Tuesday, March 24, 2009
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Welcome to the discussion group for Making of the Fittest by Sean Carroll. Prompts and posts are student generated. This is a collective effort to engage in discussions that connect the theory of evolution with the biological concepts and themes discussed in our course throughout the year.
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"Immortal Genes" are genes which have "withstood more than 2 billion years of the steady bombardment of mutation," (Carroll 79), and their meaning has not been changed "significantly" even among the various taxa that carry them.
ReplyDeleteThese genes have been able to stay the "same," so to say, through a large amount of time because their meaning has not been changed. Even though they have been mutated, the gene "as a unit endures," (Carroll 79) - meaning that although a particular letter of the genetic code for the gene may be altered by a mutation, the mutation must by synonymous. This mutation must be synonymous otherwise the function of that gene would be eliminated and the organism would not be able to survive/reproduce because such immortal genes (as stated) are "central to fundamental, universal processes in the cell...all forms of life have depended upon these genes," (Carroll 79).
The basic functions of immortal genes are those that are fundamental and universal in cells. One function includes the decoding of DNA and RNA, and the other includes the making of proteins. The reason why such genes have survived is that "[a]ll forms of life have depended upon these genes since the origin of complex DNA-encoded life early in Earth's history," (Carroll 79). Each organism that carries these immortal genes has these genes survive because they are necessary to the everyday living function of each cell. Every cell requires the necessary decoding of DNA/RNA and the making of proteins. Without such functions taking place, the cell would not be able to survive, and thus the organism/specie would not be able to survive and reproduce. Immortal genes connect to the biological themes of cells and heritable information as they are involved in key processes of the cell (otherwise the cell would not be able to function) and are heritable information that have withstood 2 billion years of time.
I think that Danyal had a good way of putting it; that the meaning (translation) of these immortal genes "has not been changed 'significantly' even among the various taxa that carry them." It is important to note that these genes have not changes within the taxa that carry them; this is not to say that these genes are present and the same in all species. Rather, an immortal gene is one that once formed, it stays with any subsequent successful offspring. To clarify, lets say that for some reason and under some set of conditions Russian humans and Russian humans alone developed (through mutation and natural selection) a gene that allowed them to live in the vacuum of space (this is obviously hypothetical). This gene would require a very specific protein to formed in the exactly correct manner, otherwise the human would suffocate, depressurize, and die in space. Now, no other nationalities have this gene, so this gene is not ubiquitous. It would, however, be immortal. Any Russian (or subsequent deviation of a Russian) who was born with this gene changed in even the slightest nonsynonymous manner would not survive. Thus, this gene is necessary for these beings, much like the gene for elongation factor 1-alpha is for all gene lines ever containing it (Carroll 81). However, unlike the gene for elongation factor 1-alpha, the hypothetical Russian "space gene" would not be immortal in every lifeform, but only in those descendents of the original Russian line that developed it.
ReplyDeleteI think that Carroll and Danyal both make it seem like immortal genes have to be present "across all domains of life", but it is important to realize that this is not true (Carroll 81). Immortal genes are simply genes that, once aquired, stay with an organism and its descendents forever.
As Danyal said, immortal genes have functions that are fundamental to the continued living of the organisms that carry them. Obvious examples include those detailing DNA and RNA transcription and translation, but these are just the universal examples. If we go further down the phylogenic trees, we will find genes that are immortal in specific gene lines. For example, it is reasonable to assume that the genes that code for endoplasmic reticulum formation in eukaryotes are immortal. Without this vital structure, eukaryotic cells would be unable to produce the compounds they need. However, prokaryotic cells, having never developed a gene for an ER, obviously do not have this gene. This is a prime example of what we can call esoteric immortality, immortality of genes that is limited to those organisms descendent of the first organisms to develop said genes.
Immortal genes are inherently related to the theme of heritable information. Immortal genes are only immortal because their inheritance is demanded by natural selection, another theme of biology more commonly termed evolution. Evolution is what makes immortal genes immortal. Selective pressures cause organisms without these genes to die out, thus making immortal genes ubiquitous.
I personally like to think of "Immortal Genes" as genes that cannot be mutated within an offspring's genome or else that offspring will have no chance to survive through the processes of natural selection. At their most basic, these "Immortal Genes" are the genes that translate into proteins that are used in the core processes of life. For example, genes that govern processes such as transcription or translation are generally immortal because any slight mutation in these that causes the polypeptide sequence to change in the proteins such as RNA polymerase, natural selection will eliminate that particular individual. The reason why certain immortal genes are present amongst nearly all life forms is exactly because no organism can survive and reproduce without them. If an organism has a mutation in an immortal gene that causes a change in polypeptide sequences of the corresponding protein, that organism will die, removing its selectively disadvantaged genome from the gene pool. Therefore, there is no way to "lose" immortal genes because "losing" them or even slightly changing them would insure the death of the organism.
ReplyDeleteHowever, there are some slight misconceptions about immortal genes that the previous comments have employed. Just because a gene can be found amongst a wide range of organisms across kingdom and even domain barriers by no means indicates that it is immortal. This merely indicates that selective pressures that have been present in these organisms' environments have favored these genes for millions of years. In Eric's "Russian Spaceman" example, the space living gene would only be immortal as long as the Russian Spacemen exist only in space. This is because of the unique selective pressures of living in space requiring that specific gene. However, if the Russians return to Earth, the selective pressures of space would become null. Therefore the gene would not be immortal because if mutations caused it to create a different protein, there would be no selective disadvantage and it is probable that this supposed "defect" will spread amongst the Russian population until this gene has degenerated and the Russians can no longer live in space.
Another example of an often misconstrued immortal gene that exists only within Kingdom Plantae is the gene coding for the enzyme ribulose bisphosphate carboxylase oxygenase (Rubisco). This gene is found across all species within the kingdom plantae because it is central to the Calvin Cycle of the dark reactions necessary for photosynthesis. Many people believe that because all plants contain this gene that it is immortal. However, over maybe another 1 billion years, this will show to be unfounded. If a new gene arises that encodes for a more useful or efficient enzyme (possibly one that does not fix oxygen and does not cause photorespiration), the rubisco gene will be unneeded and actually be "ran out" of the population because the more efficient enzyme will be better suited to standing against selective pressures. The rubisco gene will eventually mutate into a gene that encodes for a misshapen protein, but that will not matter since there is a new, more efficient enzyme that has taken over the fixation of carbon in the Calvin Cycle. After even more extended time, the former rubisco gene might even turn into junk DNA and have serve no purpose - a degenerating relic of years of selective pressures.
Now, back to Daniel's prompt.
Two examples of immortal genes are the gene that codes for DNA polymerase and the gene that codes for helicase. Note that I chose these specific genes because they code for enzymes that relate specifically to the process of DNA replication. These genes are found in all organisms becasue withouth them there would be no mechanism to replicate, reproduce, or grow (all essential functions of life). Complimentary DNA could not be built if the gene coding for DNA polymerase had a missense or nonsense mutation. Neither would DNA ever split apart to facilitate the replication and placing of new nucleotides on template strands if the gene for helicase caused a change in the function of the enzyme. Any organism that has significant mutations of these two specific genes would be unable to grow and would have no way of reproducing because the DNA would have no way of replicating. Therefore, these genes have become immortal due to their high importance nature.