The alkaline lysis method selectively purifies plasmid DNA from other cellular components of the bacterial cells including chromosomal DNA. Controlled lysis of bacterial cells using a highly alkaline (pH 12.0 – 12.5) solution of SDS results in the solubilization and denaturation of cellular components including chromosomal DNA. In the subsequent step, neutralization results in regaining normal pH that causes precipitation of chromosomal DNA due to random annealing of both strands. In contrast to chromosomal DNA, neutralization results in plasmid DNA regaining its original state as both strands of plasmid DNA lie close to each other under alkaline conditions due to its covalently closed circular structure.
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