The Influenza Genome Comprises Eight Segments
In contrast to many other viruses, the influenza genome is composed of RNA rather than DNA. The genome comprises eight segments, and only virus particles containing all eight segments are viable. New genomes can be assembled as a result of mixing between two viruses. That is, if a cell is infected with two different strains of influenza, it is possible for the segments to re-assort to create new viruses that have segments from each original virus. New combinations may be more or less virulent than the old combinations. Figure 1 below shows the relative sizes of the eight influenza segments as well as the genes that are specified by each. Figure 2 shows how the segments are arranged during viral replication.
Figure 1: The eight RNA segments of the influenza genome.
Figure 2: The systematic arrangement of genes during flu virus replication.
(Figure 2 courtesy of the National Institute of Allergy and Infectious Diseases)