Coupled leading- and lagging-strand synthesis of mammalian mitochondrial DNA

Analysis of mammalian mtDNA by two-dimensional agarose gel electrophoresis revealed two classes of replication intermediate. One was resistant to sing le-strand nuclease digestion and displayed the mobility properties of coupl ed leading- and lagging- strand replication products, Intermediates of coup led, unidirectional mtDNA replication were found in mouse liver and human p lacenta and were the predominant species in cultured cells recovering from transient mtDNA replication. Replication intermediates sensitive to single- strand nuclease were most abundant in untreated cultured cells, These are p resumed to derive from the orthodox, strand-asynchronous mode of mtDNA repl ication. These findings indicate that two modes of mtDNA replication operat e in mammalian cells and that changes in mtDNA copy number involve an alter ation in the mode of mtDNA replication.