The Dynamics of the Allomorphic Forms of Mouse Mitochondrial DNA

Mouse mitochondrial DNA can be fractionated by means of band sedimentation into the allomorphic forms supercoiled circle (34 S), open circle (24 S) and linear rod (21 S). The separation of the ring‐open form based on its hydrodynamic properties is not necessarily an indication of the chemical properties of this form, i.e. a nick in the phosphodiester backbone of at least one strand of the duplex DNA. Circular duplex DNA molecules may show an ambiguous behaviour. During sedimentation the high frictional coefficient of open circles is observed. However, as judged by other criteria the molecules behave like covalently closed circular DNA with invariant topological winding (restricted uptake of an intercalating dye). A large part of mouse mitochondrial DNA occurs as D‐loop mitochondrial DNA molecules which contain a displaced single‐stranded region in the duplex DNA molecule. The insertion of the new displacing strand reduces the rotation of the parental strands around each other and therefore reduces the number of right‐handed superhelical turns in the “covalently closed” circular DNA. For this reason, the bulk of the mouse D‐loop mitochondrial DNA molecules display a sedimentation behaviour (28 S) similar to that of ring‐open (nicked) molecules although these molecules contain a “covalently closed” phosphodiester backbone in each strand. Pulse‐chase‐pulse experiments, labeling firstly with [2‐ 14 C]thymidine and secondly with [ Me ‐ 3 H]thymidine, indicate that the different allomorphic forms of the mitochondrial DNA turn over at a different rate. The lowest turnover rate is observed in alkali‐stable supercoiled mitochondrial DNA and in D‐loop mitochondrial DNA. It is concluded that the accumulation of D‐loop mitochondrial DNA molecules in mouse cells is due to abortive replication events.