mitochondrial DNA

Jeanie

Ahnimus wrote:

I'm pretty sure they replace the nuclear DNA with your own. That was my understanding of how stem cells are created. They incubate an ovary with your DNA instead of the original. The differentiation of stem cells occurs depending on the types of neighbouring cells. Kind of like the way any gene is expressed. The gene doesn't contain an instruction set, but rather it's more complicated. I know that the 'master gene' for a mouse eye can be transmutated into a drosophila (fruit fly) and the drosophila will grow a drosophila eye, instead of a mouse eye, and vise-verca. So the expression of the genes depends on things besides the gene it's self.

But I'm curios to know the answer to your question about mitochondrial DNA as well. And Scott can correct me on the rest if I'm wrong.

Thanks Ryan. Can't stop now, but I'll be back. I have more questions.

Scubascott

All this would be a lot easier with pictures. I'm sure there must be some good websites out there that explains all this in layman's terms, but I'll have a crack anyway.

So - Following on from what I was saying before, mitochondria are like little bacterial cells that float around inside our cells (which are much larger than a bacterial cell). They are separate from the nucleus and contain their own DNA. In fact, they are so much like bacterial cells that the generally accepted theory on how they came to exist in eukaryotic cells is that back in the days of the primordial slime a large single celled critter probably something like an amoeba tried to eat some smaller bacterial cells by engulfing them. This process is called endocytosis. Rather than being destroyed and absorbed however, the bacterial cells survived inside the larger cell. Somewhere along the line a deal was struck where the bacteria were allowed to live in the favourable environment of the larger cell, in return for chemical energy, which they produced in excess to their own requirements. What we now have is the situation where the symbiotic relationship those ancient bacteria and the larger predatory cell has progressed so far that neither can survive without the other, and they are now infact just different parts of the same organism. Cool hey? Plants made an even better deal somewhere along the line. Not only do they have mitochondria cranking out energy, they also have another type of organelle descended from a photosynthetic bacteria. These are called chloroplasts, and they also have their own DNA, just like mitochondria.

So. . . stem cells. This is a complicated topic, and I need to get back to the lab, but I'll try to explain it just briefly. Ahnimus is kind of correct, but its more complex than that.

Basically the story is that every cell in your body has all the same information stored in the DNA in its nucleus (except red blood cells, but we won't go there). This information is the same regardless of whether its a muscle cell, liver cell, nerve cell, skin cell etc, and it has all the instructions required to make any type of cell in your body. But, even though all those cells contain the same information, its not possible for say, a skin cell to divide and produce a liver cell. Only special types of cells can divide and develop into other cell types. These are called stem cells.

There are several kinds of stem cells, but the ones of most interest to researchers are embryonic stem cells. When an egg is fertilised by a sperm, the fertilised egg (called a zygote) begins to divide. So one cell becomes two, which becomes four, which beomes eight, 16, 32, 64 etc etc. For the first few days after fertilisation, these cells have not yet taken on any particular function. There are no skin cells, no hair follicle cells, no pancreatic cells, just these undifferentiated stem cells. So, if you take the embryo during these first few days (I think between 3 and 5 days is the ideal time), you can extract the stem cells, and start growing them in culture. You can then (in theory) manipulate them to differentiate into whatever kind of cell you like. We're not quite there yet, but the hope is that one day we'll be able to grow new livers, hearts, lungs or whatever outside the body by directing the development of stem cell cultures.

Now, you can get stem cells from a number of sources. There are embryonic stem cells, like I described above, which can only be isolated from an embryo in the very early stages of development. There are also other types of stem cells that can be isolated from placenta and umbilical cord blood, and others that can be isolated from tissue of adults. The difference with these other types compared to embryonic stem cells is that they are limited in the number of other cell types that they can differentiate into.

So, if you want embryonic stem cells, you need an embryo. One way to get one is to fertilise a human egg with a human sperm in a test tube. This is exactly what happens in IVF. Most of the research done so far has used excess embryos produced for IVF. These embryos would have been destroyed anyway, and are donated with the consent of the couple. Another way to get an embryo is to produce one by cloning. This involves taking an unfertilised egg, sucking the nucleus out, and replacing it with the nucleus of another cell from somewhere else. This effectively replaces the 'identity' of that egg cell with that of the cell that nucleus is taken from. The egg now has a full set of two copies of DNA, and can then be tricked into developing. The neat thing about this is that if say, Jeanie, needed a new liver because she drank too much bundy rum, you could take one of her own ova (egg cells), rip out the nucleus, replace it with the nucleus of another cell from her armpit or somewhere, and hey presto! You've got cloned Jeanie stem cells, genetically identical to her. All you have to do then is tell those cells to differentiate into liver cells, grow up a new liver, and transplant it back into her. Because the new liver is genetically identical to her old one, there will be no issues with rejection.

Scubascott

Wow, that was a long post, and there's still a lot to explain that I didn't get to. I have to go again, but I'll try get back later. Note that stem cell research is not my field (I'm a microbial ecologist). But I have just marked a stack of third year student assignments on this very topic, so I can try to explain the basics.

Mookie Baylock

THC wrote:

the stuff about tracing ancestry is the really cool part. i saw a bit on this on 20/20 the other day. Its interesting though...one of their points was that if you go back say 8 generations...of your own DNA...you'd have something like 24,000 grandparents or something crazy. Which means...our genes..are very much a combination of many...many gene pools...from all over the globe.

You should read Richard Dawkins 'Anscestor's Tale'. It traces back and attempts to estimate what common anscestors of us and other animal families would be like, and used mDNA and fossil evidence to attempt to date when the anscestor would have existed, while giving lots of insight on how evolution has worked. It's qute interesting, but a bit long and heavy at times. I'm 2/3s of the way done!

My favorite fact so far: Whale's and Dolphin's closest living relitive (still on land): Hippos!

Jeanie

Scubascott wrote:

Wow, that was a long post, and there's still a lot to explain that I didn't get to. I have to go again, but I'll try get back later. Note that stem cell research is not my field (I'm a microbial ecologist). But I have just marked a stack of third year student assignments on this very topic, so I can try to explain the basics.

Thanks scott. I've got a bit of brain strain going on this week, but when I get my shit together I do have some questions if you don't mind?
I think this is very much how I understood it from the stem cell scientist guy whose public lecture I went to but I'm too fuzzy atm to hunt out the details or formulate questions.