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Endosymbiotic Theory and others Flashcards | Quizlet
In the 1970s scientists developed new tools and methods for comparing genes from different species. Two teams of microbiologists one headed by Carl Woese, and the other by W. Ford Doolittle at Dalhousie University in Nova Scotia studied the genes inside chloroplasts of some species of algae. They found that the chloroplast genes bore little resemblance to the genes in the algae's nuclei. Chloroplast DNA, it turns out, was cyanobacterial DNA. The DNA in mitochondria, meanwhile, resembles that within a group of bacteria that includes the type of bacteria that causes typhus (see photos, right). Margulis has maintained that earlier symbioses helped to build nucleated cells. For example, spiral-shaped bacteria called spirochetes were incorporated into all organisms that divide by mitosis. Tails on cells such as sperm eventually resulted. Most researchers remain skeptical about this claim.
1- If chloroplasts, in particular, were once independent cells, then there could only have been one outcome if one were swallowed by a larger cell: namely, it would have been digested by the parent cell and used as food. This must be so, because even if we assume that the parent cell in question took such a cell into itself from the outside by mistake, instead of intentionally ingesting it as food, nevertheless, the digestive enzymes in the parent cell would have destroyed it. Of course, some evolutionists have gotten around this obstacle by saying, “The digestive enzymes had disappeared.” But this is a clear contradiction, because if the cell’s digestive enzymes had disappeared, then the cell would have died from lack of nutrition.
endosymbiotic theory Study Sets and Flashcards | Quizlet
Not all eukaryotes harbor the classical aerobic mitochondria. The absence of classic mitochondria in these species led to a hypothesis in which these species, collectively called Archezoa, represent the descendants of a hypothetical lineage coined ‘the proto-eukaryote’. The proto-eukaryote supposedly had all typical eukaryotic features but lacked mitochondria. But it has now been demonstrated that these Archezoa actually contain derived mitochondria, organelles that were once mitochondria in their evolutionary past (anaerobic mitochondria, mitochondria-like organelles, hydrogenosomes and mitosomes) . This consequently suggested that the Last Common Eukaryotic Ancestor (LECA) contained the ancestor of all mitochondria and derived mitochondria and, assuming that true Archezoa have never existed, the acquisition of the mitochondrial ancestor triggered eukaryogenesis. This in turn means that the host in the mitochondrial endosymbiosis must have been a prokaryote.
One additional important conclusion that can be drawn from comparative genomics analyses addressing mitochondrial evolution, is that all mitochondria form a monophyletic group. This basically means that all mitochondria and mitochondria-like organelles from all extant eukaryotic lineages can be traced back to a single ancestor. This inherently also implies that mitochondrial endosymbiosis happened only once in eukaryotic evolution. There are several lines of evidence that support monophyly of the mitochondria :
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These questions all try to figure something out that occurred in the distant past. Therefore you would expect that if you want to answer them, you start looking for fossils or other signs from the past. But actually, the favored method is to look at modern day organisms. By comparing them, it is possible to say something about their common ancestor. Likewise, when we want to say something about the mitochondrial ancestor, we look at modern day mitochondria and Alphaproteobacteria. In this project, we have (partially) sequenced the genome a number of previously unknown Alphaproteobacteria that are candidate members of new alphaproteobacterial lineages. By comparing them with the genomes of known Alphaproteobacteria and mitochondria using phylogenomic methods, we will be able to gain new insights on the nature of the mitochondrial ancestor.
Even though the origin of mitochondria in these aspects are quite well understood, several questions remain. What kind of conditions drove the mitochondrial endosymbiosis? What was the nature of this relationship? What was the identity of the host?
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History | The Endosymbiotic Hypothesis
Mitochondria are eukaryotic organelles that are mainly known for their role as ‘the powerhouses of the cell’, supplying the bulk of energy that the cell demands. Mitochondria are also very interesting from a evolutionary point of view, because they have features that make them very similar to bacteria:
Endosymbiotic | definition of Endosymbiotic by Medical …
While the situation envisioned by the endosymbiosis hypothesis is occurring through a process of trial and error, what effects would this have on the DNA of the parent cell? As we have seen, any change in a DNA molecule definitely does not result in a gain for that organism; on the contrary, any such mutation would certainly be harmful. In his book The Roots of Life, Mahlon B. Hoagland explains the situation:
Endosymbiosis | definition of endosymbiosis by Medical …
3- There is great harmony within the cell which random mutations cannot account for. There are more than just one chloroplast and one mitochondrion in a cell. Their number rises or falls according to the activity level of the cell, just like with other organelles. The existence of DNA in the bodies of these organelles is also of use in reproduction. As the cell divides, all of the numerous chloroplasts divide too, and the cell division happens in a shorter time and more regularly.
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2- Again, let us assume that all the impossible happened and that the cell which is claimed to have been the ancestor of the chloroplast was swallowed by the parent cell. In this case we are faced with another problem: The blueprints of all the organelles inside the cell are encoded in the DNA. If the parent cell were going to use other cells it swallowed as organelles, then it would be necessary for all of the information about them to be already present and encoded in its DNA. The DNA of the swallowed cells would have to possess information belonging to the parent cell. Not only is such a situation impossible, the two complements of DNA belonging to the parent cell and the swallowed cell would also have to become compatible with each other afterwards, which is also clearly impossible.
Endosymbiosis - The Appearance of the Eukaryotes
The endosymbiosis hypothesis is based on the fact that the mitochondria of animal cells and the chloroplasts of plant cells contain their own DNA, separate from the DNA in the nucleus of the parent cell. So, on this basis, it is suggested that mitochondria and chloroplasts were once independent, free-living cells. However, when chloroplasts are studied in detail, it can be seen that this claim is inconsistent.
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