Influenza A viruses contain their genome in eight separate linear segments of negative-sense RNA, which code for ten proteins (eleven for type A if including the novel PB1-F1 protein) [7]. Each segment contains a single gene, but some can be read twice at different starting points to create two distinct proteins. The segmented nature of the genome also allows for the exchange of entire genes between different viral strains when they cohabitate the same cell. The 8 genes are:HA gene encoding hemagglutinin which produces about 500 copies NA gene encoding neuraminidase which produces about 100 copies NP gene encoding nucleoprotein. Influenza A, B, and C are distinguished by their nucleoproteins M gene encoding two matrix proteins (the M1 and the M2) by using different reading frames from the same RNA segment NS gene encoding two distinct non-structural proteins by using different reading frames from the same RNA segment PA gene encoding an RNA polymerase PB1 gene encoding an RNA polymerase and PB1-F2 protein (induces apoptosis) by using different reading frames from the same RNA segment PB2 gene encoding an RNA polymerase The genome segments have common terminal sequences, and the ends of the RNA strands are partially complementary, allowing them to bond to each other by hydrogen bonds. After transcription from negative-sense to positive-sense RNA the +RNA strands get the cellular 5' cap added, allowing its processing as messenger RNA by ribosomes. The +RNA strands also serve for synthesis of -RNA strands for new virions.
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