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The Origin of the Mitochondrion.

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Sarah Wagg 010404720 BI 211 October 21, 2002 The Origin of the Mitochondrion Introduction Cells of almost all eukaryotic organisms contain mitochondria. These organelles are vital to the process of aerobic respiration, the conversion of organic molecules to energy in the form of ATP.l. In some cases, there is a single, large mitochondrion, but more often, there are hundreds or thousands in a cell. The number is generally correlated with the metabolic activity of the cell. These and other membrane-bound organelles are not found in prokaryotic cells (those that lack a nucleus), though these more primitive organisms have respiratory membranes that are quite similar to those in mitochondria. This suggests that the mechanisms are very similar, leading to interest in tracing the evolutionary origins of the modern mitochondrion. In order to accomplish this, this paper focuses on the explanation of mitochondrial origins using molecular, genetic, and evolutionary approaches. There are very few papers that combine these approaches, often making it difficult to understand and compare the theories of mitochondrial origin. The following will present a variety of opinions on how endosymbiosis attempts to bridge the gap between the organization of prokaryotic and eukaryotic cells and will highlight the evolutionary significance of anaerobic eukaryotes that do not have mitochondria. Endosymbiotic Theory The original theory behind the origin of the organelles in a eukaryotic cell involves the infolding and specialization of the cell's membrane, a process known as direct filiation (Margulis 1981). This theory is reasonable from an evolutionary standpoint since it is a process involving selection-driven modifications, the basis of evolutionary theories. It does not, however, explain the similarities between mitochondria and free-living bacteria, nor does it account for the large gaps between aerobic and anaerobic organisms appearing in the fossil record (Schwartz and Dayhoff 1978). A more appropriate explanation of the evolution of eukaryotic cells is the endosymbiotic theory. First proposed by C. ...read more.


The bacterium would have invaded the host and reproduced itself inside, becoming the endosymbiont that supplied energy to the host if it did not kill it. This concept supports Margulis' idea that the obligate parasite Paracoccus denitrificans was the endosymbiont. Finally, in order to understand how the symbiosis may have occurred, it is vital to decide whether the association took place only once (a monophyletic origin) or if an endosymbiosis between the two cells occurred several times, each with similar results (polyphyletic origin) (Margulis 1978). The fact that the mitochondria of plants, animals and fungi are very similar in appearance and function shows that the protomitochondria were acquired symbiotically before differentiating into the various kingdoms, and before photosynthetic plastids had been acquired. Hence, the common ancestors of all eukaryotic cells were heterotrophic, mitochondria-containing protoeukaryotes, demonstrating the monophyly of mitochondria (Schwartz and Dayhoff 1975). Also, recent rRNA and protein studies have shown vast genetic and molecular similarities between mitochondria of several species, all of which have mitochondrial DNA that is a subset of that of Reclimonas Americana, an ancient organism with the greatest number of mitochondrial genes (Blackstone et al. 1995). It would not be reasonable to suggest that these similarities could exist if several separate endosymbioses occurred. There should be more genetic variation if this were the case, as the mitochondria in different organisms evolved according to their own environmental pressures. Therefore, it is apparent that the endosymbiosis that resulted in eukaryotic cells with mitochondria must have happened only once, and all other eukaryotic cells have descended from the original partnership. Maintaining the Symbiosis Once the protomitochondrion was incorporated into the host's cytoplasm, there must have been some mechanism to allow the continuation of the endosymbiosis. In other words, there must have been some selection that would have favoured the propagation of an anaerobic protoeukaryote that had an aerobic symbiont living inside of it. ...read more.


Martin mentions that the process in question is simply the reverse of when photosynthesis developed in plants, a concept that is not thought to be overly drastic (Martin 2000-1). Genetic sequencing has yet to test this hypothesis completely, but even if it does, reconstructing past events is extremely difficult and some ambiguous answers may remain. Conclusions Though the theories presented above have concrete evidence to support them, they are not intended to be the definitive answer to the question of mitochondrial origins. Instead, they are dynamic hypotheses that should be supplemented with ideas of researchers from various backgrounds. Each theory has its own flaws, which can be expected, since they predict the nature of a biological interaction that began a billion or more years ago. It seems that the hydrogen hypothesis is gaining favour in the eyes of some researchers because of the molecular evidence and the possible identity of the host, but it still fails to identify any living ancestor of the facultatively anaerobic symbiont. The endosymbiotic theory is still the most widely used, since it gives a more comprehensive, plausible explanation, but it lacks a well-defined host. Though I find the hydrogen hypothesis to be a fascinating explanation, the traditional endosymbiotic theory has been more successfully documented in scientific literature, and seems to be very promising. K.W. Jeon's experiments alone show how a parasitic relationship can become an obligate symbiosis, an event which I think to be very similar to the acquisition of the protomitochondrion. Mitochondrial genome analysis might be the key to determining how and when endosymbionts became functional organelles. It may also show that one of the two main theories is correct, or that neither are possible. The fact of the matter remains that although scientific advances are made every day, we will never be able to determine the exact sequence of events that led to the formation of our mitochondria. We can only speculate on these events since we cannot go back in time to observe them. ...read more.

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