Mitochondrial DNA
Intro
So, you’re watching Forensic Files and the narrator mentions “mitochondrial DNA”, a word that frequently makes an appearance in forensic shows but is often not explained, or at least not FULLY explained. Before researching this my only knowledge of mitochondrial DNA was that it is inherited from the mother, I presumed that this meant there would be certain limitations to its use but I couldn’t tell you what they were. This week I decided to find out.
What is this thing?
Wikipedia says: “Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, adenosine triphosphate (ATP).” If that single sentence description made your head want to explode, you are not alone. If you, like me, did not take high school level biology then that probably told you nothing, so here is a picture to help explain this a little easier.
Why is it maternal DNA?
It is often referred to as maternal DNA because it is inherited from the mother only, (unlike nuclear DNA which comes from both parents) there are a couple of reasons for this. Firstly, eggs contain a whole lot more mtDNA than sperm do so there is a significant element of dilution already, secondly, the mtDNA in sperm is located at the base of the tail – just below the head – which can break off and may not necessarily make it into the egg. Finally, the paternal mtDNA that DOES make it into the egg cell only survives for a few days before it begins to break down. Nature has pretty much ensured that mitochondria is only inherited from the mother.
So how does this help forensics?
The good thing about mitochondrial DNA is that it is often still found when only small samples or degraded samples are available, mtDNA can be found in teeth, older bone samples and hair shafts. This is because there are SO MANY copies of it. When you look at the cell you have the single nucleus which contains only two copies of nuclear DNA, but surrounding that nucleus are multiple mitochondria floating around, each containing many copies of mtDNA, meaning in a single cell there will be hundreds of mtDNA copies. In cases of old, unidentified remains, mtDNA may be extracted and used to identify maternal relatives.
But the downside is…
Unlike nuclear DNA which is unique to each individual, mtDNA is shared with all of a woman’s children (both male and female) plus her own siblings, mother and any other maternal relatives. Therefore it cannot be used exclusively to identify an individual, although it can be used to rule out an individual. When mtDNA is used as evidence in court it will form part of the argument but will never be used exclusively. Nuclear DNA is more favourable as evidence as no two people are known to share the same DNA profile. The Illustration below shows the sharing of mtDNA. All those in pink share mtDNA profiles.
Researching this certainly helped me understand and appreciate the science behind mtDNA more, I hope this can be useful for you too.