Influenza

Influenza (or as it is commonly known, the flu or the grippe) is a contagious disease, caused by an RNA virus of the orthomyxoviridae family. It rapidly spreads around the world in seasonal epidemics, imposing considerable economic burden, in the form of health care costs and lost productivity. Major genetic changes in the virus have caused three influenza pandemics in the 20th century, killing millions of people. The name comes from the old medical belief that unfavourable astrological influences cause the disease.

Genetics

Influenza A viruses contain their genome in eight separate linear segments of negative-sense RNA. 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:

Related Topics:
Genome - Negative-sense RNA - Gene - Strains

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• 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
• 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.

Related Topics:
Complementary - Hydrogen bond - Positive-sense RNA - 5' cap - Messenger RNA - Ribosome

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The RNA synthesis and its assembly with the nucleoprotein takes place in the cell nucleus, the synthesis of proteins takes place in the cytoplasm. The assembled virion cores leave the nucleus and migrate towards the cell membrane, with patches of viral transmembrane proteins (hemagglutinin, neuraminidase and M2 proteins) and an underlying layer of the M1 protein, and bud through these patches, releasing finished enveloped viruses into the extracellular fluid.

Related Topics:
Cell nucleus - Cytoplasm - Cell membrane - Transmembrane protein - Bud - Extracellular fluid

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