2.6.5: Organellar Genomes
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In eukaryotes, DNA and genes also exist outside of the chromosomes found in the nucleus. Both the chloroplast and mitochondrion have circular chromosomes (Figure \(\PageIndex{22}\)). These organellar genomes are often present in multiple copies within each organelle. In most sexually reproducing species, organellar chromosomes are inherited from only one parent, usually the one that produces the largest gamete. Thus, in mammals, angiosperms, and many other organisms, mitochondria and chloroplasts are inherited only through the mother (maternally).
These organelles are likely the remnants of prokaryotic endosymbionts that entered the cytoplasm of ancient progenitors of today’s eukaryotes ( endosymbiont theory ). These endosymbionts had their own, circular chromosomes, like most bacteria that exist today. Chloroplasts and mitochondria typically have circular chromosomes that behave more like bacterial chromosomes than eukaryotic chromosomes, i.e. these organellar genomes do not undergo mitosis or meiosis.