Mitochondrial DNA uses a different genetic code than nuclear DNA (Scheffler 1999). For example, the codon for mitochondrial-transcribed amino acid tryp-tophan is UGA while the universal (nuclear) genetic code for UGA is a stop codon. In the mtDNA genetic code, AUA codes for methionine instead of isoleucine and AGA and AGG both code for stops rather than arginine.
Fewer DNA repair mechanisms exist in mitochondria thereby leading to higher mutation rates compared to nuclear DNA. In addition, lack of proofreading capabilities in the mtDNA polymerase increases mutations during replication. However, the 10-fold higher mutation rate (relative to nuclear DNA) helps introduce more variability in samples from identical maternal lineages that otherwise would not vary. This increased variability is a good thing for most applications in human identity testing although mutations can sometimes be a hindrance when trying to definitely establish familial relationships (e.g., when comparing remains to reference samples from distant maternal relatives).
The circular nature of mtDNA makes it less susceptible to exonucleases that break down DNA molecules needed to survive until forensic DNA testing can be completed. The presence of an increased number of mtDNA molecules per cell relative to the nuclear DNA chromosomes also enhances the mtDNA survival rate, as does the fact that they are encapsulated in a two-walled organelle.
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