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MODELS OF EUKARYOTIC ORIGN

Two of the major ideas about the origin of eukaryotic cells are known as the autogenesis hypothesis and the endosymbiosis hypothesis. According to the autogenisis model, eukaryotic cells evolved through the specialization of internal membranes that originally were part of the plasma membrane of a prokaryote. The internal membrane system including the nuclear envelope, Golgi appartaus, endoplamic reticulum and other organelles enclosed by a single membrane may have resulted from the plasma membrane folding in on itself. Mitochondria and choloroplasts may have developed their double membranes through more complex folding.




According to the endosymbiosis model, the ancestors of eukaryotic cells were symbiotic prokaryotic cells, with certain species living within larger prokaryotes. One modern example of endosymbiosis is found in the digestive cells of Hydra viridis, which contain algal cells of the genus  Chlorella. These digestive cells recognize the appropriate algae and eject any others. Accepted algae are moved to the base of the digestive cell where they reproduce by mitosis until the normal algal population is reached. Therefore the algae reproduce when the host cell reproduces. The presence of the algae enables the hydra to survive when food is scarce.
Many biologist think mitochondria originated as symbiotic aerobic bactreria. The bacteria that became mitochondria were engulfed by ancestral eukaryotic cells. Before these cells acquired the bacteria they lacked the enzymes that would allow them use oxygen in respiration. The engulfed bacteria, however, were able to perform these reactions and they became the inner part of mitochondria. All the mitochondria within a eukaryotic cell arise from the division of the existing mitochondria, just as new bacteria are produced through cell division. Mitochondria divide by simple fission, splitting into two and they apparently replicate and divide their circular DNA molecule as do bacteria. Mitochondria replication, however is impossible without a cell nucleus because most of the genes required for mitochondrial division are located in the nucleus and translated into proteins by ribosomes in the cell’s cytosol. Moreover mitochondria cannot be grown in a cell free culture.
During the time that mitochondria have existed as endosymbionts in eukaryotic cells, most of their genes have been transferred to the chromosomes of the host cell. Mitochondria however still have genes of their own contained in a circular molecule  of DNA, similar to that of bacteria. This mitochondrial DNA contains several genes that produce some of the proteins essential for the functioning of mitochondria in aerobic cellular respiration.
Symbiotic events similar to those proposed for the origin of mitochondria could explain the origin of choloroplasts, which are characteristic of photosynthetic eukaryotes. Some ancestral eukaryotic cells may have ingested photosynthetic bacteria , which evolved into cchloroplasts The incorporation of photosynthetic bacteria would have conferred one great advantage on the cell; it then could manufacture its own circular DNA, but many of the genes that code for necessary chloroplast components are located in the nucleus.
It is possible that the evolution of eukaryotic cells required both autogenesis and endosymbiosis. Endosymbiosis may be responsible for mitochondria and cchloroplastand autogenesis may  be responsible for other organelles.
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