Mitochondria are organelles found in eukaryotes which are responsible for the oxidation of energy-rich substances. They are oval and have a diameter of approximately 1.5 micrometers and width of 2 to 8 micrometers.
Mitochondria have their own DNA and are thought to have evolved when an early eukaryote engulfed some primitive bacteria, but instead of digesting them, harnessed them to produce energy. Human children inherit their mothers' mitochondria, and thus mitochondrial DNA has been useful in tracing human lineages.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. Used with permission. For further information see the BioTech homenode.