Oh Mighty Chondria, who art thou?

A diagram showing a mitochondrion of the eukar...
Mitochondria are organelles surrounded by membranes, distributed in the cytosol of most eukaryotic cells. Its main function is the conversion of potential energy of pyruvate molecules into ATP. (Photo credit: Wikipedia)
The term 'mitochondria' is not alien to many. School level science introduced us to it, where we were told that these thread-granules (literal translation from Greek) were the power houses of the cell. They make energy in the form of ATP (adenosine triphosphate), something that the cells can use and that was all we ever needed to know. But these organelles have their own history and their own secrets that are so grave, that they would even put a Dan Brown novel to shame. Well, let us begin with some history.

Background (we have been outsourcing since the begining of time!!- it seems)

Although, we are very comfortable with the concept of mitochondria supplying power to the cell, many are surprised, when they get to know that the mitochondria were not originally a part of our cells. The popular theory about mitochondria is that its ancestors were probably some bacteria, who were exceptionally good at energy production and somewhere way back in time, our ancestral cells thought that it would be good idea to outsource this mechanical task of energy generation and concentrate on something that required more detailed attention, like cellular maintenance and reproduction. Of course, this would be in return for food and energy for the outsourcing staff and thus, mitochondria were officially accepted within our cell walls. 


So where is the evidence that supports this theory you might ask. Well, there are two strong points favouring this theory. The first being that even within our cells, mitochondria look and function very much like any bacteria, found in nature. The second being that all the energy production that is carried out by the mitochondria occurs as per the instructions dictated by the DNA of mitochondria themselves, a feature not seen with any other organelle in the cell. Thus, even inside our cells, mitochondria enjoy quite an independent life and are not at the mercy of the cell. Rather, recent findings have shown mitochondria to have a more dominant role in the cell, one even involving introduction of bits of its own genome into the cell's nucleus. 

Numts- hijacking the host

Yes, there is sufficient scientific evidence to show that these mitochondria send out to the nucleus, pieces of their own DNA so that it can be integrated into the nuclear DNA of its host cell. The DNA repair mechanism in the nucleus of our cells works like a charm, to incorporate these new bits and pieces of DNA floating inside the nucleus. Called, numts (short for New mitochondrial DNA), these insertions of DNA, occur much more frequently that one would like, leaving many to wonder what mitochondria are actually up to?

The mitochondrial genome is about 14,600 bases long. Interestingly, 90% of this information has already been sent out and incorporated into the human genome. Taking into consideration that the smallest protein coding gene is only 500 bases long, our mitochondria have pushed in a lot of information into our genome already and continue to do so on an everyday basis. 

Numts are not recent and are neither unique to animals

Numts have been inserting themselves into our our genome for quite some time now. A study published in 2007, has shown that 80 % of the numts seen in humans were also seen in chimpanzees at the same location on the genome. Thus, numts have been on their mission, even when chimps and humans had not separated into different species. Also, a similar phenomenon has also been reported in plant genomes, where chloroplast DNA is imprinting its nuclear information onto the host nucleus, while yeasts, honeybees and even rodents have seen numts. 

So far, research has shown that there are at least twelve sites in the human genome that have accepted numts and it is likely that this is just the beginning of the find. Interestingly, most of these sites are active genes suggesting that insertion of numts is largely a targetted exercise and not just a random event. Although, numts insertions stay largely neutral, there are also instances where such insertions can lead to diseases in individuals. Although rare, numts insertions have been established as cause for instances of Plasma Factor VII deficiency, Pallister-Hall Syndrome, Usher Syndrome type IC etc.

So what are Numts upto?

To be honest, nobody really knows why numts exist. There are a few theories though that have been put to explain the behaviour of mitochondria and the existence of numts.

A very plausible explanation is that the presence of numts is part of an extremely long process of integrating the mitochondria into our genomes. Having uploaded its genetic information into the cell's nuclear databank, the mitochondria can continue to carry out their regular function of energy production without having to bother about their own reproduction. In a way, one could also say that mitochondria are outsourcing this function just like they have outsourced production of certain proteins to the cell in the past. Over a period of time (which in this case is a few more million years), mitochondrial DNA will be entirely transferred to the nucleus, mitochondria will become a true cell organelle and its genetic control will be handled by the cell's nucleus, thereby completing the integration.

The other explanation is that numts are mitochondria's way to ensure that its genetic information is safely stored. As power houses of the cell, mitochondria are under severe oxidative stress at all times. As we all know, oxidative stress can severely damage stored genetic information and also obstruct DNA repair mechanisms.With numts, mitochondria are creating a backup of the nuclear information, so that it can be brought back later, when need be. How are mitochondria doing something of this sort, is something we do not really know at this point in time. However, one thing is for sure, these mighty chondria are pulling some extreme feats every single day. 



References:

Hazkani-Covo, E., & Graur, D. (2006). A Comparative Analysis of numt Evolution in Human and Chimpanzee Molecular Biology and Evolution, 24 (1), 13-18 DOI: 10.1093/molbev/msl149

Hazkani-Covo, E., Zeller, R., & Martin, W. (2010). Molecular Poltergeists: Mitochondrial DNA Copies (numts) in Sequenced Nuclear Genomes PLoS Genetics, 6 (2) DOI: 10.1371/journal.pgen.1000834

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