Unidirectionality of Replication in Mouse Mitochondrial DNA

Abstract

IT has been shown^1,2 that mitochondrial DNA replication begins on a covalently closed circular DNA. In the first identifiable step, a 450-nucleotide strand is synthesized by a displacement mechanism with a unique region of the light strand (L) as a template (Fig. la). After a nicking system becomes operative, displacement synthesis continues (Fig. lb), and the short heavy strand (H) is extended. Light strand synthesis occurs on the displaced heavy parental strand with the formation of a duplex. This duplex synthesis begins at 0.6 or more genome units from the origin for displacement replication (Fig. lc). The foregoing model implies that replication is unidirectional as in a→b→c rather than bidirectional as in a→b′. An experimental test of the directionality of replication is presented here.