Mitochondrial DNA (mtDNA), is a small circular DNA molecule located in mitochondria which are cell’s organelles residing in the cytoplasm. It is approximately 16,569bp (base pairs) long and consists of 37 genes and a control region which is also called a 'non-coding' region since it does not code for any gene products.
Unlike nuclear DNA, mtDNA is inherited strictly from mother to child. Any sperm mitochondria that might enter the fertilised egg are destroyed by embryo's cellular machinery due to a tag that is added onto them during spermatogenesis which marks down the sperm mitochondria for degradation. Thus mtDNA is not unique for each individual since all the maternal relatives have identical mtDNA sequence.
However, mitochondrial DNA is not the same between all populations and this is due to lack of repair mechanisms and proofreading capabilities which makes it susceptible to base substitutions, leading to high mutation rates. The mutation rates of mitochondrial DNA are 10 times higher than in nuclear DNA.
This provides mtDNA with variability useful in human identity testing and in investigating the evolutionary relationships among individuals and species, by interrogating its short variable sections. These variable sections are located in the control or 'non-coding' region of mtDNA; given that it does not code for any gene products, the limits for nucleotides mutation are fewer and the rate of polymorphisms is high between each person.
Human mtDNA was first sequenced in Sanger's laboratory in Cambridge in 1981. This first sequence was called 'Anderson' or Cambridge reference sequence (CRS) and for many years the new sequences were compared with it. In 1999 it was revised by Andrews who confirmed almost all of the original identified nucleotides. The sequences across HV1 and HV2 that are most commonly used in forensic applications were found to be identical between the revised CRS (rCRS) and the Anderson sequence.
In the forensic community two hyperviarable regions of mitochondrial DNA - HVI and HVII - are often used for human identification in cases of ancient DNA or highly degraded DNA samples. This kind of specimen fails to give DNA profiles with the commonly used STR typing technique.
Samples such as bone, teeth or hair may be the only sources of DNA left in very old or very decomposed remains but the amount of DNA that can be obtained from them is really small. Therefore mtDNA is more suitable in such cases since it can be found in high numbers within the cells and is more likely than nuclear DNA to survive after a long period of time. Furthermore the circular shape of mtDNA makes it less vulnerable to exonucleases that degrade the DNA molecules; so it is present long enough to complete the forensic DNA analysis.
Mitochondrial DNA is also used by evolutionary biologists to study the variations between human and other species in order to specify the possible relationships between them. The section most commonly used for these studies is cytochrome b gene which is considered to be useful for species identification and phylogenetic studies because it demonstrates high interspecies conservation and at the same time is variable enough to allow intraspecies differentiation.
In addition, molecular anthropologists and genetic genealogists examine mtDNA of various populations to map out ancestry and migration during the years.