September 17, 2006

Viral Action





A viral protein, known as syncytin, is hugely expressed in placental tissue – especially at the beginning of embryonic development – and is giving signs of bearing an essential role in placental architecture. Placental evolution, and indeed all evolution, results from viral actions. The virus in question is in fact a retrovirus known as human endogenous retrovirus, HERV-W. The human genome – as indeed many other eukaryotic genomes – is retroviral and viral genes added in stages to those of an amoeba. The retroviral protein syncytin is an example of how a retroviral product has been integrated in the host’s genome. The placenta is lined, on the mother’s side, by a tissue known as the syncytiotrophoblast – from where the protein’s name derives. The syncytiotrophoblast is an essential tissue for the proper development of the embryo, acting as a place of nutritional exchange between the mother and her child, as well as a center for hormone synthesis and it has a role in the fetus immunity. The syncytiotrophoblast is a one-celled layer formed via the fusion of trophoblasts. Trophoblasts are the cells that accompany the fetus in the very early stages of development giving it the help it needs to anchor itself in the womb. The fusion of trophoblasts creates the syncytiotrophoblast, the tissue that hugs the placenta. Syncytin is involved in the fusion of trophoblasts. A virus is at the heart of something so essential as the human placenta. There are a number of different HERV-W families that are found dispersed in various mammals. It is these viral families that are at the heart of the diverse types of placenta observed in mammals today.


Another example is that buried within the genetic blueprint of every human is a snippet of DNA that resembles a gene sequence from the human immunodeficiency virus (HIV). Researchers from the Howard Hughes Medical Institute at Duke University. Cullen's group published these findings in the November 9, 1999, issue of the Proceedings of the National Academy of Sciences. In fact the human genome is nothing more than genetic material contributed by viruses along with a few primitive bacterial genes and a few rare ones derived through mutation and natural selection as visualized by Darwin.


Viruses not only cause improvements of a species but also lead to the emergence of new species. A big evolutionary step has to be taken by at least two members of a breeding pair, simultaneously. This is impossible if genetic changes arise from blind mutations and recombination only. However, if infectious viruses insert new genetic instructions, then the problem of finding two members of a breeding pair, both equipped for a big evolutionary step, is solved. Viruses infect whole populations so two members of a breeding pair are likely to carry the same new instructions. This is the way for evolution to advance, and in very much larger steps than Darwin imagined.


However, what if humans got infected with genes for elephant tusks. The tusk would not appear quite the same because the entire genome, especially the switching genes, has an influence. Nevertheless it would be an odd nose. However nature has taken care that it does not happen because different viruses act on different hosts. They carry the identity codes of the life forms they emerged from and that is what they set about to reproduce. Various life forms emit plasmids during break-up that carry bits of the genetic code. These find homes in bacteria from which they emerge later as host specific viruses.


Unlike most other organisms, the genome of a retrovirus is composed of RNA instead of DNA. This means that infectious retroviral particles contain RNA. After infection of a cell by a retrovirus, the viral RNA is released into the cell along with several proteins that are required for the initial steps of viral replication. One of these proteins is called reverse transcriptase. After the release of the RNA, the reverse transcriptase makes, or transcribes, a DNA copy of the viral genome. This DNA copy is then inserted into the DNA of the infected cell. After the DNA copy is inserted into the cell's DNA, viral sequences then direct the expression of the viral genome. During this process, a complete RNA copy of the entire viral genome is produced. This RNA is then packaged into infectious viral particles, and the viral particles are subsequently start the cycle again.


In order for a virus to cause evolution in creatures, it must infect the germ line and become integrated into the genome there. This process has been proven to occur. If, for example, the DNA is injected into the nucleus of a mouse's fertilized egg, the genes will be found in many cells of the adult animal and sometimes even in its germ cells. Retroviruses can become integrated into the germ line as endogenous viruses, leading to permanent genetic consequences for the descendents of the original host. Perhaps it would be a reasonable compromise if a new virus were able to establish an infection and become widespread within the individual host's body one time only, and never again. That's the way our mammalian immune system often handles viruses.


It would make sense if the host's descendant who actually inherited the new genes were born with immunity to disease-causing, infection by the virus. In fact, it has been known since 1933 that resistance to a disease caused by a virus can be inherited. Even resistance to AIDS may be inheritable. AIDS is an incurable viral disease from which it was first thought no one is immune. Studies are now suggesting, however, that some children of mothers with AIDS are born immune to the disease. This is the story in Science, "Can Some Infants Beat HIV?"


Viruses install genetic instructions for big evolutionary steps and some of the steps require several genes so that they cannot be installed all at once but only in stages. During the long installation process, the not-yet-activated DNA remains silent, to become useful in future when the entire code is installed. This is something like downloading Windows from Micro Soft is stages. It becomes possible for very large genetic programs to be installed in stages. The parts installed first remain silent until the whole program has been installed. Sexual reproduction cleans up any errors that might occur in this code both in the expressed genes as well as the silent ones.


Things many humans consider dirty are necessary for life and life exists because of the underlying matrix of bacteria that exists in the universe.

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