Monday, April 6, 2009
Microbes exchanging genes
On page 85 of the book, Carroll states that microbes exchange genes, and some microbes live within other host species in a process called endosymbiosis. What is the advantage or benefit of exchanging genes? What are the advantages and disadvantages of living in a host species? Describe in detail how the microbes exchange genes with other organisms. Also provide specific examples of a microbe acquiring a certain gene from an outside environment or organism and the selective advantage this provides.
Subscribe to:
Post Comments (Atom)
There are three types of symbiosis: commensalism, mutualism, and parasitism. Endosymbiosis was beneficial in that it allowed the “transfer of genes between very distant relatives” that maybe otherwise would never have occurred (85). These processes help Coevolution in which a change in one species can help as a selective force on another species. These species will adapt together and require reciprocal genetic change.
ReplyDeleteThere are many advantages of living in a host species. For example, nutrients are provided from the host to the microbe, so the microbe does not have to spend time and energy obtaining food. Protection and a good environment is provided. Also, new genes can be received into the genome much quicker through endosymbiosis than waiting millions of years for a specific mutation in that gene to occur. But also disadvantages occur. For example, if the host species dies, then the microbe will most likely die unless it finds a new host very quickly. It depends enormously on the host and the microbe would not have life unless it had a host. Also, with the injection of foreign DNA, the host genome could degenerate in a bad way and harm the organism.
Microbes exchange genes with other organisms through viruses and phages. For example, bacteriophages have a circle of foreign DNA in a bacteriophage that will become part of the genome of the host cell when injected.
Examples include nitrogen fixing bacteria such as Rhizobium which live on legume roots. They help the legume by fixing nitrogen to minerals that can be used to synthesize compounds such as amino acids. These bacteria fulfill their own metabolic requirements and the excess ammonia they relaease becomes available to other organisms. The most common examples of obligate symbionts are mitochondria and chloroplast, in which the cell needs these to survive. These provide energy for the rest of the cell. Another recent example is of two Japanese scientists have discovered a heterotrophic flagellate that engulfs a unicellular green algae from the surrounding water. Once inside, the alga loses its flagella and cytoskeleton, the host loses its feeding apparatus, the host switches from heterotrophic to autotrophic nutrition, the host becomes capable of phototaxis. This shows advantages of endosymbiosis.
http://en.wikipedia.org/wiki/Endosymbiont
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endosymbiosis.html
http://www.molevol.de/research/endosymbiosis.html
There are several advantages to bacteria entering a host cell. Not only does the host cell provide nutrients necessary for the cell, but the bacteria can also engage in the horizontal transmission of genes (as opposed to vertical, which is from parent to offspring. Included in that list of advantages is the broad topic of evolution, as Bruce Levin and Carl Bergstrom state that "genes and accessory elements acquired from external sources are responsible for many of the interesting adaptations of bacteria to their environments." Also, in their study of bacteria, they stated their suprise at learning the extent to which bacteria are chimeras (an organism is given this name if it contains genetic material obtained from other organisms), an observation that directly implies the importance of integrating host genes into its own genome.
ReplyDeleteBecause bacteria reproduce through binary fission, "any given population will be composed of relatively few genetically distinct lineages, with recombination between them occurring only on rare occasions.ny given population will be composed of relatively few genetically distinct lineages, with recombination between them occurring only on rare occasions." (Levin, Bergstrom)Therefore, combining a host's genes with its own allows for greater genetic diversity, allowing for a better chance at evading potential defense mechanisms against it.
http://www.pnas.org/content/97/13/6981.full?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&author1=Levin&searchid=1016702629559_471&stored_search=&FIRSTINDEX=0#sec-3
When bacteria exchange genes, it is called conjugation. This is an advantage because it promotes genetic diversity among bacterial populations. Because it is the bacterial version of sex, it also increases the frequency that beneficial mutations will appear and spread faster through the population. Cells with an F+ plasmid are “male” and those with an F- plasmid are “female”. The F+ cell transfers the F plasmid across a temporary cytoplasmic bridge. This makes the F- cell an F+ cell. The F plasmid is then incorporated into the cell’s original plasmid.
ReplyDeleteIn transformation, bacterial cells take up naked DNA from their environments. This DNA comes from bacterial cells that have lysed. In one experiment, the harmless bacterium Streptococcus pneumoniae became pathogenic when it took up naked DNA from a medium that contained lysed pathogenic cells. The DNA that had been incorporated into S. pneumoniae’s genome happened to contain the allele for pathogenicity, a gene for a cell coat that protected the bacterium from the host’s immune system. The taking up of new DNA in this case was an evolutionary advantage because it increased the likelihood that the bacterium would survive and reproduce. This method is also used by scientists in order to make bacteria synthesize human proteins. In one case, when E. coli is placed in a medium that has a high concentration of calcium ions, the cells can be stimulated to take up small pieces of DNA. For our purposes, this foreign DNA codes for valuable proteins such as human insulin and growth hormone. This essentially turns the E. coli cell into a protein factory.
In transduction, phages carry bacterial genes from one host cell to another. The host DNA is hydrolyzed with the phage DNA. This process brings cells into contact with new genes, albeit randomly, and increasing the chances a beneficial gene will be introduced to a larger portion of the population.
Endosymbiosis is when an organism, usually a prokaryote, is incorporated into the cell of another organism, such as a eukaryote. This is the theory of the origin of mitochondria and chloroplasts. It is believed that these organelles were originally free-living prokaryotes that were engulfed by eukaryotic cells because of the efficient conversion of energy. With this endosymbiotic theory comes evidence that mitochondria and chloroplasts transferred some of their genes to their host cells. This is beneficial because it allows the mitochondria and chloroplasts to exist without sacrificing too much of their volume to the synthesis of proteins. Instead, the host cell synthesizes the necessary proteins and in return, they convert energy for the host cell. The downside of this transfer of DNA is that now, mitochondria and chloroplasts cannot survive outside of their host cell because they no longer possess the genes to synthesize the proteins necessary for their survival.
Campbell book
http://en.wikipedia.org/wiki/Endosymbiotic_theory
A great example to mention here is the Endosymbiotic theory involving mitochondria and plastids. Mitochondria contain their own DNA which is circular and different from the cell's nucleus. They feature two membranes, suggesting entry into the cell at some point in the cell's evolution. New mitochondria and plastids are also formed by binary fission, as in bacteria. All these facts point to the theory that mitochondria were free living bacteria once, but entered the first cells and formed an endosymbiotic relationship which lasts until today.
ReplyDeleteThis relationship works out great for both parties involved. The host cell is given access to the mitochondrion and plastid's immense energy generating potential. The host cell is able to power its own functions easily with the powerhouses that have grown to live inside it. The mitochondrion gets the benefits of proteins made especially for it by the cell, and the extra protection is gets deep inside the cell cytoplasm.
IN their evolution to live inside the cell, the mitochondria and plastids have shared DNA with the host cell, which poses the complications of DNA still being left in the mitochondria, as not all of it was shared. Through this sharing of DNA, the mitochondria and plastids secured themselves the perfect place for their "lives".
http://en.wikipedia.org/wiki/Endosymbiotic_theory