Changes in D-loop frequency and superhelicity among the mitochondrial DNA molecules in relation to organelle biogenesis in oocytes of Xenopus laevis.

Changes in mitochondrial DNA (mtDNA) replication activity are known to occur during oogenesis in Xenopus laevis. Electron microscopic and electrophoretic analyses carried out on mtDNA molecules at different vitellogenic stages show that 1. The frequency of displacement loop (D-loop) forms is correlated with the intensity of mitochondrial biogenesis. 2. Most of the native molecules as well as the D-loop carrying ones are superhelical. 3. Four families of different superhelicity may be distinguished and D-loops are found only in the most superhelical ones. To account for the changes in the frequencies of the D-loop forms and of the different topological types during cell differentiation, it is suggested that the initiation of a new replication occurs only on the most superhelical molecules and that some control of superhelicity may exist in mitochondria.