of Genomic DNA

[Howard T. Petrie]

In article <3hajva$j7n at mercury.hgmp.mrc.ac.uk>, mrhodes at hgmp.mrc.ac.uk
(Dr. M. Rhodes) wrote:

> I am preparing genomic mouse DNA , (which I want to keep large)
> It all goes well until I do the final ethanol ppt 
> at this point when I spin it I dont get a nice pellet
> but a globby mass...

I've seen the same thing many times, in fact, it usually happens. 
Spinning isn't the solution if you want very large DNAs.  What I have
found to work is to spool out the precipitated bits (into additional 70%
ethanol, or directly into propanol if it comes out cleanly); subsequent
rocking of the solution will result in additional precipitate, and the
process can be repeated until the glob has all precipitated. 
Alternatively, a large volume of ethanol seems to work better, but once
you hit a certain amount of DNA, the globs are inevitable.