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 Biology II (Assignment) July 15,2010

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PostSubject: Biology II (Assignment) July 15,2010   Thu Jul 15, 2010 6:21 am

1.What Is Endosymbiotic Theory ?



The endosymbiotic theory concerns the origins of mitochondria and plastids (e.g. chloroplasts), which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms that were taken inside the cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteria.


HISTORY

The endosymbiotic theory was first articulated by the Russian botanist Konstantin Mereschkowski in 1905.[1] Mereschkowski was familiar with work by botanist Andreas Schimper, who had observed in 1883 that the division of chloroplasts in green plants closely resembled that of free-living cyanobacteria, and who had himself tentatively proposed (in a footnote) that green plants had arisen from a symbiotic union of two organisms.Ivan Wallin extended the idea of an endosymbiotic origin to mitochondria in the 1920s. These theories were initially dismissed or ignored. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts (for example studies by Hans Ris), combined with the discovery that plastids and mitochondria contain their own DNA[5] (which by that stage was recognized to be the hereditary material of organisms) led to a resurrection of the idea in the 1960s.

The endosymbiotic theory was advanced and substantiated with microbiological evidence by Lynn Margulis in a 1967 paper, The Origin of Mitosing Eukaryotic Cells. In her 1981 work Symbiosis in Cell Evolution she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic spirochaetes that developed into eukaryotic flagella and cilia. This last idea has not received much acceptance, because flagella lack DNA and do not show ultrastructural similarities to prokaryotes. See also Evolution of flagella.

According to Margulis and Dorion Sagan, "Life did not take over the globe by combat, but by networking" (i.e., by cooperation).The possibility that peroxisomes may have an endosymbiotic origin has also been considered, although they lack DNA. Christian de Duve proposed that they may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular oxygen in the Earth's atmosphere. However, it now appears that they may be formed de novo, contradicting the idea that they have a symbiotic origin.
It is also believed that these endosymbionts transferred some of their own DNA to the host cell's nucleus during the evolutionary transition from a symbiotic community to an instituted eukaryotic cell. This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between prokaryotic species, even if they are not closely related. Prokaryotes can take up DNA from their surroundings and have a limited ability to incorporate it into their own genome.


A theory on the Origins of Eukaryotic Cells: Mitochondria and Chloroplasts


There are a great many differences between Eukaryotic cells and Prokaryotic cells in size, complexity, internal compartments.
However, there is a curious similarity between prokaryotic cells and the organelles of eukaryotic cells. Some of these similarities were first noted in the 1880s, but were largely ignored for almost a century!


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PostSubject: Hmmm GOOD :D   Thu Jul 15, 2010 8:26 am

Ty sa assign catiis lol! cane Very Happy
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PostSubject: Re: Biology II (Assignment) July 15,2010   Sat Jul 17, 2010 12:25 am

ty sa Assignment!!! lol! elephant
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