By Dona Hill

influenza virus

The influenza virus belongs to the Orthomyxoviridae family. It is a negative RNA virus with a segmented genome into 8 fragments that code for 11 proteins of the virus (explained during this article). In addition, it contains an envelope that protects it from the adverse factors of nature, as well as sudden increases in temperature, an increase in pH, etc. First, it is necessary to explain what the virus envelope looks like to better understand the influenza life cycle.

Influenza Virus Structure

The virus envelope is made up of a lipid bilayer (from the plasma membrane of the host cell) that contains three transmembrane viral proteins that are very important for the reproductive cycle of the virus: the hemagglutinin protein (HA), neuramidase (NA) and the protein matrix 2 (M2); with HA being the most abundant protein. In addition, the envelope contains areas called lipid rafts very rich in cholesterol and areas not so rich in cholesterol. Located just below the envelope, is the matrix protein 1 (M1) (also encoded by the virus) which has the function of maintaining the core of the virus.

The First Stage

As mentioned above, the life cycle of the influenza virus can be divided into stages: first, the virus enters into the host cell thanks to the binding between the virus HA protein and sialic acids present on the surface of the plasma membrane of the host cell. Internalization occurs through an endosome of the virus within the cell. The endosome has an acidic environment that is very important, since it activates the virus matrix protein 2 (M2), which has the function of releasing ribonucleoproteins (RNPs) that leave the cytoplasm of the host cell to travel to the nucleus, thanks to the interaction of numerous viral proteins such as nucleoprotein (NP), acid polymerase (PA) and basic polymerase protein 1 (PB1), among others.

The Second Stage

Next, transcription and replication of the viral genome takes place. Being a single-stranded negative RNA genome, it first has to become positive RNA in order to be replicated by RNA polymerase. Once the DNA is obtained, the transcription can begin, to obtain viral proteins. Six of the eight segments of the influenza virus genome code for a single protein, while segments 7 and 8 code for two different proteins; specifically, segment 7 encoded for proteins M1 and M2 and segment 8 encodes for non-structural protein 1 (NS1) and non-structural protein 2 (NS2).

The next step in the reproductive cycle of the virus consists of the export of all these encoded proteins into the cytoplasm to be able to assemble new virions. Since the influenza virus is an enveloped virus, it needs the plasma membrane of the host cell to form the virus particles or virions and to be able to leave the cell to infect neighboring cells.


This is a simple summary of the steps that occur during the reproductive cycle of the influenza virus, as the actual process is more complicated and there is still much to learn and discover. We hope that by learning how viruses replicate in host cells, we can develop better drugs and vaccines to protect us.