Organellar DNA replication in Nicotiana tabacum cultured cells

In the diploid vegetative plant cell, the nuclear DNA is present in two copies, whereas the chloroplast and mitochondria genomes are present in a higher and variable copy number. We have studied the replication of the nuclear, chloroplast and mitochondrial DNA in cultured Nicotiana tabacum cells using density and radioactive markers. Essentially all the 10,000 chloroplast genomes in a given cell replicate in one cell cycle as do all the mitochondrial DNA molecules. No measurable level of unreplicated organellar DNA molecules can be detected in these cells.