The mitochondrion of all eukaryotes and the chloroplasts of plant cells are the only organelles that have their distinct genomes. These genomes are made of a single, circular DNA molecule denoted mtDNA in mitochondrion and ctDNA in chloroplast. The replication and the mode of inheritance of organelle genomes are distinct from the nuclear genomes.

Mitochondrial genomes vary in size, among species, by up to one order of magnitude. Animal cells have a genome of approximately 16 kb (kilobases, 1000 bases) and represent the smallest mitochondrial genomes in eukaryotes. Yeast possess a much larger genome that varies among the different strains but is about 80 kb, with the whole yeast mitochondrial DNA making up 18% of the total DNA of the yeast. Plant mitochondrial genomes are the largest and most complex. They show an extremely wide range of variation in DNA size. The smallest plant mitochondrial genome...