Mitochondria and chloroplasts are semiautonomous organelles of energy conversion. They carry their own double-stranded DNA in circular or linear chromosomes whose size and gene content vary from species to species.
Translation in the mitochondria of many species depends on an alternative genetic code.
Chloroplast genomes are relatively uniform in size and carry many more genes than mitochondrial genomes. In some chloroplasts, the genes are organized in clusters that resemble the operons of bacteria.
According to the endosymbiont theory, mitochondria and chloroplasts evolved from bacteria engulfed by the precursors of eukaryotic cells. The genomes of these organelles have probably lost more than two-thirds of their original bacterial genes in the course of evolution.
In most species, organelle genomes show uniparental inheritance, mainly through the maternal line. Cells containing a mixture of organelle genomes are heteroplasmic. Cells carrying only one type of organelle DNA are homoplasmic. The genomes of heteroplasmic cells are not evenly partitioned at mitosis.
The organelles of yeast and several other species exhibit biparental inheritance. The genomes of organelles inherited in a biparental fashion show mitotic segregation.
