Mitochondrial DNA: how is it different from the nuclear DNA and why does it matter?

Figure. The mitochondrial DNA as opposed to the nuclear DNA. I suggest to read this figure starting at the bottom! Template from BioRender.com.

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Each cell in the human body contains two copies of nuclear DNA. The importance of the nuclear DNA cannot be emphasised enough. It was only in 1983, after three decades of the decoding of the DNA structure by Crick and Watson, that DNA fingerprinting was adopted as a go to identification mechanism of crime suspects. It did not stop there, genealogy and health are among the many applications of DNA sequencing.

The adjective ‘nuclear’ in nuclear DNA is to differentiate it from another DNA called the mitochondrial DNA. Mitochondrial DNAs are in hundreds even thousands in each cell, depending on the cell type. Muscle cells contain thousands of copies of mitochondrial DNA because it contains thousands of mitochondria.

While one copy of nuclear DNA is about 3 billion base pairs long, the mitochondrial DNA is about 16,500 base pairs and is circular as in bacterial cells. Also, the nuclear DNA contains between 20,000 to 25,000 genes while the mitochondrial DNA contains only 37 genes.

Now, genes are the portions of the DNA that instruct the cell to produce proteins which eventually translate into the body we live in with the few changes that occur with time. But do these two DNAs interact? Do they hold the same genes? Is the mitochondrial DNA redundant? The answers are Yes, No, No.

The mitochondrial DNA genes play the important role of producing energy for the cell to grow and divide. Those genes are only 13 genes out of the total 37 genes of the mitochondria. The mitochondria, in exchange for the energy it provides for the cell, collaborate with some genes of the nuclear DNA to ensure the well-being of the mitochondria.

Every cell has mitochondria except for red blood cells. So, in particular, the ovum, being the cell produced by the ovaries of the mother, has its mitochondrial DNA that gets passed down to the child. That is, the child inherits the mitochondrial DNA only through the mother. This is in contrast to inheriting the nuclear DNA from both parents.

As DNA testing is commonly used to learn about the genetic make-up of an individual, mitochondrial DNA is a particularly useful tool when the nuclear DNA is not available. Mitochondrial DNA is also important for determining drug sensitivity and disease risks. Determining ancestry through the mother line is another advantage for mitochondrial DNA.

In 2018, it was surprising to see that parts of the mitochondrial DNA incorporates itself into the nuclear DNA.

“Mitochondrial DNA appears to act almost like a Band-Aid, a sticking plaster to help the nuclear genetic code repair itself” said Prof Patrick Chinnery. “And sometimes this works, but on rare occasions if might make things worse or even trigger the development of tumours.”

It is amazing how this collaborative interaction between the two DNAs in our cells helps us remain healthy.

Takeaways

• Human cells carry two different DNAs: a large linear nuclear DNA and a small circular mitochondrial DNA.

• The mitochondrial DNA is solely inherited from the mother.

Further explorations

• If you are interested in a podcast about DNA and ancestry, I recommend the following BBC inside science series DNA of history.

• This is a controversial article about the use of mitochondrial DNA and the African origin of Homo sapiens National geographic article