Monday, April 13, 2009
Endosymbiosis
In the beginning of the book, Carroll slightly touches on the theory of endosymbiosis (pg. 85-86). However, he doesn’t really elaborate on the specifics of the process. Describe the theory behind endosymbiosis.What are the"requirements" (i.e. shape of DNA, ribosomes, etc). What are the specific DNA sequences that have remained the same between mitochondria and plastids and prokaryote DNA. What faults are there in the logic of endosymbiosis. How have other theories branched off this original theory in order to repair these faults in logic? Carroll states that endosymbiosis “confuses the family tree” (85). Elaborate on these confusions.What microorganism's DNA is “tainting” the evolutionary tree of eukaryotes? In addition, explain how the evolution of the endomembrane system either refutes or supports the theory of endosymbiosis.
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The endosymbiotic theory concerns the origins of mitochondria and plastids (chloroplasts). The theory involves the belief that these organelles used to be separate free living prokaryotic organisms which were taken in by a bigger cell as endosymbionts. It is believed that mitochondria developed from proteobacteria and chloroplasts from cyanobacteria. The biggest link in this theory is that mitochondria and chloroplasts both have their own distinct DNA. This is also possible because prokaryotes are able to take up DNA from their environment and incorporate it into their own genome, in the form of transformation, which is the genetic alteration of a cell due to the direct uptake and expression of foreign genetic material.
ReplyDeleteTransformation is when a live nonpathogenic cell takes up a DNA fragment that has the allele for the gene for a cell coat that protects the bacterium from a host’s immune system. The allele is then included into the bacterial chromosome replacing the native allele. This process is referred to as genetic recombination which involves the exchange of DNA segments through crossing over and now the chromosome of the DNA contains DNA derived from two different cells.
Anyways back to the link about the theory of endosymmbiosis. The main link is that the DNA in mitochondria and chloroplast both resemble the same shape and size (circular plasmids) to that of the bacteria with the closest relation. The surrounding membranes of the cell seem to derive from different origins. The outermost layer of the membrane resembles more closely to the cell’s membrane but the inner most layer of the membrane resembles more closely to that of a prokaryotic cell.
There have been recent studies where they have proven that endosymbiotic relations are definitely possible in the real world. “Recent in vitro studies have revealed that a certain Mycobacterium can survive and multiply within free-living amoebae. It is believed that protozoan’s function as host cells for the intracellular replication and evasion of Mycobacterium spp. under harmful conditions.” They had a 6-year study which definitely proves that the survival of the amoebae and mycobacterium was not just a simply fluke but a genuine relationship which allows it to coexist.
One problem that exists is neither mitochondria nor plastids can survive in oxygen nor outside the cell, having lost many essential genes required for survival. That however, can be attributed to gene fossilization where the use for that gene was no longer necessary so then in order to truly become energy efficient and produce excess energy for the cell to use, it cut down on its maintenance of transcription and translation of a specific gene.
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2526340
http://en.wikipedia.org/wiki/Endosymbiotic_theory
http://en.wikipedia.org/wiki/Transformation_(genetics)
http://jcb.rupress.org/cgi/content/abstract/13/3/383
Endosymbiosis is the theory that one organism lives in another organism, utilizing its resources and environment that it provides. This theory was first applied to chloroplasts in 1905 by the Russian botanist Konstantin Mereschkowsky. It was then applied to mitochondria in the 1920s by Ivan Wallin. At the advent of the endosymbiosis theory, it was only that, a theory. Today, however, we have detailed scientific evidence proving that this explanation of life on earth is more than just a theory. The main chunk of evidence is found in the actual genome of mitochondria and chloroplasts themselves. First, the DNA of the mitochondria does not resemble that of the cell in which it resides. Secondly, the mitochondrial DNA (mtDNA) actually closely resembles the genome of a certain living Proteobacteria called Rickettsia prowazeki. Scientists have found that this mtDNA encodes for only a limited number of RNAs and proteins that act to form a functioning mitochondria. The same goes for chloroplast DNA. In mtDNA, the specific proteins that are coded for are involved in respiration (such as NADH, succinate, ubiquitol, etc.) and the electron transport chain (ATP synthase, etc.). This would make sense because the mitochondria are the organelles where cellular respiration occurs. Also, the eukaryotic genome does not code for these proteins, thus without the mitochondria, these cells could not gain the energy they need for survival. This is where the endosymbiotic relationship comes into play. At the evolutionary time period when a eukaryotic cell came about, a eubacteria had to have fused into the membrane of the protist or whatever organism it was. This explains the mitochondrial double membrane. The eubacteria was probably in an environment where it was not gaining enough resources or protection for survival, so it was selective for it to hide and live in the membrane of another organism. Inversely, some protist was adapting to an environment where more energy was needed to move around and survive. Then, when it gained an endosymbiont capable of making energy aerobically, it lived to produce more cells that were similar, thus spawning the wide array of eukaryotic cells that exist today. (1)
ReplyDelete1. Michael W Gray, Gertraud Burger, B Franz Lang. "Mitochondrial evolution. " Science 283.5407 (1999): 1476-81. Platinum Periodicals. ProQuest. 14 Apr. 2009. http://www.proquest.com/
The endosymbiotic theory tries to explain the origins of mitochondria and chloroplasts or plastids. The theory states that these small mitochondrial and plastids were at one point free living prokaryotes. Since prokaryotes are much smaller than eukaryotes; they are able to live inside them without taking up much space. However, this does not explain how eukaryotes came to be, because not only to they contain mitochondria or plastids, they are also much bigger in size. Scienctists believe that eukaryotes came around from the fusion of two or more archae bacteria cells. Their cytoplasm fused together and their membranes combined to form. However, this still does not explain the origins of plastids and mitochondria. The theory states that proteobacteria and cyanobacteria were taking in as endsymbionts. Proteobacteria evolved to become mitochondria, and cyanobacteria became plastids. The original prokaryotes had to be engulfed by phagocytosis or endocytosis, and by doing this a double membrane is set up. The cells were engulfed and thus have a double membrane structure as we learned earlier in the year. The outer membrane is very similar to a eukaryotic membrane that would have surrounded it during endocytosis. The inner membrane, however, is very similar to a prokaryotes plasma membrane. The size of prokaryote compared to mitochondria and plastid is almost the same; they are both 1-10 microns long. A look inside the mitochondria and plastids show that; the ribosomes that are found in each are type 70s that is the same that is found in ribosomes of prokaryotes. The DNA of a eukaryote is a complex chromosome system found in a nucleus, however the DNA of prokaryotes, mitochondria, and plastids are found to be one circular chromosome. The DNA also codes for reproduction and how it will happen; eukaryotes divide by mitosis and then split the cytoplasm in half when the process is over. However, no organelles are doubled, only the DNA. This does not explain why mitochondria and chloroplasts divide by binary fission just like prokaryotic cells do; they also do this independent of the rest of the cell, like it was once its own living thing. The reason prokaryotes and eukaryotes are different is their way to get energy with an electron transport chain. Only prokaryotes have this along with mitochondria and chloroplasts. All these similarities between mitochondria and plastids with prokaryotes show that they must have been engulfed and accepted by the cell because they became a functioning unit. This also shows that together they have a mutualistic relationship because they are accepting each other, and have been together for the most of time. These show how the theory of endosymbiosis works because the similarities of a prokaryote and the mitochondria and plastids resemble each way too much.
ReplyDeleteThere has been much evidence that proves the endosymbiotic theory true, but there are still some faults in how it could exist. Mitochondria and Plastids only code for a few proteins by themselves, and most of the other uses of proteins are coded by the eukaryotic cell in its nuclear DNA. Since the DNA of prokaryotes is one singular loop, it looks to be as efficient as possible. A close study of DNA of plastids and mitochondria show that there are parts of introns located in their genetic code. This probably came from free floating RNA that was translated to DNA, or other fragments of DNA that were left alone. The most striking characteristic that goes against endosymbiosis is that no eukaryotic cell has been found without mitochondria and plastids. This means that from the creation of the first eukaryotic cell that a plastid or mitochondria was located in it. This could have happened because when the few prokaryotic cells that came together a small prokaryote was trapped in their, but that doesn’t explain the double membrane. There are faults in the theory, that right not have no answer and in the end will probably have to accept the fact that it just happened and there will be faults because man cannot go back in time to visit the creation of the first eukaryote. To account for these faults there have new hypothesis created like the hydrogen hypothesis. This states that the mitochondria were created inside a prokaryotic cell in its own phagocytosis and then as the prokaryotes came together they already had the mitochondria inside of them. This has many faults as well, because it does not explain the double membrane of mitochondria and plastids.
Overall the endosymbiotic theory is very well put together to show the creation of mitochondria and chloroplasts. Their double membrane which shows the outside being eukaryotic because of similar proteins being found as well as the many phosphates in it. The inner membrane is similar to a prokaryote because it is a plasma membrane and only they have one. The inner membrane is also home to the election transport chain only to be found in prokaryotes. This membrane structure itself if proof that endosymbiotic theory exists.
http://en.wikipedia.org/wiki/Hydrogen_hypothesis
http://www.morris.umn.edu/~goochv/CellBio/lectures/endo/endo.html
http://www.biology.iupui.edu/biocourses/N100/2k2endosymb.html