Mitochondria

Mitochondria

Mitochondria are extremely small (from .002-.008 millimeters in length) rice-shaped structures whose details can only be seen with the electron microscope . (The mitochondria shown in the top border of this page are from a mitochondria dense region of a visual cell. A single mitochondrion in the upper left is outlined in yellow.)

Mitochondria are often called the “powerhouses” of the cell because they are the site where sugar is broken down to release the energy required for cellular functions.

A cell may have hundreds or even thousands of mitochondria depending on the particular cell’s need for energy. (For example, the average human liver cell contains more than a thousand mitochondria.)

In addition to containing the many proteins needed to control the energy release process, the mitochondria also contain a small amount of DNA that is used to direct the manufacture of thirteen of the proteins needed for its activities. Mutations in the protein coding part of mitochondrial DNA can cause some human diseases, typically involving either neuromuscular dysfunction or some forms of diabetes. However, testing of mitochondrial DNA for genealogical purposes does not include this region and consequently reveals no information about medical conditions.

Of interest to the genealogist (among others) is the fact that all of an individual’s mitochondria are derived from his/her mother. Although the sperm cell tail is packed with mitochondria to power its long journey to the egg cell, the tail and mitochondria drop off of the sperm at fertilization and never enter the egg cell. Consequently, all of the mitochondria in the fertilized egg come from an individual’s mother. [A recent publication by Marianne Schwartz and John Vissing from the University Hospital Rigshospitalet in Copenhagen reports an instance of a man whose muscle cell mitochondrial DNA matched that of his father rather than his mother. This, however, is quite rare. (New England Journal of Medicine, vol 347, pp 576-580)]

As the fertilized egg divides repeatedly to produce the trillions of cells of the human body, the mitochondria also divide and pass down their bit of maternal DNA to every cell in the body of the offspring. Additionally, since each cell contains thousands of mitochondria and each mitochondrion contains a number of copies of mitochondrial DNA, that means that each cell contains thousands of copies of mitochondrial DNA but only one paired set of chromosomal DNA. This is why samples of mitochondrial DNA can be obtained from smaller amounts of tissue than samples of chromosomal DNA. For example, samples of mitochondrial DNA have been isolated from strands of hair even without follicles attached and even from ancient bits of bones that have been poorly preserved.

Since mitochondrial DNA is handed down from a mother to all of her offspring be they male or female, testing your mother’s mitochondrial DNA will not be significantly different from testing your own or your siblings or, if you are a female, your own offspring's DNA.. On the other hand, testing your father’s mitochondrial DNA may shed some dim light on your paternal grandmother’s origins. If you are male, you will share no mitochondrial DNA with your offspring.

The diagram below shows the path of mitochondrial DNA through four generations.