methylated plasmids in culture - (Jul/24/2008 )

How do plasmids in long time cultures get methylated and why do we do a DpNI digestion to such plasmids before transformation

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The cells methylate the plasmid DNA.
Dpn I only cuts methylated DNA. DNA produced by PCR, and subsequently used in ligation expts is unmethylated, so only left-over genomic DNA is digested. Basically it reduces the background

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