On page 146, Neil Shubin says that genetic mutations sometimes cancel out the functions of certain genes. When these genes are not necessary for vital functions, such as sense of smell in dolphins, the mutation will continue to be passed on from generation to generation and the gene will cease to be expressed. Assess how this viewpoint may conflict with the statement that structure of a bodily mechanism must always be related to its function. Using information from our DNA and Biotechnology units, how is it possible for chromosomes to exist in the genetic makeup of an organism if they essentially "do nothing" as Shubin says?
Christine Lin
choco_cat11@comcast.net
While the structure of a mechanism in the body is almost always related to its function, genes aren’t necessarily “part” of a mechanism per se, but rather the instructions for how to create the mechanism; the genes simply tell the ribosomes in the body cells to create certain proteins to carry out the mechanisms. If a mechanism no longer becomes useful, like the sense of smell in dolphins, the lack of gene expression will not result in a change in structure (the lack of gene expression will result in no proteins being created), and thus no change in function, so there would really be no conflict.
ReplyDeleteShubin calls useless gene DNA “silent records of evolution” (174), implying that genes that are no longer in use serve as markers in evolution, pointing out old characteristics that the organism adapted out of because of its status as a selective disadvantage. Dollo’s Law of Irreversibility states that evolution is not reversible and genes that become useless will never become useful again, but even then there are exceptions, as it has been found that some million-year-old genes can be reactivated (http://www.pnas.org/content/91/25/12283). But even though it seems that noncoding DNA sequences are useless, they not only provides insight into the evolutionary history of an organism but it also does serve a function in the body, even if it has nothing to do with direct protein creation. Noncoding DNA sequences act as gene expression regulators, as some can be translated into noncoding RNA which can be used in RNAi, when the body prevents creation of proteins through using miRNA or siRNA strands degrading or preventing the translation of mRNA, or siRNA blocking transcription directly in the chromatin (Campbell 364-366). Others can act as operators, promoters or enhancers and other control elements, regulating the production of proteins by controlling the amount of transcription (Campbell 359). Still others are able to determine where transcription factors attach, as transcription factors will only attach to certain noncoding DNA sequences.
In general, the fact that a gene “does nothing” does not necessarily mean it actually serves no purpose; it just does not directly create proteins through translation. Instead noncoding DNA usually plays a very large role in gene regulation, changing when and how many proteins can be created at any particular coding sequence on the chromosome.
Eugene Bulkin (doubleaw002@gmail.com)
As Eugene said, as we evolve, many unnecessary genes are turned off and become a record of evolution. For example, in humans we see aroudn 20-25 thousand functional protein coding genes but there are thousands more non-functional genes left in our genome. An example of this is genes used for smell. In evolution, when apes began to be trichromatic, we noticed a decrease in the number of coded olfactory receptors. For some reason, the coding of trichromacy silenced many of these genes. One theory is that of frameshift mutation. If an extra codon was added to the genome, it would have created a shift. Usually, this would lead to a missense mutation, where nothing makes sense and a useless protein is formed. However, in some very small chances, it could lead to creation of something else, in this case trichromacy. However, in the creation of the trichromacy gene, the structure of many smell receptors were destroyed in mutation.
ReplyDeleteSources : Campbell: mutation
http://www.mapoflife.org/topics/topic_308_Loss-of-olfactory-capacity-in-primates-and-cetaceans/
http://www.ornl.gov/sci/techresources/Human_Genome/project/info.shtml