Simple plasmid question

Joseph C. Bagshaw jbagshaw at
Sat Apr 26 07:09:07 EST 1997

Some plasmids form head-to-tail dimers and trimers, which migrate as
slower bands when uncut.  Even the supercoiled dimer will usually migrate
slower than the relaxed monomer.  When these various plasmid forms are
digested with a restriction enzyme, they all produce the same linear
fragment or set of fragments.  The linear form of almost all plasmids
migrates a little faster than the relaxed form of the monomer, so you will
see one or more bands in the uncut sample that migrate more slowly than
the linear monomer.

********************  HAVE GENES, WILL TRAVEL  ********************
Joe Bagshaw, Worcester Polytechnic Institute
jbagshaw at
Roadkill on the information superhighway.

On 26 Apr 1997, Michael J. Bumbulis wrote:

> I wonder if anyone can answer this simple question.  Periodically,
> I purify plasmid DNA and when I run uncut plasmid on an agarose
> gel, there are sometimes 4 or 5 bands.  I realize plasmid can exist
> in supercoiled and nick-circle forms, but what could these other bands
> represent?  I know they are all plasmid DNA because when I cut the
> plasmid at a unique restriction site, you see only one band.  And
> this linear band does not co-migrate with any of the other uncut
> bands (ruling out linear fragments).  In fact, some of the uncut
> bands migrate more slowly than the linearized (ruling out linear
> fragments again).  Any ideas about what is going on?
> -- 

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