Supporting Endosymbiosis Hypothesis

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Dr. Lynn Margulis was the first to propose the endosymbiotic hypothesis in 1967. Margulis' original hypothesis predicts that mitochondria and chloroplasts (organelles) were originally small, free-living prokaryotes that got ingested inside a larger ancestral bacterial cell, but were not digested. They survived inside the larger cell and continued to carry out their metabolic functions, i.e. photosynthesis or chemical energy conversion. Over evolutionary time, they and their DNA became integrated into the cell to form a new type of life called eukaryotes. Endosymbiosis suggests that certain organelles were once free living organisms that were taken into larger cells and the arrangement was beneficial to both organisms. First off, mitochondria and chloroplasts are similar in size and morphology to bacterial prokaryotic cells, though the mitochondria of some organisms are known to be morphologically variable. Also, mitochondria and chloroplasts divide by binary fission, just as bacteria do, and not by mitosis as eukaryotes do. Additionally, the DNA of mitochondria and chloroplasts are different from that of the eukaryotic cell in which they are found. As Margulis predicted, both types of organelles include DNA that is like that of prokaryotes- circular, not linear. The DNA of these organelles evolves independently and at a different rate from the nuclear DNA of the eukaryotic cell. Mitochondria arise from pre-existing mitochondria and chloroplasts arise from pre-existing chloroplasts (not manufactures through the direction of nuclear genes). A fairly simple piece of evidence for the endosymbiotic hypothesis is the fact that both mitochondria and chloroplasts have double phospholipid bilayers. This appears to have arisen by mitochondria and chloroplasts entering eukaryotic cells via endocytosis. Both purple, aerobic bacteria (similar

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