Mitochondrial DNA: what is it, and what are its characteristics and functions?

A review of the characteristics of this type of genetic material different from that of the cell nucleus.

When we talk about the human genome, we usually think of the DNA sequence contained in the 23 pairs of chromosomes inside the nucleus of each diploid human cell. We are not entirely wrong, for it is this genetic information that defines us both as a species and as individual entities.

Still, it is fascinating to learn that we have 300 times more genes from microbes inside our bodies than we have DNA of our own. This figure justifies itself just by saying the following: we have 100 times more microorganisms than our own cells in our body, i.e. approximately 100 trillion bacteria inhabiting in and on our body.

Beyond bacterial symbioses, there is a small portion of DNA within the cells of our body that "does not belong" to us in its entirety. We are talking about mitochondrial DNAa double strand of genetic material external to our genome. Dive with us in this fascinating journey through the mitochondrial DNA, because we assure you that we will shake the foundations of genetic preconceptions in the following lines.

Mitochondrial DNA: the foreign genetic structure

We enter directly into the subject, because in the face of such a strange phenomenon, we have a lot of ground to cover and limited space. Mitochondrial DNA is defined as the genetic material present in the mitochondria, i.e. the organelles that provide the cell with energy for its vital processes..

We could say that this genetic information is the "chromosome" of the mitochondrion. DNA inside the human cell but outside the nucleus, strange concept, isn't it? Even so, this structure is very different from the "X" that comes to mind when we talk about human chromosomes, since we are dealing with a small and circular double strand of DNA, much simpler and more basic than the genetic organization present in the nucleus of the cell.

To whet your appetite, here are some of the differences between mitochondrial DNA and mitochondrial DNA. differences between mitochondrial and nuclear DNA that put into perspective the very clear distance between them:

• Within the mitochondrial genome there are 37 genes, compared to 20,000-25,000 genes in human nuclear chromosomal DNA.
• This translates to about 16,500 or so mitochondrial base pairs, whereas human DNA contains about 3.2 billion base pairs.
• This circular double strand of mitochondrial DNA encodes a number of proteins unique to the organelle, while the nuclear DNA serves many more functions.

As we can see, we are dealing with two structures that are like water and oil. Following this parallelism, the latter two only have in common the fact that they are a fluid, just as the chains that concern us here only share their most basic structure: the nucleotides that form the DNA and the most basic structure (which are universal).

Structure of mitochondrial DNA

Having put into perspective what mitochondrial DNA is and how it differs from human DNA, it is time to take a microscope to dissect the parts of this curious structure.

First of all, let us emphasize once again that the mitochondrial genome is composed of a double strand of DNA, which is closed on itself in a circular fashion. (like a snake biting its own tail). Both chains are given special names and treatment, since they have different characteristics.

For example, the H (heavy) chain has a higher sedimentation coefficient than the L (light) chain, a value that is consistent with the fact that the coding sequences of 28 of the 37 total genes are found in this segment. We also do not want to turn this space into an advanced genetics class, so we summarize the functions of the sequences present in this DNA as nicely as possible. The genetic information of mitochondria encodes the following compounds:

• 2 ribosomal RNAs: these are RNA molecules that are part of ribosomes, which can also be found inside mitochondria.
• 22 transfer RNAs: essential for protein synthesis within mitochondria.
• 13 structural genes, which encode different subunits of the enzyme complexes of the oxidative phosphorylation system.

As we can see, the mitochondrion is an organelle that is self-sufficient to some extent, since it contains ribosomes in its interior, and can therefore synthesize proteins autonomously.and can therefore synthesize proteins autonomously. Again, we emphasize the term "to some extent", because human mitochondria contain about 1500 proteins, of which only 13 are encoded in the mitochondrial DNA itself.

Thus, most of the proteins in the mitochondria come from the genetic information encoded in the nuclear DNA (the human chromosomes in this case), as they are synthesized in the ribosomes of the cytosol (the cell medium) and then the organelle takes them for itself.

Most important characteristics

Once we have compared mitochondrial DNA with nuclear DNA in humans and reviewed its structure, the next logical stop is to discover what characteristics define this structure beyond its chemical composition. Let's get to it.

1. Polyplasmy

There are multiple copies of this DNA within the mitochondrion, as this chain of genetic information is usually associated with proteins within the organelle, forming a structure called "nucleoid". To give us an idea, there are up to 10 nucleoids per mitochondrion, which translates into about 10,000 copies of mitochondrial DNA per cell..

2. Maternal inheritance

It is fascinating to learn that the mitochondrial DNA present in each of these organelles throughout our body is inherited solely from the mother. This is because, upon entering the egg, the sperm undergoes a partial degradation in which it loses its tail and mitochondria.. Knowing this, we know that this isolated genetic information is extremely useful when inferring kinship in living beings.

3. High rate of change

Mitochondrial DNA is in the "war front", as it is very close to the cellular respiration machinery, which gives rise to the dreaded free radicals, compounds that can deteriorate DNA with certain specific interactions. Thus, this very special structure has a varied and complex repair machinery, including various forms of recombination..

Due to this continuous change and transformation, it is estimated that mitochondrial DNA has a mutation rate up to 10 times higher than that of nuclear DNA, a much faster evolutionary mechanism than we are used to seeing in the living world.

Where does mitochondrial DNA come from?

To close this tour of the "alien" genetic information found in our cells, we can emphasize that the theory of the emergence of this DNA is, to say the least, curious.

According to several experts, the mitochondrion (about 2 billion years ago) was a free-living aerobic bacterium.. At some point, an anaerobic nucleated cell engulfed this eubacterium, integrating it into its cytosol. Thus, a symbiotic relationship based on a mechanism called endocytosis would be established.

Clearly, in the course of evolution this primordial bacterium would lose many of its genes along the way to become the mitochondrion we know today, which would end up being integrated into the cell's nuclear DNA. This theory is widely supported, since mitochondrial DNA shares several features with the genome of prokaryotic microorganisms.

The mitochondrion is the organelle responsible for generating most of the chemical energy needed to activate the cell's biochemical reactions.Without it, life as we know it today would be totally impossible. Getting a bit metaphysical in this final note, it is fascinating to think that such an anecdotal event as a microorganism eating another could have triggered the current explosion of life, among which is our species.

Summary

As we promised you at the beginning, in these lines we have put into perspective the concept of the genome within the human body, including the origin of life and what could have brought us to this evolutionary point.

Even so, the uses of this knowledge are not merely conjectural. Mitochondrial DNA allows us to infer kinship relationships among the members of a population of living beings, and knowing this data is essential to be able to implement conservation plans for the species. In addition, there are several mitochondrial diseases linked to mutations in this DNA, so knowing this is the first step to tackle them.

Bibliographic references:

• Mitochondrial DNA, National Genome Research Institute (NIH). Retrieved October 16 from https://www.genome.gov/es/genetics-glossary/ADN-mitocondrial.
• Alberts, B., & Bray, D. (2006). Introduction to cell biology. Ed. Médica Panamericana.
• Mitochondria, National Genome Research Institute (NIH). Retrieved October 16 from https://www.genome.gov/es/genetics-glossary/Mitocondria#:~:text=The%20mitochondria%20are%20the%20org%20C3%A1nules,triphosphate%20of%20adenosine%20(ATP).
• Montoya, J., & Attardi, G. (1986). Human mitochondrial DNA. Research and Science, 118, 60-69.
• A cell inside your cell: genotipia.com. Retrieved October 16 from https://genotipia.com/mitocondria/.