Monday, March 15, 2010
The endogenous retrovirus
Along with Coyne's explanation of "dead genes", he briefly explains a specific type of dead gene that is present in the human genome: retroviruses. "The human genome contains thousands of such viruses, nearly all of them rendered harmless by mutations. They are the remnants of ancient infections." (69) However, as we know, there is one major retrovirus that has greatly affected some humans today: HIV. Does this mean HIV was present in our ancestors even though, according to Campbell, HIV is referred to as an emerging virus because it was just discovered in the 1980s? Can you find any specific retroviruses that are still around today that were also found in the DNA of our ancestors? What makes retroviruses different from other viruses?
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A retrovirus is an infectious particle that is made of a RNA genome in a protein capsid that is surrounded by a lipid envelope. The envelope has receptor binding proteins which serve the role of attaching to the host cell, which begins the process of infection. When retroviruses attach to a host cell, they release RNA and reverse transcriptase into the cytoplasm of the host cell. The reverse transcriptase makes complementary DNA (cDNA) using the RNA of the retrovirus. The DNA helps to continue the process of infection of the host cell, while proteins can be synthesized using the RNA from the retrovirus. Retroviruses are so fatal to humans because they often attack the immune system, preventing the body from fighting off diseases. A retrovirus that is still around today is the human T cell leukemia virus is referred to as HLTV. The Rous sarcoma virus (RSV) is another example of a retrovirus that is still around today. RSV is an oncogenic virus that causes cancer in people who are infected with it. According to Avert, HIV is commonly thought of as a descendant of Simian Immunodeficiency Virus (SIV) because certain strains show close resemblance to HIV. SIV originated in chimpanzees and has been proposed to have been spread between chimpanzees through a human touching infected chimp blood, passing the retrovirus on to humans. It is probable that humans had the possibility to be infected by a retrovirus in their gene. Since chimpanzees and humans both share ancestors in the past, it is possible their ancestors passed on a gene to both species that allowed them to be infected by retroviruses. Up until the contact of chimpanzees with monkeys, HIV did not appear because the retrovirus had not been spread to humans. Once the SIV virus was spread, it possibly changed into HIV in humans, leading to the emergence of HIV in humans at the time as HIV was spread by infected needles and donated blood.
ReplyDeleteSources:
http://www.accessexcellence.org/RC/VL/GG/diagram.php
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Viruses.html
http://www.avert.org/origin-aids-hiv.htm
Retroviruses utilize cellular mechanisms to create more copies of themselves. The first thing that they do is to insert their RNA into the cell they want to infect via an injection. The RNA then travels to cellular reverse transcriptase enzymes, where they are transcribed from RNA to cDNA, complementary DNA. The cDNA then travels to the nucleus where it is integrated into the organism’s genome via an integrase enzyme. It is now called a provirus. The provirus is then transcribed into mRNA, and it travels to the ribosomes where it is translated into the proteins that make the coat of the virus. They build up within the cell and then assemble, eventually causing the cell to lyse and release all of the new viruses. Retroviruses are difficult to treat for doctors, as the reverse transcriptase enzyme is extremely prone to errors, which causes an extremely high rate of mutation. The mutation rate means that the virus adapts far more quickly, and the immune system, once it can recognize the viral antigen, often does not recognize what that antigen has now become, due to the fact that it has mutated. In addition to being able to be translated, the provirus associated with the retrovirus often lies dormant within the genome, which can be passed down through the generations, accumulating mutations and becoming non-viral, or a “dead” gene. HIV, or human immunodeficiency virus, is indeed an emerging virus, only in existence from, at the earliest estimated date, the 1950s. When Jerry Coyne states that "The human genome contains thousands of such viruses, nearly all of them rendered harmless by mutations. They are the remnants of ancient infections" (69), he is not saying that our ancestors had to deal with HIV, but rather, that throughout the entire course of our evolution from the earliest retrovirus-infected ancestor to now, we’ve had to deal with different types of retroviruses that have inserted their genome into our DNA. These dead retrovirus genes are a way to trace evolution, because we can compare the quantities and types of retrovirus in different animals, and the more retroviral genes they share in common (and the less mutations there are between two of the same retroviral genes), the more closely they are related and the later they diverged. In addition, unless we find a way to combat HIV and AIDS, it is unlikely that the HIV retrovirus will find its way into our genome. Since it kills all infected people, and can be passed down from mother to child, people displaying the viral genes in their genome will not go on to have long-term descendents. One retrovirus that still exists today that can also be found within our genome (proof that it was in our ancestors at some point) is the borna virus. The virus affects different animals in different ways, presenting no symptoms in some animals, and causing grotesque symptoms in others, such as horses, that, when infected, smash their heads upon rocks, effectively killing themselves. The fact that the virus does not always cause symptoms and death may have let it insert itself into the genome. If it inserted itself into a sperm or an egg cell, it would be passed to a descendent, and would then continue passing on, accumulating mutations that would eventually render it a dead gene. Retroviruses are different from other viruses in the way that they replicate themselves, inserting themselves into the genome and using reverse transcriptase, as mentioned above.
ReplyDeleteSources:
http://www.nytimes.com/2010/01/12/science/12paleo.html
http://en.wikipedia.org/wiki/Reverse_transcriptase