Mitochondria are structures that are found inside virtually every cell. Of all the 250 cell types in the body, only the mature red blood cell does not contain mitochondria. Each cell may have thousands of mitochondria, or as few as two, depending on the cell type and its energy need.

The name "mitochondrion" means “thread-like” and comes from the way these structures may look under the microscope. Mitochondria can have different shapes, however, which are characteristic of the cell type where they reside.

Each mitochondrion has an outer membrane and an inner membrane. Embedded within each membrane are unique proteins that carry out specific functions required for cellular energy and metabolism. Many functions of the mitochondria are linked to the specialized cell type in which it is found.

Mitochondria possess their own DNA, unique from the cell’s nuclear DNA. Mitochondrial DNA is small and circular, not linear like nuclear DNA. It more closely resembles the DNA of bacteria. Each mitochondrion contains multiple copies of the DNA, so they may be recombined in various ways.

By Russell P. Saneto, D.O., Ph.D., Children’s Hospital and Regional Medical Center/University of Washington School of Medicine, Seattle, WA.

Topic Editor: Russell P. Saneto, D.O.,Ph.D.
Last Reviewed:6/24/04