Mitochondria are the only components of cells apart from the nucleus to possess DNA. Mitochondrial DNA is much shorter than nuclear DNA, but no less important. Most mitochondrial proteins have their origin in the nuclear DNA; these proteins are imported into the mitochondria. But a number of proteins essential to energy production come from mitochondrial DNA, and damage to this DNA can cripple the ability of the mitochondrion to produce energy.

The fact that they have their own membranes, their own DNA, and divide and reproduce themselves independent of the rest of the cells have made mitochondria the subject of much speculation by scientists. It is now widely believed that mitochondria are descended from small bacteria-like organisms that, early in evolution, invaded a nucleated cell. Gradually, this primordial cell developed in such a way that the mitochondria took over the task of producing cellular energy, while the nucleated host assumed the other duties that would maintain the health and function of the mitochondria.