In this article we will discuss about the morphology and replication of Influenza Virus.
Morphology of Influenza Virus:
Influenza virus (Fig. 14.18) is pleomorphic and its diameter is 80- 120 nm. It is an enveloped virus possessing genome segmented into eight linear single- stranded molecules ranging in size from 890 to 2341 nucleotides. The nucleocapsid of this virus is of helical symmetry, about 6-9 nm in diameter and about 60 nm in length. The envelop contains outer lipid and inner protein layers.
There are two types of spikes, hemagglutinin spikes (H-spikes) and neuraminidase spikes (N-spikes), present on the outside surface of the envelop.
These spikes are about 500 in number (N-spikes only about 100 per virion) and interact with the host cell surface. Hemagglutinin spikes (H-spikes) cause agglutination of red blood cells (RBCs). An important feature of the hemagglutinin of influenza virus is that antibody directed against it prevents the virus from infecting a cell.
Thus, antibody directed against the hemagglutinin neutralizes the virus, and this is the mechanism by which immunity to influenza is brought about during immunization process. However, neuraminidase spike is an enzyme which breaks down the sialic acid component of the cytoplasmic membrane. On the inner side of the envelop there is M-protein which provides rigidness and thus stabilizes the lipid bilayer.
Replication of Influenza Virus:
Influenza virion enters inside the host cell by the process of endocytosis. There in the cytoplasm the nucleocapsid separates from the envelop and moves into the nucleus wherein the viral genome replication takes place. Un-coating results in activation of the virus RNA polymerase. The mRNA molecules are then transcribed from the virus RNA inside the nucleus using oligonucleotide primers and they move to the cytoplasm.
However, the virus proteins are synthesized in the cytoplasm. Ten virus proteins are encoded by the eight segments of viral genome. Some of these proteins are involved in viral genome replication while others are structural proteins used in assembly of virion. Details of assembly of virions are still not known with certainty. The formation of the complete enveloped progeny virion takes place by budding process.