The Controversy of the Birth of the Modern Cell

In secondary school I remember being told wen being taught about the cell that the mitochondria, which has its own DNA and produces the energy that is required in the cell, was engulfed by a cell billions of years ago and that humans eventually evolved for that event to have mitochondria in each of our cells (except red blood cells). However due to a new study[1] from the university of Virginia we may have to rewrite the text books.

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Mitochondria are membrane bond organelles that are the “powerhouse” of the cell. In basic terms glucose that is taken in by the diet and processed to give loaded up electron carriers which are passed onto proteins in the mitochondrial membrane. Mitochondria have two membranes and a number of protein complexes used the electrons to pump hydrogen ions across the inner membrane, causing a gradient. This gradient then flows back through ATP synthase which mechanically rotates, using the energy to add a phosphate to adenosine diphosphate, making adenosine triphosphate also known as cellular energy.

It is often said that the endocytosis of a mitochondria to a proto-bacteria around two billion years ago caused a symbiotic relationship that was so favourable that the whole of our known kingdoms of life resulting from it. This hypothesis could be questioned however as it is not known how this occurred. The fateful encounter hypothesis which is what is most commonly accepted is that it was solely by chance that a single mitochondria was taken up by a single ancient bacterial cell that then divided together to give each daughter cell a mitochondria of its own. It is thought that this was so powerfully advantageous that they became the dominant cellular basis of life that developed into a eukaryotic cell.

A different theory from the fateful encounter is now emerging with evidence from this new publication. It is suggested that the mitochondria was not an innocent free living organelle that happened to become incorporated into a bacteria but it may have been a pathogen which infected the cell and by evading the host defenced became part of the cellular make up after billions of years and however many inconceivable cell divisions.

By looking at the reconstructed genome of mitochondrial ancestors these researchers can investigate the characteristics of mitochondria before they became part of our cells. Mitochondria have their own mini genome which consists of only 14 genes in humans[2] compared to the nuclear genome of ~23000 genes. This was one of the first pieces of evidence that caused scientists to think that mitochondria were once free living. These genes are now mainly involved in the mitochondria’s primary function, ATP production, however many of the original genes on the mini genome have been moved to the nuclear DNA over many years. The paper states “we identified 4,459 genes belonging to 394 families as mitochondria-derived nuclear genes” so once there were so many more mitochondrial specific genes for many different functions.

So, why do they think that mitochondria were parasitic? Firstly, in the genome of the ‘pre-mitochondria’ they could not identify the correct enzymes to synthesis all the essential amino acids suggesting that they would need to be obtained from a host cell after invasion. They also found five amino acid transporter genes which supports this idea. Secondly, in the genomes studied many were seen to possess genes encoding for flagella components (the wiggly bit on some bacteria that allow it to swim). This is often seen on most invasive bacterial types so that can direct their movement towards their host. And finally, they also found genes involved in parasite type of ATP/ADP translocase, which does the very opposite of what we know mitochondria to do today, they stole ATP molecules from the host and gave back ADP (which is much less useful). It is suggested that the ADP/ATP translocase was replaced with a protein much similar to what there is now once the flow of ATP changed direction and mitochondria started giving ATP to the cells cytosol to be used in many functions. If this theory is correct the change of ATP directional flow may be the main catalyst for the development of complex multi-cellular systems such as ourselves as we use up so much ATP in each of our cells on a daily basis.

So what if there wasn’t one fateful encounter but many encounters of the free-living mitochondria with many bacteria and it just so happened that in one type of bacteria it wasn’t destroyed by the host defences and went on to make all eukaryotic cells on earth today.

References:
[1] Wang, Z., & Wu, M. (2014). Phylogenomic Reconstruction Indicates Mitochondrial Ancestor Was an Energy Parasite PLoS ONE, 9 (10) DOI: 10.1371/journal.pone.0110685

[2] Anderson, S., Bankier, A., Barrell, B., de Bruijn, M., Coulson, A., Drouin, J., Eperon, I., Nierlich, D., Roe, B., Sanger, F., Schreier, P., Smith, A., Staden, R., & Young, I. (1981). Sequence and organization of the human mitochondrial genome Nature, 290 (5806), 457-465 DOI: 10.1038/290457a0

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