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2.9: Organellar Genomes

In eukaryotes, DNA and genes also exist outside of the chromosomes found in the nucleus. Both the chloroplast and mitochondrion have circular chromosomes (Fig. 2.22).  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.

Figure 2.22: A map of the complete mitochondrial chromosome of the woolly mammoth (Mammuthus primigenius).  The mtDNA that was used to produce this map was obtained from tissue of a mammoth that lived approximately 32,000 years ago.  The map shows the position of enzymes encoded on the chromosome including components of the NADH dehydrogenase (ND) complex and cytochrome oxidases (COX), all of which function during energy metabolism in the mitochondrion.  The mitochondrial chromosome also encodes various tRNAs and rRNAs used in translation of the genes encoded on this chromosome. Other proteins required by the mitochondrion are encoded in the nuclear genome, and are translated in the cytoplasm and imported into the organelle. Circular organellar chromosomes such one as this are typical of almost all eukaryotes. (From Rogaev et al, 2006).  Recent (Rohland et al, 2010) mtDNA work indicates that mammoths are more closely related to Indian elephants than to either of the African species.