On page 142, Moalem explains the Weismann barrier, a genetic principle that indicates how genetic information is passed down to offspring. The cells our bodies can be split into two groups: somatic cells and germ cells; the Weismann barrier stands between these two and theorizes that information in somatic cells is never passed on to germ cells (cells that contain information that is passed onto our children). While this theory was previously accepted, recent research has shown that the barrier isn’t entirely impenetrable-- it can be penetrated by viruses. This concept relates to Big Idea 1: The process of evolution drives the diversity and unity of life. As viruses and other external factors can affect not only an individual’s genetic expression, but his or her offspring because of the heritability of them, more diversity is brought to the human genome resulting in evolution.
Define a virus and explain how a retrovirus or virus can write itself into the DNA of cells of the germ groups of cells and the effects this can potentially have on an organism (positive and negative). Also, compare how penetrating the Weismann barrier with viruses that not only externally affect the evolution of humans but internally as well, is similar to the epigenetic “tags” that can be passed down to offspring even though they are acquired during the lifetime of an individual.
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ReplyDeleteA virus is an infectious agent that consists of genetic material protected by an outside coat. Viruses can only reproduce inside of a cell. This is done by integrating itself into the cells, known as the lysogenic state. During this state, the cell reproduces with the viruses DNA as part of its own. One type of virus, known as a retrovirus, is able to accomplish this by creating DNA from its original RNA code through the use of reverse transcriptase. Once the DNA is created from RNA, it becomes part of the host cells DNA through the use of the integrase enzyme.
ReplyDeleteThe virus can then exploit the host (http://www.nature.com/nrmicro/journal/v5/n4/full/nrmicro1541.html).Once the virus's DNA is part of the host's, the host follows its instructions and creates proteins for which the virus's genetic material codes. This allows a virus to have a harmful effect on the cell by interrupting normal homeostasis, covered in Big Idea 2 (Biological systems utilize free energy and molecular building blocks to grow, reproduce, and to maintain homeostasis). The virus can also have a neutral or positive effect if the virus causes the cell to produce a helpful protein.
Some viruses are able to penetrate the Weismann barrier because they become part of the host's genetic code in somatic cells. Some of these viruses have mechanisms that allow their genetic material to be passed from somatic cells to germ cells, thus breaking the Weismann Barrier. If the virus spreads from the somatic cells' DNA to the sperm or egg cells' DNA, it can be passed on to future generations (http://blog.lib.umn.edu/denis036/thisweekinevolution/2007/06/tradeoffs_in_defense_against_r.html) This can have an effect on the offspring's external and internal evolution because the virus genetic code becomes part of germ cells too, which allows for their genetic code to be passed on to the host's offspring. This is similar to how epigenetic tags work, not the mechanism of transfer but the fact that both are acquired during the host's lifetime and passed on to offspring. Although they are created throughout the lifetime of the individual due to acetylation and methylation of histones caused by certain habits (such as smoking and the person's diet). Both epigenetic tags and viruses are acquired by the individual through their lifetime, but they can have a permanent effect on the DNA passed down to offspring for many generations, thus affecting the evolution of certain populations. Therefore, the Weismann barrier is broken because if a retrovirus integrates itself into a host's genome, then it can be transferred to the next generation via mendelian genetics just like any other gene (http://www.askabiologist.org.uk/answers/viewtopic.php?id=2041)
-Mikhail Iouchkov (mikhailiouchkov@gmail.com)
A virus is a small infectious agent that is made up of genetic material, either DNA or RNA, and a protein coat to protect the genetic material. Viruses can only replicate inside the living cells of an organism. A retrovirus is a specific type of RNA virus that uses its own reverse transcriptase enzyme to produce DNA from its RNA genome. This new DNA is then incorporated into the host's genome which causes the infected cell to treat the viral DNA as part of its own, causing the cell to create the proteins necessary to create new copies of the virus. Like Mikhail said, "this part of the virus' lifespan is known as the lysogenic state" in which the virus' genetic material is being reproduced by the cell, but not the virus itself. Viruses can also enter what's known as a lytic cycle, in which the virus' proteins are actually created by the cell and assembled into the virus, which ends up lysing the host cell. The virus using cells for its own benefit relates to big idea 2, Biological systems utilize free energy to grow, reproduce, and to maintain homeostasis because the viruses are utilizing the cell's energy to survive and reproduce and by interrupting the cell's homeostasis by potentially destroying the cell.
ReplyDeleteAccording to Dr. Moalem, "The Weismann barrier stands between germ cells and somatic cells: the theory maintains that information in somatic cells is never passed on to germ cells" (142). The Weismann barrier basically supports the idea that the choices you make in life don't affect your genetics and will not be passed down to your children. According to wikipedia, "Evidence has begun to mount for horizontal gene transfer. Different species appear to be swapping genes through the activities of retroviruses" (http://en.wikipedia.org/wiki/Weismann_barrier) The horizontal gene transfer is what retroviruses do. By integrating their own genetic material into our DNA, our genetic makeup is changed pretty much permanently, which can lead to our germ cells being changed. Since our germ cells are able to be manipulated by these viruses, the Weismann barrier is able to be broken in rare occasions.
The way viruses add their genetic material to change an organism's genes during their lifetime is similar to how epigenetic tags work. Both viruses and epigenetic tags change the DNA of organism during its lifetime which causes those changes to be represented in the offspring of the organism. Epigenetic tags work by adding methyl groups to DNA to turn gene expression off. Because epigenetics and viruses can change gene expression in an organism, and this varied gene expression will be passed on to the organism's offspring, epigenetics and viruses can both affect how a species evolves over a long period of time. This relates to Big Idea 1, the process of evolution drives the diversity and unity of life, because the variation in an organism caused by viruses or epigenetics can lead to evolution over a lot of time by way of selection pressures and natural selection.
Tony Massat amassat4@students.d125.org