July112012

Mitochondrial DNA in Cold Cases

Mitochondrial DNA is found in non nucleated cells, like teeth, hair and bones. It is circular DNA, composed of  base pairs. There are only two copies of DNA in nucleated cells, whereas there are hundreds of  mtDNA, proving to be a more substantial DNA source than those currently used to find regular DNA. A major benefit of mtDNA is that it degrades very slowly. One major disadvantage to mitochondrial DNA is that all maternal relatives have the same mtDNA profile, meaning that a profile cannot match one individual. However, this is not as big of a problem as it seems, since mtDNA profiles still eliminate over 99% of the population (Melton, 2006) in most cases, and is a good method of ruling out suspects. It is also useful when suspects have been long dead (common in old cases), allowing certain family members to provide samples for mtDNA profiling.

DNA profiles are mostly obtained by electrophoresis. There are two main types of electrophoresis, gel and capillary. In both, the DNA is cut in to lots of fragments using a restriction enzyme. Gel electrophoresis involves the injection of the sample in to a well on a gel plate, submerged in a buffer solution. A current is run through the gel, moving the DNA with it through a series of mesh-like layers.

A band appears when lots of fragments of DNA can not fit through the ‘mesh’, while smaller fragments keep moving. The gel plates are treated to make the bands visible under normal light. This is what is then used to make a comparison between suspect profiles and evidence profiles.

Capillary electrophoresis is similar to gel, but fragments are separated in a capillary, not a gel plate, and separated based on charge, rather than size. Capillary electrophoresis is faster than gel, however only one sample, plus a ‘standard’ DNA ladder can be run per capillary, compared to the several that gel can handle.

PCR is used to make millions of replicates of DNA,without affecting the original copy. In some cases only a few skin cells are needed to start the  reaction. Cold case DNA usually requires special treatment, low copy number (LCN). This method detects the same ten regions of DNA, plus a gender marker, that SGM+ does, however the PCR stage uses 34 cycles, to make sure that there is enough DNA to work with efficiently.

Each PCR cycle  doubles the amount of DNA. Due to the high risk of contamination when performing LCN, duplicate tests are carried out whenever possible, to ensure that no contamination has occurred. Only if two tests return the same DNA profile can it be said that sample was not contaminated. In 2007 a review, and the suspension, of the use of LCN was carried out by recommendation of a judge. LCN use continued in 2008, after the review found that it was still a very reliable method of obtaining a profile.


References

Melton, T. (2006) Mitochondrial DNA Examination of Cold Case Crime Scene Hairs. Forensic Magazine. [Online]. Available at: http://www.forensicmag.com/article/mitochondrial-dna-examination-cold-case-crime-scene-hairs?page=0,0 (Accessed: 29th October 2011).

Forensic Science Service (2005) DNA Low Copy Number. Available at: http://www.forensic.gov.uk/pdf/company/foi/publication-scheme/communications/DNA_Low_Copy_Number_000.pdf (Accessed: 29th October 2011).

Image 1 is taken from the Wikipedia page for mitochondrial DNA.

Image 2 is taken from http://zacksprojects.blogspot.co.uk/2009_08_01_archive.html


 

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