I got this text in a lab report of one student and I don't like the part about size of the plasmid, but I'm not sure if it's not correct, so if there is someone more experienced with this I'd be glad for any insight.
Plasmid DNA is in cells in many topological forms. Due to dissociation of phosphate groups it behaves as polyanion and during electrophoresis goes to positive electrode. In such environment has the mixture of topoisomeres identical charge and thus the only thing affecting mobility of the topoisomers is the space conformation. Particular topoisomers with mass weight less than 7 kbp move in 1% agarose gel as single bands. Higher topoisomers over treshold Wr give wide continual band of superhelical DNA.
I just translated it quickly, so it may not be exactly accurate, but I hope you get the idea.
However, as I think about it now, if there is only one plasmid all the topoisomers have one molecular weight anyway. (That's what I want to hear from them, I just got confused by these)
I don't really understand the question. In agarose gels, DNA molecules of the same size, but with different linking number (Lk), they are resolved on the basis of differences in writhe (Wr) (NO MOLECULAR WEIGHT). Because the Lk for a given closed circular DNA molecule must be an integral number, adjacent topoisomer bands differ by a Lk of one.
You will find a more detailed descripcion of the DNA topology in the enclosed paper by Mirkin.
Hope this would help you
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