Mitochondria are Weird
Let’s get one thing straight: I like my mitochondria. They are a necessity for eukaryotic life (although there have been some theories about alternate high yield ATP production pathways or schemata of power management). But for all that…dang are they weird.
The situation is basically a symbiosis event. At some point the ancestor of eukaryotic cells engulfed a prokaryote (most likely Rickettsiales). However, instead of being broken down into constituent molecules, this prokaryote persisted, reproduced, and evolved within the cell. One of the most fascinating things about this arrangement is that, as time went by, the mitochondria actually began to outsource their genome to their host. This is debatably the point at which they completely lost their independence and became organelles as opposed to organisms.
The human mitochondrial genome is 16,568 bases and contains no introns. It codes for about 37 genes, a fraction of what the organelle actually requires. The rest of the proteins are made from your nuclear DNA and imported through an annoyingly complex transport system. As we look across species, we can see that the mitochondria in different organisms have retained more or less of their original genome (e.g. plasmodium have 5 genes in their mtDNA and reclinomonas have 98 genes), but no organism retains completely independent mitochondria. And there are a lot of extra tricks that the genome picks up across species: sometimes linear, sometimes circular, sometimes with introns, sometimes not, multiple copies of the genome per mitochondria, etc. As an added bonus you can wind up with multiple mitochondria per cell with multiple different genomes in that mitochondrial population.
The practical side of this is that inheritance of mitochondrial disorders is emphatically not Mendelian, and tracking such disorders can be all sorts of a headache. A headache we are going to have to work through if we want to effectively study and prevent these diseases.
Sources & Further Reading
- I highly encourage anyone with a background in genetics and some time to kill to go take a look at MitoMap. It’s the human mitochondrial genome database and includes some great references.
- One of my commenters has recommended Nic Lane’s Power, Sex, Suicide: Mitochondria and the Meaning of Life, although I personally can’t speak as to whether it is good or not. Am looking for a copy but even if I find one it will have to wait its turn in a long line of reading material.
- And because spec-fic is fun I find myself compelled to recommend Parasite Eve both the novel by Hideaki Sena and the PS1 game put out by Square. Nothing approaching scientific accuracy, just plenty of “What if mitochondria were an evil hive mind?” lovelyness and body horror. There is also a movie, but I hear that it wasn’t that great.