About DNA electrophoresis?

Question

why does open circular plasmid DNA migrate more slowly than covalently closed circular plasmid DNA, even though they have same molecular weight and base sequence?

Answer 1

Open circular is like an uncoiled elastic. It encounters more resistance migrating through a gel because it is spread out and will be in direct contact with more of the gel matrix.

Closed circular DNA (AKA Supercoiled DNA) is tightly coiled, like a balled up elastic. While the molecule may be the same size, the coiling compresses it, allowing it to encounter less resistance when migrating through the gel.

This is why if you want a true size comparison, do a single-cur restriction digest of your DNA prior to running it on a gel.