Molecular and Cell Biology of the Flu Virus

In their most basic form, viruses are made up of a genetic nucleic acid enclosed in a capsid shell, which forms a protective layer around the genetic material. The influenza virus belongs to the Orthomyxoviridae family of viruses, meaning that it has genetic material in the form of RNA and that the capsid enclosing the RNA is surrounded by a viral envelope. This envelope is typically made from the cell membrane of the infected cell from which a particular virion, or single virus particle, comes. The envelope of the human flu virus is spherical, between fifty and a hundred and twenty nanometers long, and is covered with approximately five hundred projections of hemagglutinin and neuraminidase. There is about a 4:1 ratio of hemagglutinin to neuraminidase.

It is the hemagglutinin that allows the virion to attach itself to the cell membrane of a host cell. It is a glycoprotein made up of two subunits: three identical monomers arranged in an a-helix and three spherical “heads” for bonding to sialic acid. Sialic acid is a monosaccharide that is dispersed along the host cell’s outer membrane, and the flu virus attaches itself to a host cell by latching on to the sialic acid. Because the human body is able to identify and destroy flu viruses based on their hemagglutinin, hemagglutinin is constantly mutating to stay one step ahead of the immune system. Minor changes in the coding regions of the viral RNA will produce hemagglutinin that is shaped slightly differently and is therefore unrecognizable by white blood cells.

Once attached to the host cell, the virus is able to be taken into the cell by endocytosis, where the virus’ RNA invades the host nucleus and goes to work replicating and coding proteins that will later become new viruses. When enough hemagglutinin and neuraminidase have been excreted by the Golgi apparatus and attach themselves to the host cell’s membrane, and a complete set of the viral RNA has been collected, the cell membrane closes around the viral RNA. This is when the neuraminidase goes to work, because its job is to break the bond between the sialic acid and the hemagglutinin. Once the new virion is free of the host cell, it can go off and infect other cells, thus repeating the cycle.