For ten points, name this phenomenon that manifests itself as added twists in a DNA molecule.
This phenomenon can be present in stable forms only when the ends of the DNA are not free, as in a covalently closed circular DNA molecule.  It is commonly the result of tension created ahead of the replication fork during the unwinding of the DNA during replication, and can be relaxed by the enzyme DNA gyrase.  For ten points, name this phenomenon that manifests itself as added twists in a DNA molecule.
This phenomenon can be present in stable forms only when the ends of the DNA are not free, as in a covalently closed circular DNA molecule.  
supercoiling