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of eukaryotic cells, the endosymbiotic theory ..
This site aims to offer an autogenic explanation for the origin of mitochondria, in the sense that the mitochondrial system evolved from existing eukaryotic endomembrane components. This in contrast to the endosymbiotic theory that assumes that mitochondria originated from bacteria that took residence in an unidentified other cell and became enslaved by an unknown mechanism. You can read my critique on the endosymbiotic theory . The alternative ‘eukaryotic origin for mitochondria’ presents a gradualistic scenario where small genetic changes to the existing endomembrane system lead to generation of a more-or-less independent organelle. Driving forces for such a scenario would be the advantages that a compartmentalization of metabolic function into specialized organelles offers to flexibity and robustness of the cell.
The endosymbiotic theory concerns the origins of and , which are of cells. According to this theory, these originated as , which came to live inside eukaryotic cells. The theory postulates that the mitochondria evolved from aerobic bacteria (probably , related to the rickettsias), and that the chloroplast evolved from endosymbiotic (autotrophic prokaryotes). The evidence for this theory is compelling as a whole, and it is now generally accepted.
Endosymbiosis and Eukaryotic Cell Evolution: Current …
Cyanobacteria have a close evolutionary relationship with eukaryotes. They have the same photosynthetic pigments as the chloroplasts of algae and land plants. Chloroplasts are the right size to be descended from bacteria, reproduce in the same manner, by binary fission, and have their own genome in the form of a single circular DNA molecule. The enzymes and transport systems found on the folded inner membranes of chloroplasts are similar to those found on the cell membranes of modern cyanobacteria, as are their ribosomes. These similarities between cyanobacteria and chloroplasts suggest an evolutionary link between the two, and can be explained by the theory of .
DNA evidence suggests that the first eukaryotes (green plants) evolved from prokaryotes (through events) between 2500 and 1000 million years ago. Fossils of eukaryotes that resemble living brown algae have been found in sedimentary rocks from China that are 1700 million years old, while possibly the oldest photosynthetic eukaryote, , comes from rocks 2100 million years old. Note that the diversity of modern algal groups, and particularly of their chloroplasts, suggests that these endosymbiotic events were not unusual. Modern algae comprise a range of organisms with very different structures but identical photosynthetic pigments. This suggests that very different host organisms have formed a symbiosis with the same photosynthetic cells. That is, the algal groups must have evolved through separate endosymbiotic events, and the group as a whole is identified on the basis of a similar level of structure, rather than on its evolutionary origins. Such groups, where the members have several different evolutionary origins, are described as .
Endosymbiosis and Eukaryotic Cell Evolution
The endosymbiotic hypothesis was popularized by . In her 1981 work she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic that developed into eukaryotic and . This last idea has not received much acceptance, since flagella lack DNA and do not show ultrastrucural similarities to prokaryotes. According to Margulis and Sagan (1996), "Life did not take over the globe by combat, but by networking" (i.e., by cooperation), and Darwin's notion of evolution driven by competition is incomplete. However, others have argued that endosymbiosis involves rather than mutualism.
I will try to base my research on the origin of mitochondria based on a model that sees evolution as an expanding system of functional modules that communicate through strictly defined interfaces. In such a system that grows by functional extensions, existing interfaces cannot be changed anymore because of the dependencies of downstream functions on this interface. By analyzing the functional modules, their conserved interfaces and its dependants, it then becomes possible to determine the logical sequence of events that occurred in evolution. I will look specifically to genetic control, protein import mechanisms, metabolic function and its relation with the cell cycle. I believe that these subsystems that are involved in the origin of mitochondria point to a eukaryotic origin as a functional extension of the endomembrane system.
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There is no fossil record recording the evolution of the eukaryotes
These tiny organelles in the cell not only produce chemical energy, but also hold the key to understanding the evolution of the eukaryotic cell. The complex eukaryotic cell ushered in a whole new era for life on Earth, because these cells evolved into multicellular organisms.
Endosymbiotic Theory in Eukaryotic Cells - UKEssays
Based on decades of accumulated evidence, the scientific community supports Margulis's ideas: endosymbiosis is the best explanation for the evolution of the eukaryotic cell.
Endosymbiotic Theory in Eukaryotic ..
Further evidence to support the endosymbiont theory is that mitochondria have their own DNA, in the form of a circular chromosome that is topologically like bacterial chromosomes. The sequence of the mitochondrial DNA most closely resembles the sequences of genes in alpha-proteobacteria. Mitochondrial ribosomes are structurally more similar to bacterial ribosomes than to eukaryotic ribosomes. Mitochondria reproduce in eukaryotic cells by fission, again resembling bacterial cell division.
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