I have used the kits from Qaigen, but I find that doing a "dirty" prep gives a much higher yield. Basically a phenol-chloroform extraction with an ethanol precipitation. In my experience it is sufficiently clean for many applications, though I wouldn't use it to transfect cells directly. I have gotten ~200ug DNA from 50ml culture (10X what I get with the kits).

Start the procedure the same, with P1, P2 and P3. Then spin it down 6000xg for 10-15 min to get all the fluffy white stuff out of the way.

Transfer it to a new tube (either avoid the fluffy stuff or use the filter they provide--I usually just avoid it because I don't want to buy new kits.

Add an equal amount of 1:1:1 Phenol:Chloroform:Isoamylalcohol. (If you have 6 ml in your tube, add 6 ml P:C:I.) Vortex until cloudy, let sit 5 mins. Spin MAX 3 min.

Transfer top layer to new tube (avoid getting *any* of the bottom layer, leave a little of the top layer).

Add 3 parts ice cold EtOH (I keep mine in the freezer). If you have 6 ml solution, add 18 ml EtOH. I also add 1:10 of 3M NaOAc to better precipitate my pellet. (For a total volume of 24 ml, add 2.4 ml.)

Then put the tube either in the -80 for 30 min, or in the -20 for 3h. Don't let the solution freeze solid.

Remove the tube and spin MAX for 5 mins at 4C. Check for the pellet, then pour off the supernatant.

Wash again with 70% EtOH (10 ml or so). Spin again and pour off the super.

Add an appropriate volume of TE (or water or EB) and nanodrop.

Did you get only plasmidic- or also sheared-genomic DNA?

" I wouldn't use it to transfect cells directly", but what kind of purity does it mean? I will restrict the plasmid then make chromatography to isolate the fragment afterwards, so I think the purity requirements are not as high.

thank you for your answer!

The above recommended alkaline lysis procedure is good. However, it also recovers RNA. The RNA yield from such cultures is very high, and may throw the A260nm quantification off by a large degree. This is true even if you include RNAse in the P1. This can be viewed by running out the purified nucleic acid in an agarose gel and staining with SYBR green, which will reveal the RNA. The DEAE resin in the Qiagen columns helps purify the DNA away from the RNA. Those columns can be re-generated and re-used almost indefinetly (http://bitesizebio.com/512/512/); you can find the Qiagen buffer compositions online as well. I'd recommend sticking to the column based procedures. The GigiPrep kit from Qiagen is expensive, but if carefully shepherded may be suitable for the purification you're looking for.