Argument- David Shenk states on page 22 that "Humans beings are distinct from one another not just because of our relatively few genetic differences, but also because every moment of our ongoing lives actively influences our own genetic expression." How can the alteration of genes change distinct traits? How do these traits change over time as a result of evolution? What are some ways organisms' genes has changed over time for adaptation? How are genes activated and deactivated by the environment, nutrition, hormones, and other genes and give examples of each. What is the advantage of genes being able to switch on and off?
Bing Zhao (Bingzhao9@yahoo.com)
Genes and the expression of genes can be altered throughout one’s life due to the person’s environment. Epigenetic inheritance, traits transmitted by mechanisms other than the nucleotide sequence directly, allow for the expression of genes in different magnitudes. This information is usually held in the chromatin and can be changed throughout one’s life, which could result in a change in traits (Campbell 358). Shenk explains an experiment performed on toadflax plants. Two types of the plant were studied and found to have the same genes and DNA code, but their appearances were very different. They soon discovered the difference between the flowers was in their epigenomes instead of their actual genes. The plant types became very distinct because their epigenomes allowed for different expression of the same genes. The epigenome is inherited, so as the two types reproduced, they continued to survive, resulting in a continuation of the variation. Epigenomes can also be altered by the environment, which could explain the origin of the more rare “Peloria” toadflax or why it is so rare (Shenk 157-158). The DNA sequence of the genes can also be altered by point mutations, or a chemical change in one base pair of a gene. These mutations can be passed to future generations if in a cell that produces gametes. Base-pair substitutions, switching one base pair, can be harmless because many triplets can code for the same amino acid while making proteins. Some substitutions, along with insertions and deletions, can change the amino acids that are produced by the genetic code, which completely changes the protein that is formed by the gene and the function of the gene. This causes major problems throughout the body and can lead to disease and changes in many traits (Campbell 344-346). Because these alterations are often passed down through generations, organisms who are not able to regulate these changed genes will not survive. Animals have evolved throughout history so they could survive with these new genetic codes. Many have adapted by controlling the activation of these genes. In most fungi, plants, and animals, cytosine or other methylated bases are present. When inactive, genes are usually more methylated, so removing extra methyl groups can activate genes. Many genes are only expressed for a short time during development then inactive for the duration of an organism’s life. This is often due to DNA methylation. If methylation does not happen normally, many complications can occur with embryonic development, which cannot be fixed later in life (Campbell 358). An experiment was done by Randy Jirtle, an oncologist from Duke University, to prove this. Pregnant mice with a regulated agouti gene, which gives them yellow coats and a better chance of being obese and diabetic, were used. Jirtle fed one group a diet high in B vitamins and one group a normal diet. The B vitamins caused more methyl groups to be available for attachment to the gene, causing DNA methylation and deactivating the gene. The group of mice receiving a B vitamin-rich diet gave birth to brown, normal-sized pups even though the agouti gene was present (http://www.time.com/time/magazine/article/0,9171,1952313-2,00.html). Jirtle showed diet can affect the epigenome and expression of genes through his experiment with the agouti gene in mice. Acts like smoking have also been proven to change the expression of genes.
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ReplyDeleteOne of the reasons smoking is dangerous for pregnant women is because it alters DNA methylation, which changes gene expression. A study done by Dr. Kjersti Aagaard found 25 genes with changes in methylation among non-smokers and 438 genes with altered methylation in smokers. These chemical changes cause growth restrictions on developing embryos due to the deactivation of certain genes (http://www.bcm.edu/news/item.cfm?newsID=4773). Gene expression is also regulated by protein and steroid hormones when they bind to protein receptors. Glucocorticoid is a steroid that binds to a receptor protein, releasing Hsp90, which deactivates gene expression. The receptor protein that Glucocorticoid binds with then forms a dimer with a second copy of the protein and binds to enhancer sequences, which activates gene expression again (http://www.ndsu.edu/pubweb/~mcclean/plsc431/geneexpress/eukaryex8.htm). With the ability to turn genes on and off comes the control of expression that is necessary for normal development. Different factors are able to regulate the expression of genes, but when stimuli change these factors, gene expression is altered, and this can have major, long term effects.
Jenna Sherman (jsherm013@aol.com)
According to Shenk, “we are, each one of us, the sum of our proteins” (Shenk 22). This is because our genes direct the formation of our proteins, but they are not the only thing that controls the protein-building process. Scientists have found that “genetic instructions themselves are influenced by other inputs” (Shenk 22) such as, the environment, nutrition, hormones, and other genes.
ReplyDeleteThe alteration of genes changes distinct traits because each gene consists of alleles that code for dominant or recessive traits. When the positions of genes are altered, or when a gene is turned on/off, certain traits will also be altered. The actual coding of genes is called the genotype, while the trait would be the phenotype, or what you see. Following Darwin’s theory of natural selection, these traits wouldn’t change over time as a result of evolution, but rather stay the same and just become more abundant because it is the trait that was passed on generation after generation. There can be many variations of certain traits, which occur through gene mutations. There are large-scale mutations, but point mutations are what really influence each specific trait. Point mutations are “chemical changes in a single base pair of a gene” (Campbell 344). There are different types of point mutations that each have a different level of severity in terms of how much the mutation actually changes the particular trait in an organism. Mutations occur when there is a mishap during DNA replication in the S-phase of the cell cycle. The mutated genes that allow the organism to have a selective advantage to survive and reproduce would be the genes that get passed on. Eventually, the selectively advantageous mutations would be the driving force for the evolution of the organism through generations.
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ReplyDeleteGenes can be activated and deactivated by the environment because there are many different factors in one’s environment that can significantly influence one’s traits. An example would be the height difference between Japanese kids raised in Japan and Japanese kids raised in California. William Walter Greulich, a researcher at the Stanford School of Medicine found that “the Californian-raised kids, with significantly better nourishment and medical care, grew an astonishing 5 inches taller on average” (Shenk 26) than the Japan-raised kids. Also, these kids were growing up in the same time period. This observation illustrated that genes do not dictate “any predetermined forms or figures, but interacting [interact] vigorously with the outside world to produce an improvised, unique result” (Shenk 27). Another factor that influences our traits is our hormones because many of the hormones humans produce coincide with that person’s gender. Sex-related traits are autosomal and are altered based on the hormones present. An example would be male pattern baldness, which “is influenced by the hormones testosterone and dihydrotestosterone, but only when the two levels of the hormones are high” (http://www.nature.com/scitable/topicpage/environmental-influences-on-gene-expression-536). Although the allele that controls this trait if stronger in males, females can experience hair loss if their stress levels rise high enough to produce enough “testosterone and convert it to dihydrotestosterone” (http://www.nature.com/scitable/topicpage/environmental-influences-on-gene-expression-536). Also, interactions between the genes themselves can alter certain traits because certain genes can turn another on or off. A situation in which this occurs is called epistasis. Epistasis is when “a gene at one locus alters the phenotypic expression of a gene at a second locus”(Campbell 273). An example of epistasis would be albinism because the allele causing an organism to be albino disables the gene that controls the coloring of an organism’s skin.
The advantage to being able to turn genes on and off is that organisms can adapt to their environment by reacting to it. The response an organism gives to a new environment may be to turn on or off a certain gene, which would change the trait being expressed. By changing the trait, the organism can live more optimally in the new environment and increase its chances of surviving and reproducing. The GxE equations includes dynamic processes and it helps us to understand that “genes, proteins, and environmental signals (including human behavior and emotion) constantly interact with one another, and this interactive process influences the production of proteins, which then guide the functions of cells, which form traits (Shenk 31).
(Kalista Noegroho, Kalista.dara@gmail.com)