Animation of a flu virus using a cutaway model showing internal structures.
Haemagglutinin (red) and Neuraminidase (yellow) spikes pass through the green viral envelope to dock with the underlying matrix (M) proteins shown (purple). Inside the matrix shell are the RNPs (ribonucleoproteins) containing viral genes. Flu viruses have eight separate RNPs. If two or more virus strains infect the same cell, progeny viruses can incorporate segments from more than one parent leading to new and possibly more dangerous strains. This is jumbling up of genes is called genetic reassortment and is why flu is so variable and why new strains crop up regularly. It can also mix up flu strains from different species such as pigs, birds, and humans. Flu occurs in seasonal epidemics and periodically as major pandemics.

VIRUS ANIMATION: Influenza Virus particle: model of an influenza (FLU) virion

Influenza A viruses: are covered by a characteristic array of spikes that project radially from the viral envelope. Beneath the envelope lies the matrix (M1 protein) which covers the viral genetic material (RNA).

Spikes: there are two kinds of spike: Haemagglutinin (Hemagglutinin) usually abbreviated as HA (shown in red in model) and Neuraminidase usually abbreviated as NA (shown in yellow in the model). There are several varieties of HA and NA designated by numbers e.g. the current bird flu is H5N1.

M2: this is a small protein that is embedded in the viral envelope. Four molecules associate together to create a channel through the envelope that allows protons to pass through (the M2 tetramer is shown in blue in the top image). There are only a few such pores in each virus particle. The M2 proton channel is important during replication when it allows protons to enter the virion where the acidification is thought to loosen the matrix from the underlying RNP.

Envelope: the viral envelope is derived from the host cell plasma membrane when the virus buds from the cell. In the upper images it is shown as a grey translucent layer.

RNP - ribonucleoprotein: consists of the genetic information of the virus wrapped up in protein. This combination of gentic material and protein is called the nucleocapsid. The genetic information is stored as single stranded -ve sense RNA. The full complement of genetic information is called the genome and in influenza A the genome is divided into eight segments. These segments are assumed to link together (possibly in an ordered fashion) when the virus assembles at the cell surface. Because the genome of influenza is segmented there can be a mixing of genes when two or more types of influenza infect the same cell and segments from several sources become jumbled together in the progeny virus. Such novel strains could lead to PANDEMIC INFLUENZA one of the greatest threats we currently face.

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