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The basic steps of DNA isolation are disruption of the cellular structure to create a lysate, separation of the soluble DNA from cell debris and other insoluble material and purification of the DNA of interest from soluble proteins and other nucleic acids. Historically, this was done using organic extraction (e.g., phenol:chloroform) followed by ethanol precipitation. In the case of plasmid preparations, the multiple-day protocol typically involved cesium chloride banding followed by dialysis of the plasmid DNA.

What's confusing to me is :


How is chromosomal DNA eliminated during the purification?

[rjb]

ggg...

It is during the Caesium chloride centrifugation that the autosomal and plasmid DNA are separated. When the extract is placed into a centrifuge tube along with Caesium Chloride and EtBr and spun in an ultra centrifuge at high g for 24hrs or so, a density gradienty is formed in the solution and the two forms of DNA separate out into distinct bands. From memory, plasmid DNA will tend band lower down in the tube due to its non-linear structure and the effect of the EtBr, which enhances density differences. Depending on tube material, the plasmid band can then be removed by piercing the centrifuge tube. 

Hope this helps

R