A virus, (in Latin, toxin or poison), is a sub-microscopic, acellular particle that cannot survive in the absence of a living cell/host cell. Viruses cannot reproduce on their own as they are dependent on host metabolic machinery and ribosomes to replicate and reproduce. Since antibiotics do not harm a virus, treatment for viral diseases such as flu mainly helps ease the symptoms rather than to kill the viruses. Most viruses cause generally mild diseases like the common cold and some even don't cause any symptoms and may go unnoticed, but some cause diseases that can be severe and deadly like Avian influenza, AIDS, and some forms of cancer.
Influenza viruses are pleomorphic (variable),
mostly spherical or ovoid and filamentous, ssRNA (single-stranded
RNA) enveloped viruses with a helical symmetry (Fig. 2). They
are covered over by lipid/lipoprotein envelope.
The viral envelope has lipoprotein membranes that enclose nucleocapsids and nucleoproteins.
The diameter of each enveloped virus ranges from 50-120 nanometers
(nm) and filamentous virions are 20nm in diameter and 200-300nm
long (Fig. 3). The genome is in the form of eight negative-sense
ssRNA fragments (seven for Type C). The total genome length is
12000-15000 nucleotides (nt), the largest segment being 23-25
and the smallest being 800-900 nt. All have terminal repeats at
their 5' end and 3' end about 9-13 nucleotides long (Fig. 4).
The 5' and 3' terminal sequences of all RNA strands are highly
conserved. The longest RNA strand is closely associated with the
nucleoprotein to form helical symmetry.
There are some 500 distinct spike-like surface proteins of the viral envelope, each projecting 10-14 nm from the surface. There are mainly four types of glycoproteins /antigens:
- Hemagglutinin (HA) is a 135Å trimer, a major glycoprotein present on the viral surface as rod-shaped projections, and mediates the attachment of the virus to the cellular receptor. There are 16 types of HA reported.
- Neuraminidase (NA) is a 60Å tetramer, a kind of glycoprotein, and 9 types have been reported. The ratio of HA to NA is about 4-5 to 1.
- Nucleocapsid protein (NP) It coats the RNA strands.
- Matrix protein (M) The inner side of the viral envelope is lined by the matrix protein.
Go To Replication, transcription & translation
List of Visuals
- Fig. 2. Electron micrograph of
the Influenza virus (~200nm in diameter)
Linda Stannard (Department of Medical Microbiology, University
of Cape Town, South Africa)
- Fig. 3. The influenza (flu) virus
Molecular Expressions, The Florida State University (Michael
W. Davidson, National High Magnetic Field Laboratory, 1800 East
Paul Dirac Drive, The Florida State University, Tallahassee,
- Fig. 4. Different types of mRNAs formed
in the host nucleus by influenza viruses
MicrobiologyBytes (Dr. Alan Cann, School of Biological Sciences,
Learning & Teaching Research Group, Universty of Leicester,
University Road Leicester LE1 7RH United Kingdom)