While we inherit equal parts of our nuclear DNA from both our parents, we are thought to inherit a disproportionate amount, if not all, of our mitochondrial DNA from our mothers—sorry, Dad!
Of all the organelles inside of a cell, perhaps the most notorious are the mitochondria. There are entire memes dedicated to poking fun at their oversimplified and underappreciated job description, that being the “powerhouse” of the cell. Common memes might say something to the effect that students don’t learn much practical knowledge applicable to everyday life from school like how to manage personal finances, buy a house or get a job, but they do learn to memorize random facts like “mitochondria are the powerhouses of the cell.”
Mitochondria are indeed powerhouses, but they are so much more than just that. Mitochondria hold clues into the origins of the first eukaryotic cell (the types of cells that we find in our bodies) and the most recent maternal ancestor of all living humans called Mitochondrial Eve.
Their DNA has also been used in forensics cases when no other DNA samples were available or intact and to identify archeological remains and to better understand ancient civilizations. Most recently, scientists sequenced the mitochondrial DNA of a victim from the historical Mount Vesuvius eruption in Pompeii. There has even been speculation in the Star Wars universe that midichlorians, the microscopic life forms responsible for the supernatural powers of the Force often wielded by Jedi and Sith Lords, are really mitochondria.
Other popular memes on mitochondria abound as well, for example, one that often circulates on or around Mother’s Day is one that shows a picture of a mitochondrion and claims that Moms give their children something that Dads cannot: mitochondrial DNA.
While DNA is the molecule of information stored inside the nucleus of our cells, mitochondria have their own DNA that can be inherited too. How’d it get there, and why is it there? Why do we have two separate compartments of DNA inside our cells? Do we really only inherit mitochondrial DNA from our Moms? Why not Dad too?
In order to answer these questions, we first need to look more closely at the anatomy and evolutionary biology of mitochondria.
Mitochondria are indeed powerhouses, but they are so much more than just that. Mitochondria hold clues into the origins of the first eukaryotic cell and the most recent maternal ancestor of all living humans called Mitochondrial Eve.
The Mitochondrial Matrix and Membrane
Mitochondria come in a variety of shapes and sizes, but typically in their textbook depictions they look more like a football or Cheeto puff. If you look inside with a high-powered microscope you’d see their intricate and beautiful maze-like structure.
The maze is formed by an inner membrane that folds in on itself forming their characteristic zig-zag shape. This inner maze is called the matrix and the infoldings of membrane are called cristae. The membrane surrounding the matrix is called the inner membrane, and the membrane surrounding the exterior of the entire mitochondria is called the outer membrane or the mitochondrial outer membrane (MOM for short! Again, sorry Dad!). Essentially, mitochondria are a double-membrane bound organelle because they have two membranes. Most organelles have a single membrane, but you can find double membranes in organelles like plastids and chloroplasts, the solar-powered kitchens cooking up the “food” plants, some animals and protists need to survive.
The outer membrane is studded with tiny openings called pores, which allow things to travel in and out of the mitochondria. Embedded in the inner membrane are specialized proteins that are the biochemical work horses that produce the energy (in the form of a molecule called ATP) mitochondria are well known for. The inside of the mitochondria is filled with a gooey fluid called cytoplasm, which also fills our cells.
The Origin of Mitochondria
Scientists believe that modern mitochondria descended from free-living ancient organisms closely related to bacteria. These ancient ancestors of mitochondria may have been engulfed by an early cell, and instead of being “eaten”, they survived and became a permanent resident inside. These tenets form part of what is known as the Endosymbiotic theory. There isn’t a consensus on the particulars of the theory, for example, the type of cell that engulfed the ancient ancestors of mitochondria, but the general idea that mitochondria descended from some type of free-living ancestor is the best explanation we have for their origins. And just because the origin of mitochondria is referred to as a “theory”, does not mean it is a shot-in-the-dark guess, there is scientific evidence.
Support for this origin story comes from the fact that mitochondria have their own DNA and double membrane, unlike most other organelles, suggesting they could have been an independent entity at one point. Mitochondrial DNA also resembles more closely the DNA found in bacteria than our own cells in structure. Further, their DNA sequence is related to those found in bacteria, and mitochondria are similar in size to bacteria as well. There is quite a bit of additional evidence too that you can read about in more detail.
Because both mitochondrial DNA and nuclear DNA can mutate, this also means that their genomes can evolve because mutations are one of the selection pressures, or driving forces for evolutionary changes in organisms and populations of them.
Mitochondrial DNA, like the DNA inside our cell nuclei, can acquire mutations, basically misspellings in their sequence. Not all misspellings are bad, some are harmless, but some can have drastic consequences like diseases. Because both mitochondrial DNA and nuclear DNA can mutate, this also means that their genomes can evolve because mutations are one of the selection pressures, or driving forces for evolutionary changes in organisms and populations of them. Interestingly, mitochondrial DNA has a higher mutation rate than the DNA inside our nucleus.
Mitochondrial DNA and Dad
We used to think that mitochondrial DNA was only passed down in humans from the mother, and that DNA from the nucleus was inherited from both parents. I remember learning this as dogma in college. It was thought that paternal mitochondria were discarded and eliminated, while maternal ones were kept and preserved.
Cells often recycle and dispose of old or damaged organelles and other parts as a quality control mechanism. In some sense, cells literally “eat” a part of themselves that is damaged to prevent further widespread damage. One type of cellular eating is called mitophagy, it specifically refers to cells eating old or damaged mitochondria. Scientists had observed cells using this process to selectively eliminate paternal mitochondria and their DNA from offspring.
Just like mitochondria are more than powerhouses, Dads are much more than just their mitochondrial DNA.
As to why there’s a biological preference for maternal over paternal mitochondria, this is an active area of research. It does suggest that there might be something advantageous though to mostly maintaining Mom’s mitochondria than Dad’s…sorry, yet again, Dads!
However, a study published in 2018 found that mitochondrial DNA can be passed down from Dads. The authors reported that it is a rare event, but that they believe that this phenomenon could be more widespread, just often overlooked and therefore undetected.
In the field, this study remains somewhat controversial, because more recent studies building on this work don’t support it. An earlier study published back in 2002 that identified evidence for paternal inheritance of mitochondria has yet to be repeatable as well.
So, whether mitochondria and their DNA can be paternally inherited remains a mystery. Perhaps, it is a rare event, but possible. We find exceptions to rules in nature, but dogmas that have been around for years and decades require more validation and support before the scientific community changes its consensus.
The popular meme of mitochondria then remains mostly true for now, but perhaps we will have more clarity on this matter in the future. Until then, lack of paternal inheritance for mitochondria and their DNA does not make Dad’s genetic contribution any less important. Just like mitochondria are more than powerhouses, Dads are much more than just their mitochondrial DNA.
Red and green fluorescently labeled mitochondria in cells, iStockphoto.com/HeitiPaves
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