SIMPLE Q ABOUT EXTRACTION - (Jan/23/2008 )
BY USE KIT FOR PLASMID EXTRACTION FROM E.COLI . HOW THE KIT CAN EXTRACT THE PLASMID WITHOUT THE CHROMOSOME
The kits contain lysis buffers that are relatively mild. So as long as you follow the instructions and don't overdo the lysis, the large genomic DNA should pellet at the bottom of your tube and the small plasmid DNA should be present in the supernatant. If you shake the cells while lysing you can shear the genomic DNA and contaminate your plasmid prep with genomic fragments.
Try to search for this paper by Birnboim and Doly (1979) Volume 7 Number 6 in Nucleic Acids Research. It explains more on the reason. Basically is the alkaline condition that remove chromosomal DNA but not the plasmid DNA. Something to do with the denaturation.
Sorry.. can't remember the details. Hope this answers your question.
thank you all