Practice question

In alkaline lysis for plasmid isolation, covalent closed circular supercoiled plasmid DNA and linear chromosomal DNA behave differently due to topology. Under alkaline conditions with SDS, both are denatured. Upon rapid neutralization, small supercoiled plasmids renature quickly because their two strands remain interlinked topologically, becoming soluble. Large chromosomal fragments cannot correctly reanneal and form an insoluble network with SDS-protein complexes and precipitate. Thus separation is based on topological differences and renaturation kinetics, not merely size variation or charge differences alone.