An organelle found in eukaryotes 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. Offspring inherit their mothers' mitochondria, and thus mitochondrial DNA has been useful in tracing human lineages. Compare chloroplast.