By Laura Domicevica
This seemingly easy question has proved to be a rather tough nut to crack. It is well known that mitochondria produce power in the form of ATP which is the main energy currency in a cell. That seems really important, since without the energy the cell would stop dividing or die. In either case, the consequences of loosing mitochondria function are quite serious. In humans, problems in mitochondria function cause devastating disorders affecting muscles and brain the most, as these cells use great amount of energy.
What came first – the nucleus or the mitochondrion?
The way eukaryotes acquired mitochondria reminds something from Star Wars. The ancestor of mitochondrion was an ancient bacterium that lived in symbiosis with the early eukaryote. As the part of the cell that produces most of the energy, it sounds to me like a midichlorian – bacterium channelling the Force in Jedi.
The structure of mitochondria shows some proof of its bacterial ancestry, as it has double membraneinstead of the single lipid bilayer present in other subunits of the cell and even separate genome, encoding many important proteins.
Even with the extensive research done in this field, it is still impossible to tell when exactly the mitochondrion was acquired. For some time it was thought that eukaryotes formed endosymbiosis at some stage of their evolution. Consequently, this could be proved by finding a cell with a nucleus and without mitochondria. So far, no luck here.
Simple parasitic cells, for example, well known disease causing microbe Trichomonas vaginalis, do not have mitochondria in the traditional sense. They were found to have extremely reduced organelles with double membrane and some functions characteristic to mitochondria.
Elements we cannot live without – iron and sulphur
Researchers from Newcastle University looked at a parasites called Microsporidia that have lost many features common to a eukaryotic cell. Nevertheless, these obligate intracellular parasites that cannot live outside of a host cell have mitosomes – a reduced form of mitochondria. The question they were trying to answer was what is the ultimate role of the mitosome i.e. mitochondria in essence. Surprisingly, the parasite did not need it for the production of energy, as it was possible to steal it from the host.
Actually, the one of the main features still present in this organelle was the production of iron-sulphur (Fe/S) clusters. For those who do not know, Fe/S clusters are essential for all cellular life, as they keep maintenance of the protein production. They are themselves a part of integral proteins, such as polymerases replicating DNA and RNA and proteins involved in transcription and translation. This study provides more evidence of the importance of this process that now seems to be required in all forms of nuclear life.
In the end, the question I proposed in the title is not easy to answer, but looking at the simplest cells that try to lose as much as they can in order to survive might just give a few hints.
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