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12.2.2.3.1. Flu virus life cycle

Flu virus life cycle

There are several types of influenza virus (influenza) but basically it is a virus with a membranous envelope, with glycoproteins, a capsid and RNA as genetic material, associated with the reverse transcriptase enzyme.  

  • Adsorption phase. The fixation of the virus occurs when the glycoproteins  of the virus envelope come into contact with the glycoprotein receptors of the membrane of the cell to be infected, which will induce the cell to introduce it by endocytosis.
  • Penetration phase. In the endosome , the viral coat fuses with the cell membrane, releasing viral RNA molecules , accessory proteins, and RNA polymerase into the cytoplasm. Decapsidation then occurs, leaving the viral RNA released from the capsid.

      These proteins and RNA form a complex that is transported to the cell nucleus, where RNA polymerase begins to transcribe positive complementary copies of the reverse RNA (antisense). 

  • Eclipse phase. In this phase, viruses are not seen inside the cell, but in it the cellular metabolism is directed by the viral nucleic acid, to form the components of the new viruses from the materials and energy of said host cell. Initially, the RNA, thanks to reverse transcriptase, gives rise to a copy in DNA. From this transcription occurs, which will direct the replication of new viral RNAs and translation, which gives rise to new viral proteins , reverse transcriptase and glycoproteins of the  virus envelope. Cellular DNA is not destroyed.

Viral RNA can be returned to the cytoplasm and translated, or remain in the nucleus. The newly created viral proteins are secreted through the Golgi apparatus to the cell surface or transported back to the nucleus to bind to the vRNA and form new virus particles. Other viral proteins have multiple actions in the host cell, including the degradation of cellular RNA itself in order to use the resulting nucleotides for the synthesis of more vRNAs and inhibiting the transcription of cellular RNA. The viral genome is composed of eight single-stranded (single-stranded) RNA segments.

  • Assembly of new viruses. When the viral components are synthesized, the capsid is formed, including within the viral RNA associated with a reverse transcriptase. The virus then moves to the cell periphery. Viral glycoproteins enter the host cell membrane. 
  • Lysis or release. Mature viruses cause budding, with small vesicles appearing on the cell membrane. They are then separated from the host cell surrounded by membrane with viral glycoproteins. The flu virus is released without causing lysis of the cell.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Navarra, June 2020, option A, question 9

The flu has its maximum incidence in the first months of the year.

a) What pathogen causes it?

b) Describe its structure.

c) Why is it necessary to update the vaccine against this disease every year?