How is plasmid removed from DNA?

Alkaline lysis is the most common method used for removing plasmid from DNA. This method involves denaturing the plasmid and genomic DNA using an alkaline lysis buffer that contains sodium hydroxide and sodium dodecyl sulfate (SDS). 

The sample is then neutralized in a potassium acetate solution in order to renature the plasmid. The plasmid DNA, which is smaller, can easily reanneal, forming double-stranded DNA, but the genomic DNA, which is longer, gets tangled making it difficult to re-anneal properly so that the complementary strands remain separate. 

On centrifugation, the soluble plasmid DNA and the high molecular weight genomic DNA globules separate from each other. Ethanol precipitation is then used to collect and recover the layer containing the plasmid DNA.