When we talk about the human genome, we usually think of the DNA sequence contained in the 23 pairs of chromosomes in the nucleus of each diploid human cell. We are not entirely wrong, because it is this genetic information that defines us both as a species and as an individual.
Yet it is fascinating to know that we have 300 times more microbial genes in our bodies than our own DNA. This figure is justified in itself by simply saying the following: we have 100 times more microorganisms than our own cells in the body, that is, around 100 trillion bacteria living inside and above our body.
Beyond bacterial symbiosis, there is a small portion of DNA in the cells of our body that “doesn’t belong 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 on this fascinating journey through mitochondrial DNA, as we guarantee that we will undermine the foundations of genetic prejudice 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 long way to go and a limited space. Mitochondrial DNA is defined as genetic material found in mitochondria, the organelles that provide the cell with energy for its life processes.
You could say that this genetic information is the “chromosome” of mitochondria. DNA inside the human cell but outside the nucleus, a strange concept, isn’t it ?. However, this structure is very different from the “X” that comes to mind when we talk about human chromosomes, because we are dealing with a small circular double strand of DNA, much simpler and more basic than the organization. Genetics present in the nucleus cell.
To whet your appetite, we show below a series of differences between mitochondrial and nuclear DNA which put into perspective the very clear distance between the two:
- There are 37 genes in the mitochondrial genome, compared to 20,000 to 25,000 genes in human nuclear chromosomal DNA.
- This translates to around 16,500 more or less mitochondrial base pairs, while human DNA collects around 3.2 billion base pairs.
- This circular double strand of mitochondrial DNA encodes a number of proteins unique to the organelle, while the nuclear performs many other functions.
As we can see, we are faced with two structures which are like water and oil. According to this parallelism, these last two have in common only the being of a fluid, just as the chains which belong to us here only share their most basic structure: the nucleotides which make up DNA and the most basic structure. basic (which are universal). .
Mitochondrial DNA structure
Once we put into perspective what mitochondrial DNA is and how it differs from humans, it’s time to use a microscope to dissect parts of this curious structure.
First of all, we emphasize again that the mitochondrial genome it is made up of a double strand of DNA, which closes in on itself in a circular fashion (Like a snake biting its tail). The two channels are given a special name and treatment because they have different characteristics.
For example, H (heavy) chain has a higher sedimentation coefficient than L (light), a value consistent with the fact that in this segment are found the coding sequences for 28 of the total 37 genes. We also don’t want to turn this space into an advanced genetics class, so we are summarizing the functions of the sequences present in this DNA in the nicest way possible. Mitochondrial genetic information encodes the following compounds:
- 2 Ribosomal RNAs: these are RNA molecules that are part of ribosomes, which are also found in mitochondria.
- 22 RNA transfer: essential for protein synthesis within mitochondria.
- 13 structural genes, which code for different subunits of 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 can therefore synthesize proteins autonomously. Again, we emphasize the term “to some extent” because human mitochondria contain around 1,500 proteins, of which only 13 are encoded in mitochondrial DNA itself.
So most mitochondrial proteins come from genetic information encoded in nuclear DNA (human chromosomes in this case), as they are synthesized in the ribosomes of the cytosol (the cell medium).) Then the organelle takes them for herself.
Most important features
Once we have compared mitochondrial DNA with nuclear DNA in humans and examined its structure, the next logical stop is to find out what characteristics define that structure beyond its chemical makeup. Let’s do this.
There are multiple copies of this DNA in the mitochondria, as this chain of genetic information is usually associated with proteins in the organelle, forming a structure called a “nucleoid”. Because we have 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 know that the mitochondrial DNA present in each of these organelles throughout our body is only inherited by the mother. This is so because upon entering the egg, the sperm undergoes partial degradation in which it loses its tail and mitochondria. Knowing this data, we know that this isolated genetic information is extremely useful in deducing the relatedness of living things.
3. High exchange rate
Mitochondrial DNA is on the “total war front” because it is very close to the machinery of cellular respiration, giving rise to the dreaded free radicals, compounds that can damage DNA with certain specific interactions. So this very special structure it has a varied and complex repair machinery, which includes various forms of recombination.
Due to this continuous change and transformation, mitochondrial DNA is estimated to have a mutation rate up to 10 times that of nuclear DNA, of course, a much faster evolutionary mechanism than that of nuclear DNA. that we are used to seeing in the world of living things. .
Where does mitochondrial DNA come from?
To close this path to “extraterrestrial” genetic information found in our cells, we can emphasize that the theory of the emergence of this DNA is curious to say the least.
According to several experts, mitochondria (about 2 billion years ago) was an aerobic bacteria living freely.. At one point, an anaerobic nucleated cell swallowed this Eubacterium, integrating it into its cytosol. Thus, a symbiotic relationship would be established on the basis of a mechanism called endocytosis.
It is clear that during evolution, this primordial bacterium would lose many of its genes along the way to become the mitochondria we know today, which they would eventually integrate into the nuclear DNA of the cell. . This theory is well founded because mitochondrial DNA shares several characteristics with the genome of prokaryotic microorganisms.
Mitochondria is the organ responsible for producing most of the chemical energy needed to activate the cell’s biochemical reactionsSo without it, life as we know it today would be utterly impossible. Going a little metaphysical in this final note, it is fascinating to think that such an anecdotal fact that a microorganism while eating another could have triggered the current explosion of life, among which is our species.
As we promised you at the beginning, in these lines we put into perspective the concept of genome in the human body, even the origin of life and what could have brought us to this evolutionary point.
However, the uses of this knowledge are not just conjectural. Mitochondrial DNA allows to deduce kinship relationships among members of a living population, and knowledge of these data is essential to be able to implement conservation plans for the species. Additionally, there are several mitochondrial diseases linked to mutations in this DNA, so knowing is the first step to reducing.
- Mitochondrial DNA, National Genome Research Institute (NIH). Retrieved October 16, from https://www.genome.gov/es/genetics-glossary/DNA-mitochondrial
- Alberts, B. and Bray, D. (2006). Introduction to cell biology. Pan American Medical Ed.
- Mitochondria, National Genome Research Institute (NIH). Collected October 16 at https://www.genome.gov/es/genetics-glossary/Mitocondria#:~:text=Las%20mitocondrias%20son%20los%20org%C3%A1nulos,trifosfato%20de%20adenosina% 20 ( ATP).
- Montoya, J. and Attardi, G. (1986). Human mitochondrial DNA. Research and Science, 118, 60-69.
- A cell in your cell: genotipia.com. Collected October 16 from https://genotipia.com/mitocondria/