In a normal cell, DNA (Deoxyribo nucleic acid) is present in nucleus and also in a cell organelle known as mitochondria. DNA present in the nucleus is known as nuclear DNA and DNA which is present in the mitochondria are called as mitochondrial DNA (mtDNA). Nuclear DNA contains two sets of DNA (twenty three chromosomes), each set originating from the paternal and maternal copy. But in mitochondria many copies of DNA are present. That is the copy number of mitochondrial DNA is very high, which makes it useful in cases with small or degraded sample. In addition, biological materials like hair shafts, teeth and bone do not contain nuclear DNA but mitochondria may be present and mtDNA analysis can be performed to solve the case.
As when a sperm fertilizes the egg, only the head of the sperm which contains a single set of DNA fuses with the egg, tail and midsection of the sperm are left outside of the egg. In a sperm mitochondria are present in the tail and midsection of the sperm, as these parts require a lot of energy to propel the sperm. As a result mitochondria or mitochondrial DNA never enters the egg. Mitochondrial DNA is inherited from a mother to child. That is mitochondrial DNA of the mother and child is identical and can be used for solving maternal relationships in forensic investigations.
Mitochondrial DNA is circular and approximately 16,569 base pair long. mtDNA can be divided into two parts, coding region and control region. Coding region codes for the biochemical products required to produce energy molecules. The control region regulates the production of the biochemical products from the coding region. Control region also contains two highly polymorphic variable regions, known as Hypervariable region 1 and Hypervariable region 2 or HV1 and HV2. These variables can also be used to solve cases in the forensic science.
Method Used for analyzing mtDNA:
1. Sample is first visually analyzed then cleaned. Cleaning is done by immersing the sample in detergent and an ultrasonic bath. Dentin and pulp part of the teeth are used in analysis. Sample is powdered and then extraction solution is added to release the cellular content including all the cell organelle and also the mtDNA.
2. Extracting the mtDNA from mixture of cellular material is done by adding mixture of chemicals which separate DNA from other organic molecules and then mixture is ultracentrifuged. After ultracentrifugation mtDNA is concentrated in the top layer of the solution, they are extracted and then purified.
3. mtDNA are amplified using a technique known as polymerase chain reaction. PCR uses heating and cooling along with bases and other enzymes to amplify the mtDNA. These mtDNA are purified.
4. Final step is the sequencing of the mtDNA using Sanger's method. This results in many DNA strands with varying length. These DNA are then electrophored to separate them based on their length. Computer software is used to reconstruct the sequences of mtDNA.
5. Finally, DNA examiner verifies the mtDNA sequence.
mtDNA are used to solve crime since 1996. Mitochondrial DNA has been used to solving cases such as missing persons, identifying disaster victims.
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