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Description of the Viral Replication Cycle by XXX Description of the Viral Replication Cycle By XXX Introduction Scientists have long debated over the issue of life and what criteria must be met for something to be alive. For example it was proposed that an entity must move for it to be alive, but by this description things like trees would be non-living. Another less controversial criterion for defining life is the ability to reproduce independently. A virus is an organism without the means to reproduce by itself so it must overtake the host cell’s (infected cell) mechanisms to replicate. So for this reason a virus is not considered “living,” but due to its efficiency in replication (producing thousands of new particles by the second or third replication cycle), viruses pose a severe threat to multi-cellular organisms. The purpose of this document is to inform the general reader about the mechanisms viruses use to replicate. If the reader is familiar with some basic cell biology much of the discussion in this document should be easy to comprehend and the author will try his best to simply explain the biological terminology for those readers who are not. The viral replication process consists of six key steps: attachment of the virus, penetration into the cell, transportation to the nucleus, integration with cell DNA, replication of the genetic material and assembly and release from cell, see figure below for process flow. Process flow of viral replication cycle Image from <http://www.food-info.net/uk/virus/biochem.htm> Attachment to the cell Before a virus starts its replication it must find a suitable “host cell”, the cell the virus will infect. Many cells have plasma membranes which mediate what enters and exits the cell, similar to a gate. The plasma membrane is the first obstacle the virus must overcome in order to replicate. Plasma membranes contain bio-molecule (proteins, lipids and sugars) receptors which act as “markers.” The virus, which also expresses markers, identifies a cell by the cellular markers. The virus then attaches to the cell via the markers. Penetration into the cell Once the virus has found the cell to infect, it must get its genetic material, which contains the instructions to build more virus particles, into the host cell. There are several mechanisms a virus uses to enter the cell. One mechanism is by forcefully puncturing the cell membrane and injecting the viral DNA or RNA (the genetic material), see figure below. Another mechanism is by getting encapsulated by a membrane vesicle (the membrane pinches in on itself in order to sample the outside environment) which then carries the virus into the cell. Forceful injection of genetic material into the cell Image adapted from http://creationwiki.org/File:Nbt0104-31-F1.jpg Transportation of genetic material Once inside the cell, the virus must transport its genetic material to the cell nucleus in order to replicate. Some viruses (those that entered through vesicles) need to remove its genetic material from the viral coat; special proteins in the cell will automatically dissolve this coat. The virus which is only a few nanometers long usually has to travel far inside the cell (few micrometers in length) to get to the nucleus. The inside of the cell is very crowded, so the virus cannot simply diffuse through, some type of active transport is needed. Usually the virus uses the cell’s system of actin and myosin filaments (these filament act like the vehicles and roadways to facilitate transport) to be transported to the nucleus. Integration with cell DNA When the virus reaches the nucleus, it must again overcome a membrane. However, this is easier than before as the hijacked transport mechanism will usually deliver the viral genetic code into the nucleus without much resistance. One issue however is that some viruses, called retroviruses, contain RNA as its genetic material, but in the cell DNA is the information carrier. These retroviruses make use of more special proteins (reverse transcriptase) inside the cell that copy the information from RNA into DNA. The reverse transcribed DNA is then integrated into the cell, the cell now believes the viral DNA is part of the cell’s DNA. Replication of genetic material After the viral genetic code has been integrated into the cell’s genetic code, the virus once again hijacks the cell’s mechanism to carry out its purpose. At periodic intervals various enzymes and proteins are used to read and replicate DNA. The viral genetic code is usually also replicated and transcribed at this time. Assembly and release of virus Befitting of the virus, for the last step of the replication process the virus once again commandeers the cellular components to do its bidding. While viral DNA is being copied it is also being read by the cell’s mechanism which follows the instructions contained in the viral genes. The viral genetic material is copied and transcribed into RNA, a copy of the instruction to build the virus. The RNA is then shipped out of the nucleus and into the cytoplasm (outside the nucleus but inside the cell). Here the RNA is read by specific proteins and chemicals, which follow the instructions to construct the various parts of the virus. In some cases the instructions also shut down the other functions of the cell; by diverting all resources to viral production the virus optimize viral replication, see figure below. Shutdown of cellular mechanism to produce viral particles Image adapted from http://creationwiki.org/File:Nbt0104-31-F1.jpg Then the parts are assembled by other cellular mechanisms. The replicated genetic material is integrated into the assembled virus. Finally the viruses then burst out of the cell, releasing several new viral particles while killing the host cell in the process. The newly released particles then go and find other cells to infect and start the process again. Conclusion Reproduction is one of the defining criteria for life, and our genetic material carries instruction on how to reproduce and keep the species alive. Viruses also contain genetic material, but have no means of autonomous reproduction so viruses must use a host cell’s mechanism in order to replicate. The viral replication process involves six steps: attachment, entry of the virus, transportation to the nucleus, integration with cell DNA, replication of the genetic material and assembly and release from cell. Ultimately as viruses replicate, they end up destroying the host cells, and if enough of these cells are destroyed the body becomes diseased. .
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