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 single-strand nuclease digestion and displayed the mobility properties of coupled leading- and lagging- strand replication products. Intermediates of coupled, unidirectional mtDNA replication were found in mouse liver and human placenta 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 presumed to derive from the orthodox, strand-asynchronous mode of mtDNA replication. These findings indicate that two modes of mtDNA replication operate in mammalian cells and that changes in mtDNA copy number involve an alteration in the mode of mtDNA replication.