Science Sundays: Mitochondrial Eve

By: Meradeth Houston on 10:54 AM

It's that time of the week again! Today I thought I'd delve a little into my own research and talk about mitochondrial DNA and mitochondrial "Eve"--both are quite a bit of fun, if I do say so myself! :)

Okay, let's start off with the basics: what the heck is a mitochondria? Well, it's a small organelle in the cell (possibly incorporated into cells long, long ago in a symbiotic relationship, but that's a whole other story) and is often referred to as the "power house" of the cell. It makes the ATP, or energy, all cells in our bodies run on. Pretty important right? Well, it also has its own little genome--a small circular bit of DNA that it uses for purposes within the organelle. It's this little piece of DNA that I allows us to trace human history through the females of human history.

There are some really unique features about mitochodrial DNA (mtDNA) that I should point out:

• It is inherited solely through the matriline, so everyone on earth has the same mtDNA as their mom, but only daughters will pass it along to the next generation (basically because the sperm don't introduce any--or very little--mitochondria to the egg cell in fertilization).

• It mutates rapidly. Meaning that changes between mother and daughter's mtDNA happen more often than changes in nuclear DNA. This is mostly harmless (with a few exceptions), but allows for looking at differences between groups that are much more recent, as if there weren't any mutational differences, it would be really hard to tell groups/people apart based on their mtDNA.

• There are lots of copies per cell, which is really handy when working with old samples because more mtDNA copies means there's a higher chance of recovering the DNA to analyze it.

There are lots of other interesting things about mtDNA, but I'll leave it at that for now :)

So, with mtDNA, it's possible to trace ancestry. Chances are, if you've sent in a spit-sample to trace your ancestry to one of the many companies that does this, they told you what haplogroup you belong to. These haplogroups, lettered by the alphabet, are a way researchers have broken down the different lineages of mtDNA. I personally belong to haplogroup X (sounds much more mysterious than it really is :), which is found in Native American populations and at low frequency in some Europeans. I'm from the European branch, which I can tell based on the specific haplotype (or sub-haplogroup) I belong to.

How do these haplogroups, or lineages, work? Well, as I mentioned, mtDNA mutates from time to time--meaning that at one of the base-pairs (the A,T,C, or G that makes up all DNA) spontaneously changes into one of the other bases. So, think of it this way: a mother has five daughters, and she passes along her mtDNA to each of them. But due to some random chance event, in one of her daughters, the mtDNA she passes along has a small change in it, so all the daughters and granddaughters from that line now carry a small difference. This mutation that happened splits the lineage, and it's possible that the mother and daughter would be placed in different (though related) haplogroups. It kind of works like a family tree, only with DNA.

Today, when we look at lots and lots of people's different mtDNA's, and the mutations they all have different from one another, it's possible to group them into lineages, and then trace these mutations backwards through time. It's kind of like grouping everyone that has one specific mutation in common because they are all related matrilineally, and then noticing that this group only has one mutational difference from another group, so they must be closely related, and nesting these relationships further back until eventually you get to the original root of the family tree. 

In humans, we can do this with all mtDNA to finally reach a single haplogroup that is found in Africa. Sometime a long time ago (based on how fast mtDNA mutates, probably about ~200kya), when modern humans first appeared, there was a small population with the same mtDNA haplotype, and this was passed along to all modern humans that live today. This is often confused with a single person who lived back then, referred to as mitochondrial Eve, though in reality it was probably a small group of people. Still, it's pretty cool that it's possible to trace the matrilineal lineage of all modern humans to one spot and group!

The same thing can be done with the Y-chromosome, which can be traced through the patriline in much the same way, and all male Y-chromosomes go back to a single individual, also in Africa, who lived a little more recently (~150kya). 

Have you had your mtDNA or Y-chromosome DNA tested?