Evolution of Eucaryotic Cell

The profound differences between eucaryotic and procaryotic cells have stimulated much discussion about how the more complex eucaryotic cell arose. Some biologists believe the original ‘protoeucaryote’ was a large, aerobic archaeon or bacterium that formed mitochondria, chloroplasts, and nuclei when it plasma membrane invaginated and enclosed genetic material in a double membrane. The organelles could then evolve independently it also is possible that a large cyanobacterium lost its cell wall and became phagocytic. Subsequently, primitive chloroplasts, mitochondria, and nuclei would be formed by the fusion of thylakoids and endoplasmic reticulum cisternae to enclose specific areas of cytoplasm. By far the most popular theory for the origin of eucaryotic cells is the endosymbiotic theory. In brief, it is supposed that the ancestral procaryotic cell, which may have been an archaeon, lost its cell wall and gained the ability to obtain nutrients by phagocytosing other procaryotes. When photosynthetic cyanobacteria arose, the environment slowly became oxic. If an anaerobic, amoeboid, phaocytic procaryote-possibly already possessing a developed nucleus- engulfed an aerobic bacterial cell and established a permanent symbiotic relationship with it, the host would be better adapted to its increasingly oxic environment. The endosymbiotic aerobic bacterium eventually would developinto the mitochondrion. Similarly, symbiotic association with cyanobacteria could lead to the formation of chloroplasts and photosynthetic eucaryotes. Some have speculated that cilia and flagella might have arisen from the attachment of spirochete bacteria to the surface of eucaryotic cells, much as spirochetes attach to themselves to the surface of motile protozoan Myxotricha paradoxa that grows in the digestive tract of termites. There is evidence to support the endosymbiotic theory. Both mitochondria and chloroplasts resemple bacteria in size and appearance, contain DNA in the form of a closed circle like that of bacteria, and reproduce semiautonomously. Mitochondrial and chloroplast ribosomes resemble procaryotic ribosomes more closely than those in the eucaryotic cytoplasmic matrix. The sequences of the chloroplast and mitochondrial genes for ribosomal RNA and transfer RNA are more similar to bacterial gene sequences than to those of eucaryotic rRNA and tRNA nucltargeles. Finally, there are symbiotic assciations that appear to be bacterial endosymbioss in which distinctive procaryotic characterisics are being lost. For example,the protozoan flagellate cyanophora paradoxa has photosynthetic organelles called cyanellase with a structure similar to that of cyanobacteria and the remain of peptidoglycan in their walls. Their DNA is much smaller than that of cyanobacteria and resembles chloroplast DNA.

Source – Prescott, Harley, and Klein’s Microbiology

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