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

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

Still, it's fascinating to learn that we have 300 times more genes from microbes inside our bodies than we do from our own DNA. This figure is justified by itself just by saying the following: we have 100 times more microorganisms than cells own in the body, that is, approximately 100 billion bacteria living inside our body and about him.

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

• Related article: "Mitochondria: what they are, characteristics and functions"

Mitochondrial DNA: the foreign genetic structure

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

We could say that this genetic information is "the chromosome" of the mitochondria. DNA inside the human cell but outside the nucleus, strange concept, right? Even so, this structure is very different from the "X" that comes to mind when we talk about human chromosomes, since we are before 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, we show below a series of differences between mitochondrial and nuclear DNA that put into perspective the very clear distance between the two:

• 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 mitochondrial base pairs or so, while human DNA makes up about 3.2 billion base pairs.
• This double circular strand of mitochondrial DNA encodes a series of proteins unique to the organelle, while the nuclear one has many more functions.

As we can see, we are facing two structures that are like water and oil. Following this parallelism, these last two only have in common being a fluid, like the chains that concern us here. they only share their most basic structure: the nucleotides that make up DNA and the most basic structure (which are universal).

Mitochondrial DNA structure

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

First of all, we reemphasize that the mitochondrial genome It is composed of a double strand of DNA, which is closed on itself in a circular way. (like a snake biting its tail). Both chains receive a special name and treatment, since they present different characteristics.

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

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

As we can see, mitochondria is an organelle that stands on its own to some extent, as it contains ribosomes inside, and therefore, can synthesize proteins autonomously. Again, we emphasize the term “to a certain extent”, since human mitochondria contain about 1,500 proteins, of which only 13 are encoded in the mitochondrial DNA itself.

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

• You may be interested in: "Differences between DNA and RNA"

most important features

Once we have compared mitochondrial DNA with nuclear DNA in humans and have reviewed its structure, the next logical stop is to discover what characteristics define this structure beyond its composition chemistry. Go for it.

1. polyplasmy

There are multiple copies of this DNA within the mitochondria, 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 about 10 nucleoids per mitochondria, which translates to 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 bodies is inherited solely from the mother. This is so because, upon entering the ovule, the spermatozoon undergoes partial degradation in which it loses the tail and mitochondria. Knowing this fact, we know that this isolated genetic information is extremely useful when it comes to inferring relationships in living beings.

3. high exchange rate

Mitochondrial DNA is on the "full front line" as it is very close to the breathing machinery cell, which gives rise to the dreaded free radicals, compounds that can damage DNA with certain interactions specific. Thus, this very special structure has a variety of 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 DNA. Nuclear DNA, of course, a mechanism of evolution much faster than what we are used to seeing within the world of beings alive.

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 various experts, the mitochondria (about 2,000 million 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 would be established based on a mechanism called endocytosis.

Clearly, over the course of evolution this early bacterium would lose many of its genes along the way until become the mitochondria that we know today, which would end up being integrated into the nuclear DNA of the cell. This theory is widely supported, since mitochondrial DNA shares various features with the genome of prokaryotic microorganisms.

mitochondria It is the organelle in charge of generating most of the chemical energy necessary to activate the biochemical reactions of the cell., so without it, life as we know it today would be totally impossible. Getting a bit metaphysical on this final note, it is fascinating to think that a fact as anecdotal as a microorganism eating another may have triggered the current explosion of life, among which is our species.

Summary

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

Still, the uses of this knowledge are not merely conjectural. Mitochondrial DNA allows 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 in the species. In addition, there are various mitochondrial diseases linked to mutations in this DNA, so knowing it is the first step to tackle them.

Bibliographic references:

• Mitochondrial DNA, National Genome Research Institute (NIH). Collected on October 16 in https://www.genome.gov/es/genetics-glossary/ADN-mitocondrial
• Alberts, B., & Bray, D. (2006). Introduction to cell biology. Pan American Medical Ed.
• Mitochondria, National Genome Research Institute (NIH). Collected on October 16 in https://www.genome.gov/es/genetics-glossary/Mitocondria#:~:text=Las%20mitocondrias%20son%20los%20org%C3%A1nulos, adenosine%20%20triphosphate%20(ATP).
• Montoya, J., & Attardi, G. (1986). human mitochondrial DNA. Investigation and Science, 118, 60-69.
• A cell inside your cell: genotypia.com. Collected on October 16 in https://genotipia.com/mitocondria/