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What is the Influenza Virus?
The influenza virus is responsible for the seasonal flu. There are three types of influenza virus: Type A, Type B, and Type C. Type A and B cause epidemics, while Type C is limited in range. Type A influenza viruses are placed into subtypes, depending on the proteins lining the surface of the virus. These include neuraminidase and hemagglutinin. The influenza virus infects a host cell, replicates within the host cell and spreads to nearby cells. The replication of the influenza virus is dependent on the host cell's translational process.
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The Influenza Virus Enters the Cell
The replication of influenza virus starts with the virus entering a host cell. It utilizes the haemagglutinin to attach itself to the surface of the cell. The cell responds by sinking that portion of the cell membrane. Eventually, the virus is encapsulated into a vesicle that detaches from the cell membrane and enters the cytoplasm. This process is called endocytosis.
The vesicle is acidic, which changes the structure of the haemagglutinin. This causes the virus to push itself toward the membrane of the vesicle until the two merge. Then, the contents of the virus enters the cell cytoplasm.
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The Influenza Virus Enters the Host Cell Nucleus
Several components of the virus are free floating in the cytoplasm. The genetic material is contained within the nucleocapsid, and it heads toward the nucleus. It enters the nucleus through the nuclear pore. Inside the nucleus, the viral RNA is transcribed into mRNA, which leaves the nucleus and heads toward the ribosome for translation. Several viral proteins are produced, including nucleoproteins, matrix proteins and transmembrane proteins. Some of these proteins travel back to the nucleus, while others remain in the cytoplasm.
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Creating Positive Sense RNA from Negative Sense RNA
The viral RNA of the influenza virus is negative sense. For the virus to replicate, that is create another negative sense RNA strand, it must first produce a positive sense RNA strand. One of the proteins translated from the viral mRNA transcribes the negative sense strand. The resulting positive sense strand is then transcribed, creating a copy of the original negative sense viral RNA. This negative sense RNA is shuttled out of the nucleus and into the cytoplasm by viral proteins.
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Assembling the Virus
In the cytoplasm, the new viruses are assembled. Some proteins are processed through the endoplasmic reticulum and golgi apparatus after leaving the ribosomes. Nucleocapsids form around the viral genetic material, either during the transport process out of the nucleus or in the cytoplasm. The various parts of the virus come together at the host's cell membrane.
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Exiting the Cell
As the viruses assemble, they begin to attach to the cell membrane. In a process called budding, the transmembrane proteins of the virus embed themselves in the host's cell membrane. This region of the host's cell membrane bulges outward. The nucleocapsid containing the viral genetic material enters this bulge. Eventually, the transmembrane proteins of the virus surround the nucleocapsid and break away from the host cell. A new virus cell has been replicated, and it will search for a host cell.
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1. "Seasonal Influenza (Flu)." http://www.cdc.gov/flu/about/viruses/
2. "Replication." http://www.stanford.edu/group/virus/1999/rahul23/replication.html
3. "Influenza virus Replication or Flu Life Cycle." http://www.rkm.com.au/virus/influenza/lifecycle.html