Simple, inexpensive and RNase-free purification of plasmid DNA by fractional precipitation with isopropanol.

We present a modified alkaline lysis method for purification of plasmid DNA (pDNA) from bacterial extract using fractional precipitation with isopropanol (FPI). This method includes two successive precipitations with 0.33 and 0.36 volumes of isopropanol and separates pDNA from total RNA and most of the lipopolysaccharides. Using different quality control tests, we demonstrate that plasmids purified with FPI show superior quality compared to plasmids prepared with commercial kits based on spin-column chromatography.