In class today we were studying cloning and the ethical issues associated with taking an egg from a mammal, for example a rabbit, and putting the nucleus from a human cell onto that egg cell in replacement of the rabbit nucleus. One of the issues raised was whether the new cell is a human cell because the nucleus contains human DNA or a combination of a human cell and a rabbit cell because the mitochondrial DNA is from the rabbit.
However, I don't understand how the new cell could be considered to be partly a rabbit cell because I thought that mitochondrial DNA was prokaryotic and not specifically the "rabbit DNA". Wouldn't human mitochondrial DNA be the same as the rabbit mitochondrial DNA in that case and if not why not?

Hi Elie,
Our mitochondira are indeed descended from ancient prokaryote ancestors that formed a symbiotic relationship with our eukaryote ancestors and they have the ability to replicate independently inside our cells. So in a sense we are all hybrids of a sort. However, modern eukaryotic cells and their mitochondria are so dependent on one another that one simply could not survive without the other. So it makes more sense to consider them part of the same organism. In other words, your mitochondrial DNA is just as much a part of you as your nuclear DNA. These categories do get tricky though when you mix DNA from different species in situations such as the nuclear transfer procedure you described. Rabbit mitochondrial DNA is different from human DNA (although they must be similar enough that a rabbit mitochondrion can survive and funciton in a cell with a human nucleus and vice-versa). Wo it would not be unreasonable to say that such a hybrid cell is not completely human.