Mitochondrial DNA- Your Other Genome
Who do you share more DNA with, Mom or Dad? You might think that since you’re a spitting image of your father, it must be him. Or maybe since you act just like your mom, you assume it’s her. Well, when it comes to the DNA in your chromosomes, it’s pretty close to a 50/50 split, but did you know that you have a whole other genome apart from your chromosomes? It’s called mitochondrial DNA, and it all came from your mama.
Most of your DNA is inside the nucleus of your cells. We sometimes refer to it as nuclear DNA. That’s where the X and the Y and all the other chromosomes live. A cell is a pretty complicated thing, almost like a body unto itself. It has different components called organelles (literally “little organs”) that have different functions. One of those organelles is called “mitochondria.” The mitochondria are like the cell’s power plant. They take sugar and turn it into a form of energy the cell can easily use (like a power plant converting energy from coal into electricity). Mitochondria are weird because they have their very own genome. Unlike nuclear DNA, the mitochondrial DNA is in a single ring, just like in bacteria. In fact, scientists think that the first mitochondria were actually bacteria living symbiotically inside a cell. The mitochondria’s ring of DNA is 16,500 letters long, and it holds 37 genes in all.
Mitochondria are found in all cells of the body, except the red blood cells (they don’t have a nucleus either. Weirdos.). Powering a cell is pretty important work, so if there are problems with the mitochondria, you might end up with a wide range of some very serious symptoms. Muscle weakness, fatigue, loss of motor control, developmental delays, and even seizures can be caused by dysfunctional mitochondria. There are several syndromes caused by mutations in mitochondrial DNA including Kearns Sayre syndrome (vision loss, coordination problems, among other symptoms), and Leigh syndrome (severe, progressive loss of muscle function, which is usually fatal in childhood), and Cyclic Vomiting syndrome (which is exactly what it sounds like).
There aren’t cures for these conditions yet, but there are therapies and lifestyle changes that may help. Some people find their symptoms improve with certain diets or supplements designed to support mitochondrial health. They can also try to avoid things that put extra energy demands on their bodies, like fasting (even 8 hours of fasting while sleeping is too long for some people), or getting too hot or cold.
Different cells have different numbers of mitochondria. It can range from practically none, to thousands. It turns out that sperm have mitochondria, but they’re all packed into the tail where they’re needed. Those don’t make it into the egg, so all of the mitochondria a baby gets is whatever was present in Mom’s egg already (though new evidence suggests there may be rare exceptions to this rule).
On its surface, the way mitochondrial conditions get passed down through families sounds like it must be pretty straight forward. If mom has it, she’ll give it to all of her kids. If Dad has it, he’ll give it to none of them. But as in most things genetic, it’s almost never that easy. First of all, some of the genes responsible for having healthy mitochondria are actually in the nuclear DNA, so some mitochondrial conditions are actually due to non-mitochondrial genes.
Second, remember I said each cell can have many, sometimes thousands of mitochondria. Well, there’s no guarantee each one of those mitochondria are going to be identical. Sometimes there’s a mix of those with the mutation and without the mutation. We call this “heteroplasmy” and it makes trying to figure out what’s going on in a family tough. Each time a cell divides, the mitochondria are randomly split between the two new cells. That includes egg cells. If 50% of mom’s mitochondria have a mutation, there’s no way of knowing if her egg ended up with the same proportions, or a higher or lower ratio. This can lead to one person in a family having a full-blown genetic syndrome, while their sister might suffer from chronic fatigue, and their brother has no noticeable symptoms at all.
For moms who want to avoid passing down mitochondrial conditions to their children, but still want to have biological children, they may have the option of using donor mitochondria. This is done by a process called cytoplasmic transfer. To do this, they take the nucleus out of a donor egg and replace it with the nucleus from Mom’s egg. In these cases, the resulting baby has DNA from three people: Mom, Dad and a donor. This has already been done in a handful of people. Time will tell if I becomes a readily available option in the future.