Mitochondria float around in the goo of your cells, tirelessly making the molecules that power your body. But these mitochondria used to be independent of your body; they were bacteria, floating free in the world. You are, at a fundamental level, the result of symbiosis — the interdependence of two life forms.
Symbiotic relationship between clownfish and coral by Ethan Daniels via Shutterstock
Of Mums and Mitochondria
Mitochondria make your body's energy. Floating in the cell's cytoplasm, inside the cell wall but outside the nucleus, they toil away, making adenosine triphosphate, or ATP. It's ATP that cells use as an energy source, deriving energy from breaking it down to its components. You need a continuous supply, and mitochondria provide it.
These organelles are so vital that they predate conception. Mitochondria swim around in unfertilized ova, waiting for sperm to show up. Mitochondria have their own DNA, known as mitochondrial DNA or mtDNA, and can reproduce themselves completely independently of any kind of deoxyribonucleic shenanigans going on inside the cell nucleus. Because mitochondria have their own DNA and don't need to be fertilised, when it comes to your mitochondria, you are your mother's clone. You have exactly her mtDNA, and her mother's mtDNA, and her mother's mother's mtDNA. This is why mtDNA is of such interest to genetic researchers. It does sometimes mutate, but it doesn't get juggled around as much as DNA in the cell nucleus.
The fact that mitochondria have entirely separate DNA from the nucleus of the cell is odd, but pales in comparison to the shape of the DNA. Mitochondrial DNA is circular - a helix twisted around into a ring. Mitochondria get even stranger when we look at the barrier between them and the rest of the cell. Prokaryotic cells are just a cell wall around some free floating junk. Eukaryotes - a group that includes pretty much everything except bacteria - have cells in which DNA is contained inside a nucleus, not floating around free in the cell cytoplasm. Most eukaryotes have a lot of different organelles, each one of which has a membrane that keeps its insides packaged up. Mitochondria are slightly different from other organelles. They have a double wall, an inner one and an outer one.
The circular DNA, the double cell wall, and the fact that mtDNA is different from the DNA inside the nucleus, makes mitochondria look less like a part of human cells, and more like some bacteria put a scuba suit on over its own membrane and plunged into our cells. Scientists think that this may be exactly what happened.
The Bacteria Who Came To Dinner
The current theory on how we acquired our mitochondria is the "endosymbiotic theory." A prokaryotic cell grows, and as it grows encounters a problem. As long as it stays roughly spherical, its volume will grow much faster than its surface area. There is a solution to this problem. Its outer membrane wrinkles deeply, giving it more surface area. These wrinkles get deeper, and can even pinch off, like bubbles flowing from the membrane on a bubble wand.
One day a hungry, or merely parasitic, bacteria approaches. It lodges in one of the wrinkles and the wrinkle pinches off behind it, trapping it in a little "bubble" inside the cell. What happens now? The cell might consume it, or just break it down. The bacteria, on the other hand, might consume the cell. Neither of those things occur. Instead, the bacteria supplies something the cell desperately needs - energy. The cell, meanwhile, provides the bacteria with a safe home in which it can continue reproducing, as long as it doesn't reproduce too much. Over time, these two creatures become one, each with its own DNA, one still wearing the "bubble" it got when it came in from the cold.
It's interesting to consider what this relationship is. Is the mitochondria a vanquished foe, forced for eternity to make power for a hungry eukaryotic cell? Or are two cells amicably working together, each providing something that the other one needs? You decide.