The cleared lysate method (seeChapter 25 ) is not usually very effective for isolation of plasmids larger than about 20 kb. Recovery of plasmid DNA is often poor, presumably because high molecular weight plasmids are removed by the clearing spin. An alternative procedure, therefore, is to load the complete cell lysate onto a cesium chloride-ethidium bromide gradient that will separate the plasmid DNA from the chromosomal material and also from other cell components.
