The present invention relates to a method for isolating and purifying nucleic acids, preferably comprising genomic DNA, from biological samples comprising the steps of lysing the sample using a lysis buffer comprising a source of anionic surfactant ions, optionally disintegrating the RNA present in the lysate, precipitating the surfactant ions from the lysate, and separating the nucleic acids from the precipitate and further contaminants by size-exclusion chromatography. The invention furthermore relates to a lysis buffer, a method of lysing cells and a kit for the isolation and purification of nucleic acids.