What is Mitochondrial DNA?
Updated on April 25th, 2019
In early studies of human DNA, it was believed that our DNA existed only in the control center of our cells…the nucleus. But in 1963, scientists at Stockholm University discovered DNA residing outside the nucleus.
They found DNA fibers in the energy centers of human cells. These energy centers are called mitochondria.
Mitochondrial DNA, often referred to as mtDNA, accounts for a small sampling of a person’s total DNA. It contains just 37 of the 20,000 to 25,000 protein-coding genes in the human body.
It is distinct from DNA found inside the nucleus and as researchers continue to study it, they are beginning to unlock many mysteries of genealogy, aging and illness.
The human body is made up of eukaryotic cells that contain a nucleus and other organelles. Organelles are mini organisms that carry out specific functions.
Mitochondria is one of these organelles and its role is to generate energy. What makes mitochondria so unique is that they have their own genetic material that is independent from the nucleus.
Each cell contains hundreds to thousands of mitochondria and these cells reproduce by splitting in two after they make a second copy of the DNA.
Mitochondrial DNA is different from nuclear DNA in a number of ways:
- Nuclear DNA comes from both parents, but mtDNA only comes from the mother.
- Visually, nuclear DNA is linear while mtDNA is circular.
- Nuclear DNA has 3.3 billion DNA base pairs that are the building blocks of DNA but the mtDNA genome is made up of about 16,500 base pairs and only encodes for 37 genes.
How Can You Test for mtDNA using DNA Tests?
A mitochondrial DNA test can be taken by both men and women because mtDNA is passed down from a mother to both sexes.
Parties can choose to either examine a limited region of their mtDNA that will help identify their basic haplogroup and migration paths of their ancestors. A subject can also choose to have a full mtDNA sequence test performed.
This examines all regions of a person’s mtDNA and gives much more refined results for genealogical purposes.
A person’s mtDNA has three regions. They are known as the two hypervariable regions, HVR1 and HVR2, and the Coding Region.
A basic mtDNA test only looks at the hypervariable regions while a full sequence test includes the Coding region as well.
After a test is performed and a perfect match is shown for another person’s mtDNA, it is possible to assume that both subjects have a common ancestor and are related on their mother’s (matrilineal) side. Test results can also help identify living relatives with similar mtDNA.
What Does Mitochondrial DNA Tell Us?
Because mtDNA mutates slowly, it allows a person to find out ancient ancestral information but will not help them learn about more recent origins.
This makes it especially useful for determining migration patterns of a person’s ancestors and can determine which haplogroup a person is from.
When humans left Africa eons ago, they traveled in smaller groups to different parts of the world. Over time, they evolved as their DNA produced mutations to help them adapt to their surroundings.
Based on these mutations, groups of people share common characteristics that are known as haplogroups.
It a test result determines that a person has several mtDNA matches, it means they belong to a more common haplogroup, indicating that their lineage has survived and reproduced well.
Why has Mitochondrial DNA Become so Important?
In addition to unlocking the keys to a person’s ancestral beginnings and genealogical links, mtDNA is also helping to advance science the study of the origins of many diseases and conditions.
By studying these things at a DNA level, scientists hope to unlock what triggers the diseases which can hopefully lead to effective treatments and cures.
For example, scientists already know most pancreatic cancers have mutations in mitochondrial DNA. However, they still do not know whether mitochondria have a special role in producing cancers or if they are just a normal part of the aging process.
Part of the focus in studying cancers has been on mitochondrial DNA telomeres. Telomeres are the ends of chromosomes that act like caps and keep chromosomes from falling apart.
When mitochondria lose telomeres, scientists have found that early forms of pancreatic cancer have extremely short telomeres. It is theorized that this produces changes in the pancreatic duct that can lead to the evolution of cancer in a person.
Scientists further theorize that without the loss of so much telomeric DNA, people might not ever get pancreatic cancer.
Scientists are also making advancements in the study of telomeric length and the overall aging process. The recent evidence suggests the existence of a strong linkage in this area.
This type of research is being replicated in a number of ways and in the study of a number of diseases and conditions, making advancements in understanding mtDNA and how it functions in the body a critical area of study.
It’s important to note that any mutation is a change in a DNA sequence and is not necessarily always bad. But 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 children inherit a mother’s mitochondrial DNA, fathers with mitochondrial diseases aren’t at risk to pass on the disorder to offspring.
Treatment options for mitochondrial diseases remain limited but as the understanding of mtDNA and its effect on diseases grow, it will lead scientists to make advancements in targeting and creating treatments in the fight against diseases.
Is Mitochondrial DNA Inherited from the Mother or Father?
Mitochondrial DNA is normally transmitted and inherited exclusively from the mother. There are about 200,000 molecules of mtDNA in an egg, but only about 5 mtDNA molecules found in human sperm.
It is believed that mtDNA found in sperm are usually destroyed by the egg cell after it is fertilized. This is because paternal mtDNA is marked in such a way that it is selected for destruction at a later date inside the embryo.
There are a few rare cases of male mitochondrial inheritance that have been documented but many of these cases involve cloned embryos or subsequent rejection of the paternal mtDNA.
mtDNA is passed down from mothers to both sons and daughters, but sons cannot pass along their mothers’ mtDNA to their children. The mtDNA transmitted through the female egg is nonrecombinant.
This means it does not combine with any other DNA so that it is passed down virtually unchanged through the direct maternal line over generations.
Because only the maternal mtDNA survives, it allows genealogical researchers the ability to trace a person’s maternal lineage back in time. In addition, because mtDNA mutates at a very slow rate, this makes it possible to research back for many generations with a high degree of accuracy.
Biologists are also able to identify and compare mtDNA sequences among different species. They use this information to build evolutionary trees that allows them to look back in time.
For example, studying mtDNA allows scientists to trace the matrilineal descent of domestic dogs from wolves.
Using this same concept, researchers are also able to trace the concept of the Mitochondrial Eve as an attempt to discover the origins of humanity.
Genetic Health Conditions Related to Changes in mtDNA
Mutations of mtDNA can lead to a number of illnesses. Although research still continues, there is evidence to support the fact that mutated mtDNA is also a contributor to the aging process and to diseases that are associated with growing older.
Mitochondrial DNA damage is thought to be a factor in several neurodegenerative diseases. This is characterized by a progressive loss of structure or function of neurons and as advancements in the study of these types of diseases moves forward, there is a growing body of evidence to suggest that these are related to each other on a sub-cellular level.
The greatest risk factor of these diseases is aging. mtDNA damage accumulates and together with oxidative stress, it may trigger the onset of several conditions such as Parkinson’s disease, Alzheimer’s disease, Huntington’s disease and ALS (amyotrophic lateral sclerosis).
One theory suggests that aged mtDNA is the critical factor that in the origin of neurodegeneration in Alzheimer’s disease.
Mutant Huntington protein promotes oxidative damage to mtDNA and nuclear DNA that may contribute to Huntington’s disease.
When DNA oxidizes, it produces a marker called 8-oxogunine (8-oxoG). In persons with ALS, the enzymes that normally repair 8-oxoG damage in the mtDNA of spinal motor neurons are not able to perform this critical function.
This means that oxidative damage to mtDNA in motor neurons may be one of major contributing causes of ALS.
In general, diseases and health conditions caused by mutated mtDNA often involve multiple organ systems such as the heart, brain and muscles. Although symptoms and severity can vary widely, specific conditions include diabetes, kidney failure, heart disease, hearing loss, eye problems and loss of vision, and diminished intellectual functions such as dementia.
Because mtDNA also has a limited ability to repair itself, somatic mutations that are not inherited can build up over time and can result in some forms of cancer and an increased risk of heart disease, Alzheimer’s and Parkinson’s diseases.
According to the U.S. National Library of Medicine, Genetics Home Reference, many other diseases can be traced back to mutated mtDNA and include:
- Cyclic vomiting syndrome
- Kearns-Sayre syndrome
- Leber hereditary optic neuropathy
- Leigh syndrome
- Mitochondrial complex III deficiency
- Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS)
- Myoclonic epilepsy with ragged-red fibers (MERRF)
- Neuropathy, ataxia, and retinitis pigmentosa (NARP)
- Pearson marrow-pancreas syndrome
- Progressive external ophthalmoplegia
mtDNA’s Role in Genealogical Research, Explained
By itself, mtDNA testing has a number of drawbacks where genealogy is concerned. For example, because mtDNA is passed down only from a mother to her offspring, it is impossible to tie genealogy to a specific surname because women in many cultures change their names when they get married.
This means a person’s mother did not have the same surname has her mother, who did not have the same surname as her mother, and so on.
In addition, because mtDNA mutates very slowly, it means that a person’s mtDNA is likely to have nearly identical mtDNA to a maternal ancestor who lived thousands of years ago. It also means a person’s mtDNA is going to have literally thousands of matches to people living today.
mtDNA is more useful for tracing ancestors back thousands of years to discover what region they migrated from. Known as haplogroups, a person who tests their mtDNA may find out they were descended from relatives in Northern Europe or from various parts of Asia as an example.
A full sequence test will be able to drill down a much more specific region.
Examining mtDNA actually works best by disproving relationships rather than proving them. For example, if a child’s mtDNA is not the same as his or her mother’s mtDNA, this means she is not the child’s biological mother.
However, it is much more difficult to tell from a person’s mtDNA alone exactly who their biological mother really is, because that mtDNA is going to match the mtDNA of thousands of women.