The DNA discussed so far is found in the nucleus of the cell, in the chromosomes. In addition to this nuclear DNA, cells also contain DNA in small structures outside the nucleus called “mitochondria.” The accompanying diagram of a generalized animal cell shows the nucleus and mitochondria, as well as the other cell structures. Mitochondria contain the cell’s energy-processing machinery, but they also contain a small amount of DNA. Mitochondrial DNA (mtDNA) is inherited entirely from one’s mother. There is no paternal contribution.
Mitochondrial DNA, unlike its nuclear counterpart, is circular. In that respect it is similar to some bacterial genomes, which consist of a single circular DNA molecule. It is now known that over long periods of time mtDNA occasionally undergoes mutations and that these mutations are stable and passed along from mother to offspring. Anthropologists, scientists who study the variation and evolution of human beings, use mtDNA to follow patterns of human migration over time. Some of the mtDNA codes for specific proteins, but mtDNA also has a region (called the “control region,” or “D-loop”) that, like some sections of nuclear DNA, is subject to a great deal of polymorphism. It can be divided into two hypervariable regions, designated HV1 and HV2, which are 342 and 268 base pairs (bp) in length, respectively. Hypervariable means that these regions are especially prone to random mutations over time, and most of the variability in mtDNA from person to person is found here.
Forensic mtDNA typing is sometimes used in cases when nuclear DNA typing fails or cannot be done. It is also used in trying to identify human remains. Mitochondrial DNA is quite robust in some tissues, especially older or weathered specimens such as old bones. That is the basis for its use in the identification of skeletal remains, which include no soft tissues (and hence no nuclear DNA). In addition, hair shafts contain mtDNA but no nuclear DNA. (Hair roots have nuclear DNA, but many hairs found as evidence have no roots. They are shed from the human body.) So, any DNA analysis on hair shafts must be mtDNA typing.
