Genetics of the Flu Virus

The influenza, or flu, virus can infect both birds and mammals. This is not a genetic disease. Instead, it is passed from infected mammals through the air by coughs and sneezes and from infected birds through their droppings. It can also be transmitted if to a mammal if it touches a surface that has been contaminated with an infected mammal’s saliva, nasal secretions, feces and/or blood. The virus can stay infectious for about a week at a human’s body temperature, over a month at zero degrees Celsius, and even longer at lower temperatures. However, disinfectants and detergents do kill the virus.

When the virus enters a mammal’s or bird’s body, it must be able to replicate in order to survive. It does this by binding through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells (shown in Figure 1 as stage one). These cells are normally found in the nose, throat and lungs of mammals and in the intestines of birds. The cell brings in the virus by the process of endocytosis. Then part of the hemagglutinin protein that has been fused with the viral envelope is released into the cytoplasm as viral RNA (vRNA) molecules along with the RNA-dependent RNA transcriptase. The proteins and vRNA form a complex that is transported into the cell nucleus where the RNA-dependent RNA transcriptase begins transcribing it. Then it is either transported to the cytoplasm to be translated or left in the nucleus. The newly synthesized viral proteins are either secreted through the Golgi apparatus onto the cell surface or transported back to the nucleus to create more viral particles. The vRNA that forms the genomes of future viruses, RNA-dependent RNA transcriptase, and other viral proteins are assembled into a viron. Then hemagglutinin and neuramindase molecules cluster together in the cell membrane, and the vRNA and viral core proteins leave the nucleus to enter the membrane protrusion. The mature virus then buds off the cell. Once the new virus is released, its host cell dies off.

RNA-dependent RNA transcriptase makes an insertion error about every ten thousand nucleotides which is also about the length of the influenza vRNA. Therefore, this virus is a mutant known as antigenic drift. This happens because there are no RNA proofreading enzymes. Once vRNA assortments have occurred and more than one viral line infects a single cell, antigenic shifts occur, and this allows the virus to infect new species hosts and overcome protective immunity.

Here is a pretty good video.
Here is a pretty good diagram of the replication process.
This site has some more diagrams on the flu virus.