Monday, March 23, 2009
Endosymbiosis
On page 88, Carroll attributes our eukaryotic origins to the "fusion of genomes between an endosymbiont and its host". How is this possible? What key mechanisms would need to be involved for this fusion to occur? Is this fusion possible in an endosymbiotic relationship between eukaryotes?
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First, an endosymbiont is an organism that lives in the body cells of another organism therefore forming an endosymbiosis. In most cases, the host and endosymbiont need each other in order to survive. Organelles from the eukaryotic cell, like mitochondria and chloroplasts, originated as bacterial endosymbionts. This is also known as the endosymbiotic theory.
ReplyDeleteThis theory states that chloroplasts and mitochondria evolved from bacteria that were taken in by prokaryotic cells through endophagocytosis. The bacteria lived within the prokaryotic cell, making this an endosymbiotic relationship. The mitochondria formed by first an anaerobic cell ingested an aerobic bacteria which then grew within the cell and digested the oxygen molecules to gain energy. The bacteria leaked Adenosine triphosphate into the cyctoplasm which allowed the anaerobic cell to now digest food aerobically, and then the bacteria became an endosymbiont of the cell and became the mitochondrion. Therefore, the endosymbiont is the bacteria and the host is the cell and this proves that such a fusion can occur to create the eukaryotic cell.
Both mitochondria and chloroplasts have their own DNA, which are fragments of their genome that they had when they were independent bacteria. It has been shown that mitochondria and gene sequences that are linked to a group of bacteria called the alpha-Proteobacteria while chloroplasts have DNA related to cyanobacteria.
Even today endosymbiotis is happening through intermediates. Such examples are corals, clams, and snails. Also, it is shown that the eukaryotic inner membrane is the original membrane of the prokaryotic cell while the outer membrane is the food vacuole.
The cell nucleus originated in archaea, while proteins are closely related to bacteria. Therefore, eukaryotes formed from the fusion of an archaeon and a bacterium.
Therefore, for this fusion to occur, a host an an endosymbiont is needed, so two organisms that are lacking something can come together for their benefit in order to survive and become a higher organism. Two eukaryotes can fuse together if they survive and reproduce better when combined and working off of each other. I couldn't find an example of such a fusion that has been indicated in a scientific lab but it is possible.
As Jackie mentioned, endosymbiosis is the fusion of two organisms into one. The most well know example is the endosymbiotic theory which is defined in our textbook as “a hypothysis about the origin of the eukaryotic cell, maintaining that the forerunners of eukaryotic cells were symbiotic associations of prokaryotic cells living inside larger prokaryotes,” (Campbell Glossary). This theory has been explored greatly through he study of mitochondria and chloroplasts which have been shown to have their own DNA, different from that of the rest of the cell.
ReplyDeleteEndosymbiosis is actually much more common than people may think. Oftentimes is occurs when neither organism can survive without the other but other times only one organism may benefit without the knowledge of the other or possibly even by harming the other organism. E. coli bacteria, for example, live inside of the human digestive system. They benefit from living inside of the large intestine in that it provides an environment rich in nutrients which is maintained at constant temperatures and is free from predators and in exchange they produce vitamin K, amino acids, and other growth factors which are absorbed and used by the human. The disease malaria, which is also discussed by Carroll, involves an enosymbiosis between the disease and mosquitoes. Another possible example is of the origin of cilia, flagella, centrioles, and microtubules. Some scientists believe that they came about form the fudion between an early eukaryotic cell and spirilla-type bacterium. Centrioles have been found with a small amount of their own DNA which leads scientists to believe that they once may have lived on their own.
Phagocytosis is a cellular process in which a cell basically engulfs a particle by stretching its cellular membrane around it and forming a vacuole. Usually the engulfed substance is digested but when endosymbiosis occurs, as in the case of the formation of mitochondria and chloroplasts, it is not digested and instead gives the cell a selective advantage and eventually becomes part of the cell. Phagocytosis is how endosymbiosis probably first occurred. However, with modern-day organisms that are multicellular this is not the only way. As in the case of the E. coli in humans, they made their way into the digestive system not through phagocytosis but through the mouth and traveled through the digestive system, eventually staying in the large intestine.
Sources:
http://www.abbysenior.com/biology/digestive_system.htm
http://www.iscid.org/encyclopedia/Endosymbiosis
http://en.wikipedia.org/wiki/Phagocytosis
http://www.encyclopedia.com/doc/1O142-endosymbiosis.html
According to the endosymiotic theory, the mitochondria and chloroplasts of today's eukaryotes evolved from bacteria. The evidence for this lies inside the cell itself. First, both mitochondria and chloroplasts must be made from pre-existing mitochondria and chloroplasts, as the nucleus does not contain all of the required DNA to successfully create more of these organelles. On top of that, both have their own genome, and it more closely resembles that of bacteria, as they not only have a single circular molecule of DNA, but they also lack histones, proteins that play a role in cell division. Mitochondria and chloroplasts are also equipped with the ability to produce their own proteins. This part of the organelles also has ties to bacteria, as their start sequence is fMet, as opposed to Met, the start sequence for normal eukaryotic proteins. The differences outlined above make it clear that the mitochondria and chloroplasts are foreign objects, and it appears that they were engulfed through some form of endocytosis. The reasoning behind this lies in the fact that both mitochondria and chloroplasts are able to make their own food, which is a selective advantage, because they are not reliant on other organisms for their energy. Instead, they utilize respiration and photosynthesis to autotrophically produce their own food. For this reason, heterotrophs engulfed either the mitochondria or the chloroplast in order to "exploit (its) autotrophic way of life". In order for this endocytosis to occur without complete digestion occuring, the two must have been able to resist the digestive enzymes. This then led to a symibiotic relationship, as the cell would provide nutrients and in return, the organelle would provide bundles of ATP for usage.
ReplyDeletehttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endosymbiosis.html
http://en.wikipedia.org/wiki/Histone
http://en.wikipedia.org/wiki/Mitochondria#Origin
http://encarta.msn.com/encyclopedia_761582165/mitochondria.html