"Describe The Theory Of Endosymbiosis And The Arguments For And Against This Theory"

1129 words - 5 pages

Dr Lynn Margulis, a biologist from Boston University, proposed in the 1960's, the theory of endosymbiosis. It was then officially published in her book "Symbiosis in Cell Evolution". The theory suggests that cells originated as communities of interacting entities a could join together in a specific order. Organelles like mitochondria and chloroplasts were themselves prokaryotic cells before the development of eukaryotic cells. It was believed that a host cell, probably similar to amoeba, ingested a smaller cell by phagocytes. They ingested in a way that the host cells are unable to digest them and they were allowed to survive. Also, the DNA of the ingested cell would be coded into the host ...view middle of the document...

Eukaryotic cells like animals, fungi and some protists, then evolved to have cells that contains mitochondria and, or chloroplasts, which are now instead of a separate cell, an organelle.Evidence supporting the theoryThere are many evidence that supports the theory by suggesting that mitochondria and chloroplasts could be prokaryotic cells. This is due to their similarities in structure and reactions with prokaryotic cells. Both mitochondria and chloroplast are similar in size with prokaryotic cells, which are on average, 10 times smaller than eukaryotic cells. They lack a nucleus but have a circular DNA which lacks histones, similar to prokaryotic cells. This independent DNA from the nucleus DNA allows them to reproduce by binary fission, which is also the way prokaryotic cells reproduce. Mitochondria and chloroplasts have the 70S ribosomes like prokaryotic cells, unlike the 80S ribosomes found in the cytoplasm of eukaryotic cells. This allows them again, to synthesis some of their own proteins. Mitochondria and chloroplasts have two or more membranes, with their inner membrane always different from other organelles but similar to prokaryotic cell membrane. The protein transcriptions of the first amino acid for mitochondria and chloroplasts are similar to prokaryotic cells as it is always starting with fMet instead Met for eukaryotic cells. Antibiotics like streptomycin which blocks protein synthesis in bacteria also works in mitochondria and chloroplasts, which on the other hand, does not affect protein synthesis in the cytoplasm of eukaryotic cells. Also, antibiotics like rifampicin that inhibits RNA polymerase of bacteria and mitochondria does not affect that of eukaryotic nucleus.Other evidence supporting the theoryThe relatively small genomes of mitochondria and chloroplasts compared with other organisms supports the theory that the 'organelles' and the host cells have became dependant towards one another through the relationship. On the other hand, the lack of complete DNA for the organelles within the nucleus and the existence of their independent DNA suggests that they had been independent cells.Evidence against the theoryThe reliability of the theory raises many questions. How did the ingested cell manage to reproduce in such tight space, inside another cell? How did the DNA of the ingested cell able to be passed into the host's DNA? As...

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