Mechanism of mitochondrial DNA replication in mouse L-cells: introduction of superhelical turns into newly replicated molecules

The first closed circular product of mouse L-cell mitochondrial DNA synthesis is a zero superhelix density molecule. Both of the asynchronously synthesized mitochondrial DNA daughter molecules pass through the zero superhelix density state. These molecules have a mean lifetime of approximately one hour before conversion to supercoiled molecules containing approximately 100 superhelical turns. A low frequency of intermediates in the conversion of these two closed circular forms is demonstrable by agarose gel electrophoresis. The degree of sensitivity to alkali has been used to demonstrate that newly replicated mitochondrial DNA has the same content of ribonucleotides as mass-labeled mitochondrial DNA.