Question

Define about mitochondrial DNA. What is the forensic significance of mtDNA.

Answer 1

mtDNA (mitochondrial DNA):

Human mitochondrial DNA (mtDNA) is a circular double-stranded molecule, 16,569 base pairs (bp) in length that codes for 13 subunits of the oxidative phosphorylation system, 2 ribosomal RNAs (rRNAs), and 22 transfer RNAs (tRNAs). It is present in hundreds to thousands of copies in each cell, not within the nucleus, but within the cell’s energy-generating organelles, the mitochondria. MtDNA consists predominantly of coding DNA, with the exception of a ∼1100- bp long fragment that has mainly regulatory functions and is therefore termed the control region.

Mitochondrial DNA (mtDNA), an extra-nuclear genome, has certain features that make it desirable for forensics, namely, high copy number, lack of recombination, and matrilineal inheritance. mtDNA typing has become routine in forensic biology and is used to analyze old bones, teeth, hair shafts, and other biological samples where nuclear DNA content is low.

Properties of Human Mitochondrial DNA:

High Copy Number:

MtDNA is present in high copy number in human cells. The average somatic cell has just two copies of any given nuclear gene or DNA segment, but hundreds to thousands of copies of mtDNA

Maternal Inheritance: The uniparental mode of inheritance is one of the great advantages of mtDNA, as it enables researchers to trace related lineages back through time, highlighting the

maternal ancestry of a population, without the confounding effects of bi-parental inheritance and recombination inherent in nuclear DNA.

Mutation Rate: The mutation rate of mtDNA is several orders of magnitude higher than that of nuclear genes, with an estimated rate of 0.017 × 10−6 substitutions per site per year for the whole genome excluding the control region

The strengths of mitochondrial DNA (mtDNA):

1) Successful recovery from highly degraded sources where nuclear DNA typing may fail.

2) Application to cases where the only reference samples available are from matrilineal relatives.

3) Application to samples where nuclear DNA is virtually absent.

Forensic Significance of mtDNA:

mtDNA enables identification of otherwise unidentifiable victims, if maternal relatives are alive for comparison. This is becoming increasingly relevant for victims of war or terror.

mtDNA is increasingly used in so-called personalized genetic histories. This is the use of genetic testing to investigate individual genealogies, including tracing the origins of immigrant/slave ancestors.

mtDNA typing analytical methods have been used worldwide to resolve identity issues in violent crimes, lesser crimes, acts of terrorism, missing persons cases, and mass disasters.

Because of the high copy number of mtDNA molecules in a cell (16), typing mtDNA is particularly advantageous, compared with nuclear DNA, for certain kinds of forensic analyses.

In cases where the amount of extracted DNA is very small or degraded, it is more likely that a DNA typing result can be obtained by typing mtDNA than by typing polymorphic markers found in nuclear DNA.