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Mitochondria

Page history last edited by PBworks 13 years, 3 months ago

Mitochondria

 

Origin

As mitochondria contain ribosomes and DNA, and are only formed by the division of other mitochondria, it is generally accepted that they were originally derived from endosymbiotic prokaryotes. Studies of mitochondrial DNA, which is often circular and employs a variant genetic code, show their ancestor, the so-called proto-mitochondrion, was a member of the Proteobacteria. In particular, the pre-mitochondrion was probably related to the rickettsias, although the exact position of the ancestor of mitochondria among the alpha-proteobacteria remains controversial. The endosymbiotic hypothesis suggests that mitochondria descended from specialized bacteria (probably purple nonsulfur bacteria) that somehow survived endocytosis by another species of prokaryote or some other cell type, and became incorporated into the cytoplasm. The ability of symbiont bacteria to conduct cellular respiration in host cells that had relied on glycolysis and fermentation would have provided a considerable evolutionary advantage. Similarly, host cells with symbiotic bacteria capable of photosynthesis would also have an advantage. In both cases, the number of environments in which the cells could survive would have been greatly expanded.

 

This relationship developed at least 2 billion years ago and mitochondria still show some signs of their ancient origin. Mitochondrial ribosomes are the 70S (bacterial) type, in contrast to the 80S ribosomes found elsewhere in the cell. As in prokaryotes, there is a very high proportion of coding DNA, and an absence of repeats. Mitochondrial genes are transcribed as multigenic transcripts which are cleaved and polyadenylated to yield mature mRNAs. Unlike their nuclear cousins, mitochondrial genes are small, generally lacking introns, and many chromosomes are circular, conforming to the bacterial pattern.

 

A few groups of unicellular eukaryotes lack mitochondria: the microsporidians, metamonads, and archamoebae. On rRNA trees these groups appeared as the most primitive eukaryotes, suggesting they appeared before the origin of mitochondria, but this is now known to be an artifact of long branch attraction — they are apparently derived groups and retain genes or organelles derived from mitochondria (e.g. mitosomes and hydrogenosomes). There are no primitively amitochondriate eukaryotes, and so the origin of mitochondria may have played a critical part in the development of eukaryotic cells.

 

 

Mitochondrial genomes have many fewer genes than do the related eubacteria from which they are thought to be descended. Although some have been lost altogether, many seem to have been transferred to the nucleus. This is thought to be relatively common over evolutionary time. A few organisms, such as Cryptosporidium, actually have mitochondria which lack any DNA, presumably because all their genes have either been lost or transferred.

 

Hydrogen hypothesis

The hydrogen hypothesis is a model proposed by William Martin and Miklos Muller in 1998 that describes a possible way in which the mitochondrion developed in the first eukaryotic cell within the endosymbiotic theory framework.

 

According to the hydrogen hypothesis the first eukaryotic cell did not appear as a consequence of a primitive host cell engulfing a primitive bacteria, which wasn't fully digested and eventually became the mitochondrion as the current endosymbiotic theory suggests. It claims instead that the host - a methanogen archaea which used hydrogen and carbon dioxide, producing methane - and a primitive eubacteria, the future mitochondrion, which produced hydrogen and carbon dioxide as byproducts of anaerobic respiration, started a symbiotic relationship based on their byproducts.

 

The idea originated when Martin assisted at a talk by Muller on hydrogenosomes. These occur in anaerobic eukaryotic cells replacing the mitochondrial ATP production role, and producing large amounts of hydrogen and carbon dioxide. One of Muller's slides presented a cluster of methanogens around a hydrogenosome inside a eukaryotic cell they had invaded.

 

If the hypothesis is correct it would imply that eukaryotes are very close to archaea and appeared relatively late. This contradicts the current view which states that archaea and eukarya split before the modern groups of archaea appeared.

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