When we think about DNA, we automatically think of the DNA housed in the nucleus of our cells. This is the DNA inherited from our parents, half from mom and the other half from dad. The DNA in the nucleus codes for most of the proteins that bring about our traits, health, and development. As you noticed, most was italicized because DNA is actually found in two places within a cell: the nucleus and the mitochondrion.
Mitochondrion, or more commonly in plural form as mitochondria, contains DNA. The DNA within the mitochondrion differs from DNA in the nucleus by shape, size, location, and origin (see table below). The origin of all mitochondria DNA (mtDNA) comes from mothers, which is how scientist trace maternal ancestry back thousands of years. mtDNA is passed to all offspring of a mother, therefore males and females can have their maternal ancestry traced.
|
|
Shape |
Size in Base Pairs | Location |
Origin |
|
DNA |
Linear | 3 million | Nucleus |
Each Parent |
| mtDNA |
Circular |
16,500 |
Mitochondria |
Mother |
Why does only the mother’s mtDNA get passed on?
Both reproductive cells, sperm and egg, contain mitochondria. Mitochondria is needed in reproductive cells, as well as the cells in our body, to break down food into energy. This is where 90% of the cell’s energy comes from. As the egg is leisurely waiting for a sperm, the sperm is high-tailing itself to the egg, in order to beat out the other sperms. Boom! When the two collide, the sperm’s mitochondria self-destruct and a vesicle (aka garbage man) comes to remove it. This prevents additional DNA from entering the embryo for development.
If mtDNA is maternally passed on, do siblings have the same mtDNA?
Brothers and sisters all carry the same mtDNA. And not only that! But if your mother has a sister, you and your cousins would have the same mtDNA. Or, the mtDNA of your great-great-great grandmother and her sister’s children would all have the same mtDNA as you. mtDNA transfer stops at males. Therefore, all the women and offspring on your maternal side (who did not marry into the family) would have the same mtDNA.
How is Maternal Ancestry traced through mtDNA?
All DNA, whether it be in the nucleus or mitochondria, goes through mutations. When a mutation occurs in the DNA or mtDNA of a reproductive cell, the mutation will be passed on to the offspring and all the cells in the offspring will have that mutation.
This mutation will pass on through generations, which allows scientist to use these mutations to follow migration patterns of women and approximate when these mutations occurred. These mutations take years to establish. Currently, scientists can trace these mutations starting over 75,000 years ago to the last thousand years.
To help organize these mutations, scientists established haplogroups. Haplogroups are formed by looking at a large population of individuals and analyzing their mtDNA. For example, if 50,000 people have a mutation on marker X and another 20,000 people has a mutation on marker X and marker M, then mutation on marker X most likely happened before marker M. Mutations of marker X would become its own haplogroup and mutations on marker X and M would become another haplogroup. This builds into migration patterns because if people with marker X mutation predominately live in central Europe and people with marker X and M mutations live in northeast Europe, then it is most likely that individuals from Central Europe migrated to northeast Europe thousands of years ago.
Mutations in mtDNA are happening today, but they will not become haplogroups for another hundred to thousands of years down the road, when enough people have today’s new mutation.
