Viruses are different from all the organisms that are dealt with in Biology.
They are described as acellular (not composed of cells) and are non-living.
This means that they do not carry out the normal day-to-day processes that living organisms do.
They do not function unless they are inside a living cell of a true biological organism.
Here they take over the cell's processes and use them to make more of themselves.
Virus particles - also known as virions - are built up from some of the same biochemical compounds that living organisms consist of.
These are extremely small and only visible using the electron microscope.
Viruses are not classified in the way that living organisms are. There is no viral Domain/Kingdom, but they may be categorised according to some of their characteristics.
The Baltimore classification - named after David Baltimore, not the place - clusters viruses into families depending on their type of genome.
Virus graphic images - all from ViralZone
Click on pictures for enlarged version (opens in separate window)
(Source: ViralZone www.expasy.org/viralzone, Swiss Institute of Bioinformatics)
The Ebola virus
The Zika virus
Note the stalked attachment proteins
The herpes simplex virus
This virus infects nerve cells in the face, including some near the lips, causing cold sores. HSV can remain inactive inside the body for years.
The Influenza A virus
On the surface of the influenza virus are several proteins, notably H and N antigens, of which there are many versions and combinations, e.g. H5N1.
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This virus infects bacterial cells. The tail fibres are responsible for the attachment of the virus particle to the host cell.
The structure of a virus particle
Virus particles consist of genetic material enclosed in a coat or capsid.
Virus particles contain either DNA or RNA.
This can be used to categorise viruses: DNA viruses, RNA viruses (riboviruses) or retroviruses.
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The outer coat of a virus is composed of protein. This is composed of sub-units called capsomeres
These assemble into geometic shapes. Many are polyhedral - with flat faces, making a regular hollow structure. The most common shape is an icosahedron (20 sides) but some are cylindrical/helical and bacteriophages have a tubular section as well as an icosahedral section.
Animal viruses may have an outer phospholipid-based layer: an envelope. This originates from a previous host cell: the outer plasma membrane or even intracellular membranes.
Many viruses have special attachment proteins projecting from their capsid or envelope. These bind to extracellular molecules on the surface of host cells, and assist the virus in attachment to and and entry specifically into the host cell (and in exit after infection).
adenovirus (viral fibre glycoproteins)
influenza virus (haemagglutinin)
bacteriophage (tail fibres)
The proteins on the outer layer of a virus can trigger a response from the immune system of the infected organism. Different strains of virus can be distinguished by the presence of these proteins.