9.

Covalent, double-stranded circular DNA molecules

• a. are not typically observed in nature.
• b. display free rotation about their phosphodiester bonds.
• c. with different numbers of supercoils cannot be isolated by electrophoresis.
• d. can be negatively-supercoiled by topoisomerase II in the presence of ATP.
• e. can become supercoiled even when nicked by DNase.