Endosymbiosis

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In the field of evolutionary biology, endosymbiosis is the hypothesis that the larger, more complex organelles within the cell actually came to be by one cell ingesting another. Specifically, those organelles are the chloroplast and the mitochondria. As these organelles are found in most eukaryotes (and mitochondria are found in all), the endosymbiotic events are very important steps in the evolutionary history of "higher" life.

Contents

[edit] The organelles

Endosymbiotic theory accounts for the origin of two centrally important organelles: the mitochondria and the chloroplasts.

[edit] Mitochondria

It has long been known that mitochondria have their own particular genetic code (mitochondrial DNA) which is independent of the DNA within the nucleus of the cell. The general hypothesis is that the ancestral mitochondrium was simply a cell endowed with special proteins involved in the electron transport chain. This cell was consumed by another one, but instead of destroying it, the larger cell preserved the ATP-producing cell and gained a massive competitive advantage. At first, this would have been considered simply a symbiotic relationship, but the lives of the two cells became so intertwined that they now function as a single entity.

The precursor to the mitochondrium was likely an aerobic bacterium that lived inside a large anaerobic cell.[1] Although the lives of the two were independent, they did share a common channel for reproduction (that's important...see "endosymbiositic theory and symbiosis", below).

[edit] Chloroplasts

As with the mitochondria, chloroplasts[wp] would have given a large competitive advantage to any cell which consumed but did not destroy the photosynthetic cell. Also like mitochondria, chloroplasts have their own particular genetic code and structure. Comparisons of photosynthetic lineages also suggest that the variety of chloroplasts may have arisen from multiple endosymbiotic events.

The various precursors to modern chloroplasts were all likely some form of cyanobacteria[wp] (blue-green algae) and, like the chloroplasts, they were originally responsible for their own replication.

[edit] Evidence for endosymbiosis

The primary evidence for endosymbiotic events are the genetic make-ups of the organelles in question. Not only do they have their own, separate DNA, but this DNA is also held in a circular loop, an arrangement characteristic of prokaryotes. Furthermore, the organelles have their own support structures, such as ribosomes, DNA/protein synthesis mechanisms, and even an independent plasma membrane.

[edit] Endosymbiotic theory and symbiosis

Endosymbiosis may be simply considered a special form of a mutualistic relationship. In the beginning, the two partners benefited each other, but they were not necessary for each other's survival. After all, prokaryotic organisms are able to survive without mitochondria or chloroplasts, using only glycolytic metabolism. Now, however, the relationship is so intimate that they cannot survive without each other (to the point of being one organism). Why is this the case?

As mentioned above, mitochondria and chloroplasts share a reproductive channel with their host cell. That is, when the host passes on its genes, the mitochondria/chloroplasts also get a chance to pass on their genes. Using selfish gene theory as his base, Richard Dawkins suggests that such a system would almost certainly promote increasingly mutualistic relationships, until the point of mutual necessity.[2] In other words, because the actions of the chloroplasts benefit the host and in turn ensure the chloroplasts' successful reproduction, natural selection will shift the nature of the relationship toward the extreme mutualistic end of the symbiotic continuum.

[edit] Footnotes

  1. Solomon, Berg, and Martin, Biology: Seventh Edition
  2. Richard Dawkins, The Extended Phenotype
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