Question about Mitochondrial DNA

Im watching this incredibly frustrating and poorly done documentary on the theory that we are all descended from one single black female that lived 150k years ago. If you know enough to answer this question then you probably know the theory. What first vexed me was that they used the probable mitochondrial dna to reconstruct the face of what the woman might have looked like. But they also say that mitochondrial dna is passed down rote from mother to daughter with no changes (sans mutation) . That would mean that daughter, granddaughter, etc would probably have the same mitochondrial dna. If that’s the case, how can this dna tell what someone might look like if it could conceivably be passed unaltered from one generation to the next for a while? Mothers and Daughters can certainly look very different. My guess is they grossly overexaggerated the accuracy of the likeness, even though I well know it was supposed to be a gross likeness.

There is absolutely no way that knowledge of mitochondrial DNA would allow one to reconstruct what a face looked like - that information simply isn’t there. Even complete knowledge of a nuclear genome wouldn’t allow reconstruction of a face - we aren’t anywhere near knowing how genes code for that.

In any case it isn’t that mitochondrial DNA is passed down without mutation. It’s that the (slow) mutation rate in mitochondrial DNA allows us to calculate that these mitochondrial lineages began diverging about 150,000 years ago.

Colibri- not sure what you meant on one aspect. I think we might be agreeing. My understanding is that mitochondrial dna does’nt mix (meiosis whatever) with the males and therefore the ONLY way it could change woulld me mutation.

ah nm, i see, you thought i meant “sans mutation” meant “without mutation” rather than “excluding”

Yes, that’s what I assumed you meant.

Mitochondrial DNA is passed down from the mother - to both daughters and sons - in the cytoplasm of the egg cell and doesn’t undergo recombination. Males have it, they just don’t pass it on to their offspring since it isn’t transmitted with the sperm.

Well, perhaps they do.

So is everyone’s mitochaundrial DNA pretty much the same then?

I haven’t seen the show you’re talking about, but I am a bit familiar with the Eve theory. The key points and gist of it per my understanding:

  1. Mitochondria are organelles, which are inside of most cells and synthesize ATP, which is what powers almost all biological processes. It is thought that back in the primordial soup days, mitochondria were free-living organisms, like bacteria. In fact, there is a striking resemblance between these two types of teeny tinies. Sometime early in the game, a symbiotic relationship developed between mitochondria and another early life-like form, and this partnership then went on to become the progenitors of eukaryotic cells as we know them. Thus became us.

  2. Included in the aforementioned resemblance between mitochondria and bacteria is the fact that both contain DNA. That means in eukaryotic cells, such as us, there are two collections of DNA, one in the cell nucleus (nuclear DNA) and one in the mitochondria. The latter, however, is a relatively paltry collection containing only a few genes which, I believe, contribute only to the functioning of the mitochondria and not to, say, the structure of external features, such as the cut of one’s jib.

  3. Whole mitochondria, not just their DNA, is transferred between mother and offspring, not just daughter. This is because the mitochondria live inside the ova (egg cells), and are incorporated into the embryo, etc… There are also mitochondria in the squirmy swimmers, but they are usually not allowed in the house.

  4. While this mitochondrial DNA might be passed from mother to offspring unaltered*, it does undergo mutation. Furthermore, the rate of this mutation is fairly constant. Thus, by comparing the mitochondrial DNA of two persons, you can get a good idea of how long it’s been since they shared a common pool of mitochondria—in other words, when they had their last common female progenitor. By applying this line of reasoning to the human race at large, you can, in theory, trace everyone’s lineage back to a single mother (you know what I mean!) who seemed to live in the suprizingly not so distant past. (*This is in contrast to how nuclear DNA is passed, as there are mechanisms in sexual reproduction that recombine maternally and paternally derived DNA. Thus the nuclear DNA of offspring is necessarily different than that of either parent, disregarding rare exceptions which don’t concern us here.)

  5. While mitochondrial DNA does not code for any externally recognizable features, it can be correlated with nuclear DNA more or less by association. So, as the mitochondrial DNA is followed back, the association with nuclear DNA can be appealed to to see who currently on the planet has nuclear DNA which is associated with the most common form of mitochondrial DNA. This narrows things down to a group of people rather than an individual.

  6. All this is to say, I don’t know where the show got off making a face from mitochondrial DNA, if that’s what they did. The genes that contribute to the mug are located nowhere near the mitochondrial DNA and are passed down in entirely different, albeit associable, ways. What they probably did was construct a face which resembled that of someone from the group mentioned above. If that’s what they did, they wouldn’t’ve needed to synthesize some hypothetical face- they could have just used the actual appearance of someone from that group.

I wouldn’t worry too much about the face part of it. (I for one am starting reach the regrettable conclusion that I might can’t believe everything I see on TV.) It’s my understanding there’s some fairly big holes in that whole theory anyway. I read an article about that a while ago, but have been unable to find it again. If anyone has a good reference I’d be grateful.

I finished watching this show. It was on The learning channel. Worst documentary on a fascinating subject ive ever seen. One of the things I thought was strange is they said they knew the rate of mutation for mitochondrial dna to be "once every 20,000 years and that this African “Eve” from whom the entire world population presumbably descends, lived only about 150,000 years ago. Now dumb math tells me that’s 7 mutations though they would increase at the fibonacci ratio or something liike that. Doesn’t seem like enough to get the variability in the M-Dna that they were talking about that exists today.

The “mitochondrial Eve” theory isn’t accepted by the experts. See:

SCI. NEWS: DNA’s Evolutionary Dilemma

Oh, and the mystery of the separate DNA? That was another instance of “ridiculed vindicated science” similar to Wegener’s plate tektonics. Dr. Lynn Margulis proposed that chloroplasts and mitochondria are actually symbiotes living within cells, and that’s why they have their own DNA. It was blasphemy at the time, since everyone “knew” that evolution requires competition, never cooperation. Today it’s textbook science. Yet Margulis’ support for another theory of bio-cooperation, the Gaia Hypothesis, is still widely ridiculed.