Obviously, all of you; but where did it come from? There’s quite a lot about you that’s interesting, and might be a surprise to you. It was to me and this is why I’ve embarked on writing this and the next few posts.
You probably already know that you get half of your DNA – the parcel of information that encodes your biology, where things go, how and when they work – from each of your parents.
Well, that’s not quite true.
You actually get just slightly more of your DNA from your mother. While most of the DNA you have lives quite happily parcelled up in a tightly wound package inside the nucleus of your cells, there is a little more sitting in ‘organelles’, compartments of a cell that have specific functions, like the organs in your body, called mitochondria. They essentially have their own, independent, genome, and are passed on from mother to child from the mother’s ovum and not the father’s sperm. For some reason, currently unknown, the sperm mitochondria are destroyed and don’t make it into the final, err, product: you.
I think that’s kinda interesting pedantic pub quiz stuff in itself, but it poses a question: why is all the rest of the DNA in the nucleus, but this organelle has its own?
Well the story gets even more interesting because the mitochondrial genome itself shares a surprising similarity to bacterial genomes, but is also quite different. So, being the attractive and astute reader that you are, you’ll no doubt be wondering: how did something with its own genome get inside my damn cells?!
Around that time there were no mitochondria and it’s thought that they arose from two different types of early cell; one called a eukaryote the other called a prokaryote. The prokaryote evolved a mechanism of efficiently producing a molecule called ATP (this is used as fuel by most of the cells processes that need energy). Around the same kind of time frame the eukaryote had evolved a flexible membrane and size, allowing them to go about munching up and digesting other bugs and things, this allowed them to steal the hard-won energy of others to supplement its own.
Credit Dr David Furness, Wellcome Images. Three mitochondria (blue) fractured to reveal the internal structure.
The surrounding cytoplasm is shown in gold.
Somehow, over a long time, a relationship began between the eukaryotes and the prokaryotes: vows were given, rings were swapped and eventually the prokaryote began an existence as an endosymbiont (living inside the eukaryote). The relationship probably worked as it may have been easier to keep a few of these ATP producing guys around rather than digesting them all the time, what with them producing all this lovely ATP and all.
The number of mitochondria in a cell can vary depending on the cell type, for example liver cells, which are hugely metabolically active have over 1,000 in each cell alone, or heart cells, which clearly need a lot of energy to keep going all the time day and night, whereas skin cells, which mostly just kinda sit there, need far fewer.
Ever heard of brown adipose, or brown fat? This is a type of fat that’s primary purpose is to produce heat – it’s one of the things that keeps us warm blooded – and they have a much higher number of mitochondria that normal fat, this is what makes it brown. Quite how they produce this heat was a bit of a mystery at first. The path to the solution started in World War I. The French were using a chemical called DNP (di-nitrophenol) in the production of explosives. The women working in the munitions factories reported having high temperatures and were losing weight; it seemed that the DNP was speeding up their metabolism in some way. The reason, it turned out, was that the DNP had interfered with the way that protons moved through the mitochondria; normally the energy from this is harnessed to produce ATP, but DNP disrupts the normal process and the protons were just slipping through the membrane and releasing their energy as heat instead. It was later discovered that mitochondria have a way of doing this deliberately; to produce heat in a controlled way using a protein called thermogenin.
There’s another interesting thing mitochondria give us too: Mitochondrial Eve. Because mitochondria are only inherited from our maternal side by studying the genetics of mitochondria between people, and through populations, it’s possible to determine if we all have a single common ancestor, and most evidence suggests we do: Mitochondrial Eve. She is one of our most recent common ancestors (another being Y-chromosome Adam…if you’re a dude, of course), living around 200,000 years ago, from whom all of us eventually descended on our maternal side.
Mitochondrial Eve also suggests something else interesting; that the descendants of the human race appear to have gone through an evolutionary ‘bottleneck’ at some point, where some disease or process reduced the population significantly, leaving relatively few survivors to pass on their DNA, and mitochondria.
We are not the only life forms to have accomplished this kind of trick, as no mention of mitochondria is complete without mentioning their plant-based equivalent, the chloroplast, the bits of plant cells responsible for capturing the energy of the sun, for which a similar evolutionary path must have been taken.
Mitochondria also play a role in the antiviral immune system, calcium signalling and apoptosis (a form of ‘programmed cell death’), among other things.
Mitochondria, in five words: They’re all kinds of awesome.