Self replicating organelles (such as mitochondria and chloroplasts) often contain their own DNA. A common theory for the origin of this DNA is Endosymbiont Theory. The theory states that eukaryotic organelles are thought to have evolved from prokaryotic cells that lived inside ancient eukaryotic cells in symbiosis. Mitochondria, for example, are believed to have evolved from aerobic bacteria nearly two billion years ago .
- ↑ Hartyl D. L. and Ruvolo M. (2012) Genetics: analysis of genes and genomes, 8th Edition, Burlington: Jones and Bartlett Learning. Page 598.