Mitochondria (singular - mitochondrion) are organelles responsible to convert the energy from food into simpler energy forms that can be used by the cells. Hence, mitochondria are known as the 'powerhouses of the cell'. Other than this function, mitochondria are associated with certain cell-specific functions as they contain RNA (ribonucleic acid) and DNA (deoxyribonucleic acid).
The small circular DNA present in mitochondria is known as mitochondrial DNA (mtDNA). The total mtDNA genome accounts to about 16,000 base pairs, which contributes only a fraction of the total DNA content (about 1 percent) in a cell. Let's take a look on how mitochondrial DNA is used in forensic studies.
Use of Mitochondrial DNA in Forensics
As far as forensic science is concerned, investigators leave no stone unturned, while solving cases. DNA-based technologies have been used to identify victims of a crime. In such methods, the DNA sequence of individuals or samples available from the crime scenes, also known as DNA fingerprint, is scanned to identify the victim. Most popular and effective DNA technologies that are used in forensics include RFLP (restriction fragment length polymorphism), PCR (polymerase chain reaction) and STR (short tandem repeat) analysis.
In case of very old biological samples, which are devoid of nucleated cellular material, mtDNA analysis is used. In recent times, this analysis technique is used to identify victims of cold cases, mass disasters (tsunami and earthquakes), historically important cases (war soldiers) and cases with limited biological material. Another advantage of mtDNA is that it can be isolated from samples like hair shafts, bones and teeth. Thus, investigators can study these samples by extracting mtDNA.
In comparison to extracting nuclear DNA, mtDNA isolation is more easy due to the presence of multiple copies in a cell. In humans, there are two copies of nuclear DNA (nDNA) in a cell; whereas, the mtDNA copies range from 100-10,000 per cell (except in egg and sperm cell). Hence, very small samples can be tested for victim identification. In general, a mitochondrion contains at least 2-10 copies of DNA.
Since mtDNA is inherited from the mother's egg cell, it can be used to establish the maternal lineage. In simpler terms, the mtDNA of both mother and daughter are same. Hence, mtDNA analysis serves as a valuable tool, while investigating the case of a missing person. The mtDNA from the unidentified remains can be compared with the mtDNA profile of a maternal relative (such as brother and sister, mother and her daughter). If the respective profiles of the two individuals match, then they are relatives. Thus, any maternal relative will be able to provide a reference sample.
These are the advantages of using mtDNA as a crime-solving tool in forensic studies. Because of its circular structure and location inside the cell, it is more stable and strong than the nuclear DNA. It is shown that mtDNA is protected from degradation, even when exposed to prolonged environmental conditions, which is not the case with nDNA. This is the reason why, mtDNA is present even in older and degraded specimens, whose nDNA is already degraded. The only limiting factor of its analysis is that there is little discrimination between the mtDNA sequences of the same maternal lineage.