Anyone who knows me knows that mitochondria are pretty much my life. My Ph.D. focuses on making these little cellular power plants more efficient at producing energy so that they can help muscles heal faster after traumatic injury. More than that, though, I want to teach people that mitochondria are more than just “the powerhouse of the cell.” Mitochondria harbor their own evolutionary secrets and memory, all neatly packed away. The memory I am talking about is mitochondrial DNA.
Whenever we think of DNA, our mind tends to instantly conjure up that double-stranded, twisted ladder that seems obligatory to every scientific illustration. That is nuclear DNA, a set of instructions on how to build cells that we inherit from our parents; half comes from our mother and half from our father. This type of DNA is linear, containing a beginning and end, and is folded tightly together in structures called chromosomes. Mitochondrial DNA (mtDNA), on the other hand, is circular. DNA inside of bacteria has a similar structure, leading to the creation of the theory that mitochondria originally were bacteria that entered into a symbiotic relationship with another cell and maintained a separate set of DNA instructions. Over time, mtDNA has evolved to become much smaller than the nuclear genome, accounting for a paltry 1% of all genetic material in the cell. While nuclear DNA comes from a combination of maternal and paternal genes, all the information contained in mtDNA is passed down through a single parent–the mother.
To understand how this single parent inheritance happens, we have to look back to when you were young. Like, barely a single cell, young. Human oocytes, or female egg cells, can contain more than 100,000 mitochondria, while an individual sperm cell contains around 100. When the sperm enters the egg during fertilization and releases all the male nuclear genetic material, it also releases some of its mitochondria along with it. However, the fate of the mitochondria that enter the egg is not well understood.
Some theories state that sperm mitochondria break down shortly after entering the egg or are flagged as foreign material and removed. Another theory proposes that because there are so many more maternal mitochondria present, the lack of paternal mtDNA in offspring is simply a matter of dilution. That is not to say that children can’t inherit mitochondria from their fathers, it’s just unlikely. Heteroplasmy describes having mitochondria with different genomes, whether through biparental inheritance or mutation. Though research on biparental heteroplasmy is still in its infancy, there seems to be no negative impact on having two different, healthy mitochondrial genomes.
Mutations or errors in mtDNA can be deadly. Only 37 genes are encoded in mtDNA, but the secrets they contain cannot be found anywhere else within the cell. Within these small loops of DNA resides your body’s recipe for making and maintaining mitochondria’s internal architecture. It also contains the blueprints for the molecular machinery that breaks down fuel to create the molecule ATP. Mutations in these genes cause a severe lack of ATP, depriving the body of energy. These mutations are the main culprit of diseases such as Leigh and Pearson syndrome, and have been implicated in others like diabetes, Alzheimer’s, and liver failure. Before anyone panics, it is important to note that these diseases are, in many cases, inherited and only present themselves if an overwhelming majority of the mitochondria are damaged. The likelihood of spontaneously developing one of these diseases is low, as your cells routinely quarantine and dispose of poorly functioning mitochondria.
The story of Us
Mitochondria can also tell the story of our ancestors. Because mtDNA is passed down from mother to child, very few genetic changes occur each generation. Companies like 23andme and FamilyTreeDNA are able to extract mtDNA from saliva samples and use it to trace your ancestry back several generations and narrowed down to a single continent or region. In fact, mtDNA analysis technology has advanced in such a way that scientists have been able to identify our oldest common female ancestor, dubbed Mitochondrial Eve, who lived some 200,000 years ago in Africa.
If there is one thing you take away from this article, let it be an appreciation for how important mitochondria are. They are the ink on a page of living history and the sole guardians of instructions that we need to live. Though mitochondria only make up a small part of our body, they have a large impact on how it functions. Passed from mother to child, their memory spans generations and can pinpoint the birthplace of your family. To put it simply, you wouldn’t be the person you are today without your mitochondria.