Mitochondrial growth and DNA synthesis occur in the absence of nuclear DNA replication in fission yeast

Cell growth and division require the doubling of cellular constituents followed by their equal distribution to the two daughter cells. Within a growing population, the ratio of mitochondrial to cellular volume is maintained, as is the number of mitochondrial genomes per cell. The mechanisms responsible for coordinating nuclear and mitochondrial DNA synthesis, and for balancing increases in cell and mitochondrial size are not well understood. In studies of the fission yeast Schizosaccharomyces pombe we quantified cellular and mitochondrial DNA content by both Southern blot analysis and flow cytometry of cells stained with a variety of DNA-binding fluorochromes, which we show are able to detect nuclear and mitochondrial DNA with different efficiencies. In the conditional cell division cycle mutant cdc10, which is unable to initiate nuclear DNA synthesis, we found that there was an increase in the mitochondrial DNA content in the absence of nuclear DNA replication. This demonstrates that mitochondrial and nuclear DNA synthesis are not obligately linked. We also show that mitochondrial DNA replication is not required for the increase in mitochondrial size that occurs as cells elongate, although this results in a decrease in the ratio of mitochondrial DNA to mitochondrial volume.