Re: I need to know is our mitochrondria strong enough to evolutionalize again?

Hi Jeffie,

First off, from your question I can suggest that you might want to take a look at our MadSci Evolution FAQ. This FAQ covers some of the basics of evolution, and I think you will find it generally useful for your studies.

Now, on to your question. To be brief, I would say that you don't have to worry about your mitochondria "turning on you" and "deciding" to evolve in their own self interest. That is because your health, welfare, and reproduction are the things that are in your mitochondria's self interest! Besides, as you will see from the MadSci Evolution FAQ, evolution is not a conscious or intrinsic process (not that your mitochondria are conscious, no matter what Madeleine L'Engle would have you believe :). Evolution is what happens when some organisms leave more offspring to the next generation than others. With that in mind, let's stop being brief.

Mitochondria are organelles found in the cells of almost all eukaryotes, and carry out the processes of oxidative phosphorylation and so-called aerobic respiration for our cells. In addition to carrying out these marvelous functions for us, mitochondria are also interesting in that they contain DNA, when almost all of the DNA in a eukaryotic cell is found in the nucleus. In humans, this DNA constitutes a small (16,569 bp long) circular chromosome, and includes, 22 functional tRNA genes, 13 functional protein coding genes, and 2 functional rRNA genes. These 37 genes and their products work together with the products of a large number of nuclear genes to express the NADH dehydrogenase, cytochrome oxidase, ATPase, and cytochrome b protein complexes, which are all key to the processes of oxidative phosphorylation.

According to the so-called endosymbiont theory, our mitochondria are descended from a bacterial organism that once lived free, but which was long ago incorporated into the cytoplasm of another ancient cell, probably an archaean. In addition, it seems likely that mitochondria don't exist as multiple, separate organelles in our cells, but rather as one large connected network that spans the cytoplasm.

When we compare the genomes of modern bacteria and modern mitochondria, we immediately notice that quite a lot of bacterial genes are missing from the mitochondrial genome. Remember, there are only 13 protein coding genes in the human mitochondrial genome. For example, looking at the NCBI's Entrez Genome database, I can see that the chromosome of Bacillus licheniformis is 4,222,336 bp long and encodes 4161 proteins, and that the chromosome of Bacillus cereus is 5,300,915 bp long and encodes 5134 proteins! So, right there you can see that there has been quite a lot of evolution in the history of mitochondria. In fact, we have a great answer in our archives (by Mike Klymkowsky) describing that evolution.

The gist of all of that evolution is that most of the genes that are important for mitochondrial function were moved to the nucleus, while those that were not important for mitochondrial function were lost. This means that mitochondria are no longer independent organisms; they're integral parts of the cell, as dependent on the nucleus and cytoplasm as the rest of the cell is dependent on the mitochondria. As I suggested above, the NADH dehydrogenase, cytochrome oxidase, ATPase, and cytochrome b protein complexes could not be expressed without the contribution of many nuclear genes.

Now, while mitochondrial DNA can accumulate mutations, we don't see evidence of those mutations unless they are passed on to the next generation. This is because (as I stated above) evolution occurs when some organisms contribute more offspring to the next generation than others. However, since the mitochondrion is an integral part of the cell, the only way that mitochondria make it to the next generation is as an organelle in the cytoplasm of the ovum. Therefore, only those mitochondrial mutations that allow the entire cell to function will make it to the next generation. So, that is a long winded way of saying that our mitochondria can't "turn on us." I love my mitochondria, and my mitochondria love me. :)