Mitochondrial DNA and Genetics

The Mitochondrial Genome Informs Genetics, Evolution and Forensics

Aug 27, 2008 Tamara Golden

Mitochondrial DNA is inherited only from the mother, and is present in higher abundance than the nuclear genome, making this molecule useful in genetics studies.

The mitochondrion is a sub-cellular structure found in all human cells except red blood cells. There are multiple mitochondria per cell, and each mitochondrion contains multiple copies of a circular piece of DNA, which is known as the mitochondrial genome or mitochondrial DNA. There can be hundreds of copies of mitochondrial DNA in a single cell. This is in striking contrast to the nuclear DNA, which is present in only two copies per cell in most cell types.

Mitochondrial Genome

The mitochondrial genome is a small piece of DNA, containing about 16,000 base pairs. In contrast, the nuclear genome, which is what is commonly referred to by the phrase "human genome", contains over three billion base pairs. Clearly, the mitochondrial genome contains much, much less information than the nuclear genome. However, the mitochondrial DNA is present in many more copies per cell than the nuclear DNA, and thus is easier to extract from tissue and use in forensic or other tests.

Mitochondrial DNA and Evolutionary Genetics

The mitochondrial genome is inherited only from the mother. Therefore, all women who are descended from a common female ancestor will have mitochondrial DNA with the same nucleotide sequence. The mitochondrial DNA does not undergo recombination with every generation as the nuclear genome does, so the sequence of the mitochondrial DNA remains relatively constant from generation to generation.

Changes in the mitochondrial genome within a family will only occur as a result of random mutations, and these mutations will be inherited by subsequent generations. This makes the mitochondrial DNA an interesting subject for studying evolution and population migration.

Comparing mitochondrial DNA sequences can provide a measure of how closely related two people are, or how recently two groups of people had a common ancestor. Based on the rate of mutation of the human mitochondrial genome, and on measurements of the differences between the mitochondrial genomes of many people around the world, researchers have estimated that all humans presently on earth are descended from a common female ancestor, the so-called "Mitochondrial Eve", who lived about 170,000 years ago (1).

Mitochondrial DNA and Forensics

Because the mitochondrial genome is present in more copies than the nuclear genome, it is easier to obtain nucleotide sequence information from the mitochondrial genome than the nuclear genome in a small number of cells. This becomes especially important in a forensic setting if only a small amount of tissue is available, or if a sample has been kept under conditions where DNA may have degraded.

Mitochondrial DNA can be useful in the identification of remains, because it can be compared to DNa from living female relatives to determine whether the unidentified person and a specific family are related. Mitochondrial DNA can also be used to rule out a suspect, if the sequence of the DNA found does not match that of the suspected individual.

References

(1) Ingman et al. (2000) "Mitochondrial genome variation and the origen of modern humans". Nature. 408,708-713

The copyright of the article Mitochondrial DNA and Genetics in Genetics & Evolution is owned by Tamara Golden. Permission to republish Mitochondrial DNA and Genetics in print or online must be granted by the author in writing.

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