Nuclear DNA, or DNA that’s found in the nucleus, is probably the only type you learned about in high school bio. It carries all of an organism’s genetic information, which is used for proper growth, development, functioning, and reproduction. We get a set of DNA from each of our parents: one set from mom and one set from dad. In this Ask A Scientist piece, we delve into research on our third set of DNA: mitochondrial DNA.
Recently, there was news of the first baby born with DNA from three parents. The mother lost her first two babies to Leigh syndrome, which is associated with mutations in mitochondrial DNA. In attempt to circumvent this, doctors removed the nucleus from one of the mother’s eggs, placed it in a donor mother’s egg (after removing the donor egg’s nucleus), and fertilized this egg with the father’s sperm. The successful transplant led to the world’s first baby with DNA from three parents: mom and dad’s nuclear DNA and a donor’s mitochondrial DNA.
What is mitochondrial DNA?
Eukaryotic cells (which we are made of) contain a nucleus and other organelles. Organelles are “little organs” in the cell that carry out specific functions. Mitochondria, one of these organelles, generate energy for the cell in the form of ATP. This is why know it as the “powerhouse of the cell.” However, mitochondria are unique organelles because they have their own genetic material — independent to that in the nucleus!
Mitochondrial DNA (mtDNA) is different from nuclear DNA in a lot of ways. mtDNA is generally circular, while nuclear DNA is linear. Nuclear DNA has 3.3 billion DNA base pairs (the building blocks of DNA) – the mitochondrial genome is only made up of ~ 16,569 base pairs and only encodes for 37 genes. You might remember that there’s only one nucleus in a cell, where the DNA is tightly packed into chromosomes. We have two copies of each chromosome (46 chromosomes total). However, a single cell can have multiple mitochondrion and each of them has dozens of copies of the mitochondrial genome. Plus, mtDNA isn’t in the nucleus.
Why do we have mtDNA?
Given the shape of mtDNA and its independence from nuclear DNA, it’s possible that mitochondria came from bacteria. This theory claims that over a billion years ago, mitochondria were actually small aerobic (oxygen related) bacteria that were engulfed by a larger cell. Instead of digesting the small cell, the big and small cells developed a relationship. Perhaps that first big cell benefited from the smaller cell’s ability to use oxygen to produce energy! This theory originated from professor Lynn Margulis in the mid-1900s.
Thank your mother for your mtDNA!
Interestingly, unlike nuclear DNA, which comes from both your mother and father, only maternal mtDNA is passed down (maternal inheritance). Since this is unique to the maternal lineage, scientists think that they can trace human lineage through mtDNA inheritance. This might help us identify one ancient woman who is the most common female ancestor of all modern people. She is aptly named “Mitochondrial Eve” (the male equivalent is the “Y-chromosomal Adam”).
Even though mtDNA is passed down by maternal inheritance, males still have it. We know paternal mtDNA gets eliminated but nobody knows why. Excitingly, earlier this year a group of scientists found a potential explanation. They observed that in C. elegans (a type of roundworm), paternal mitochondria are eliminated when the sperm and egg fuse. If this process was disturbed, embryo survival rates decreased. This is the first time a study showed experimental evidence to suggest that maintaining paternal mtDNA may be harmful.
mtDNA and biomedical research
Because of the number of mtDNA, there’s a much higher mutation rate in mitochondrial DNA. A mutation is a change in DNA sequence and is not necessarily always bad. However, when a mutation occurs in an important gene and alters the ability of the gene to function normally, it can contribute to genetic diseases. These mutations can occur spontaneously, due to errors in DNA replication and repair, or as a result of exposure to chemicals.
Since offspring inherit mom’s mitochondrial DNA, fathers with mitochondrial diseases aren’t at risk to pass on the disorder to offspring. However, since a single cell contains many mitochondria, each with multiple copies of mtDNA, symptoms of mitochondrial diseases can vary. Given that the role of mitochondria is to produce power for the cell, mitochondrial DNA diseases often affect tissues requiring lots of energy like the heart, brain, and muscles. Just last month, a group from Cornell University published a study suggesting a link between mtDNA and some forms of autism spectrum disorder. Right now, treatment options for mitochondrial diseases are limited but as the understanding of mtDNA and its effect on diseases grow, so does our ability to target and come up with potential treatments.