Virion

Schematic structure of the HI-virion.

A virion (plural viria, virions or virions), rarely also viron, is a single virus particle that is located outside a cell. Virions or viruses are not considered living things (organisms), particularly because they do not have their own metabolism and can only replicate (multiply) parasitically using foreign cells.

While the term "virion" is used solely to describe the structure of virus particles outside of cells, the broader term "virus" also includes biological properties such as the infectivity of a virion.

Components

A virion consists of one or more nucleic acid molecules (genome: single-stranded or double-stranded RNA or -DNA). The virion may contain other proteins (for example with enzymatic activities) and/or nucleoproteins.

capsid

Main articles: capsid and capsid protein

In the vast majority of viruses, these components are packed into a protein capsule, the capsid. The capsid proteins are often differentiated into major and minor capsid proteins (MCP and mCP).

In exceptional cases, there are also viruses without a capsid (i.e., true virions), such as the RNA viruses of the Narnaviridae and the viroids of the Pospiviroidae (with the Citrus Exocortis Viroid and the Citrus Bark Crack Viroid).

If the genome consists of several segments, these are usually packaged together in a capsid (example influenza viruses), in some viruses the segments can also be individually packaged in capsids (example Nanoviridae).

In many viruses, the virions have icosahedral symmetry, which can be ideally isometric or elongated. Many virions also have other shapes:

• Inoviridae and Filoviridae: thread-like/filamentous/helical
• Ampullaviridae: bottle-shaped
• Bicaudaviridae, Fuselloviridae, Halspiviridae and Thaspiviridae: spindle- to lemon-shaped
• Poxviridae and Ovaliviridae: ovoid to ellipsoid
• or as in the genus Gammaretrovirus and other Retroviridae such as HIV, roughly round to complexly multiform (pleomorphic).

From microscopy there are indications of many more special morphotypes.

Tail

In some groups of viruses - such as the class Caudoviricetes ("tail viruses") and the genus Tupanvirus - the capsid carries an appendage called the "tail".

Morphology of the Caudoviricetes using the example of the Enterobacteria phage T2

The tail of the Caudoviricetes is usually divided into

• neck, possibly with collar a m.o.w. long, possibly contractile tail sheath (engl. )
• a base plate
• possibly tail fibers/tail spikes (engl. ).

The latter are used to establish contact with the host cell. The tail of these viruses serves as an injection device to introduce their own genome into the host cell. The Caudoviricetes tail is also divided into and (MTP and mTP), for example in the Enterobacteria phage lambda, in addition there may be a (TSP) or (TFP).

Even in viruses with helical morphology (such as the Rudiviridae and Ahmunviridae), the terminal fiber proteins responsible for the receptor binding are called tail fiber proteins (tail fiber proteins).

Spikes

Main article: Peplomer

So-called spikes (peplomers) can protrude from the capsid, as in the Coronaviridae, the Tectiviridae and others. These are used to establish contact with the host cell.

In viruses of the genus Chlorovirus, the virions have a single spike that serves as an injection site. pparat serves; extendable injection apparatus are found in virions of the family Tectiviridae.

Viral envelope

Main article: Viral envelope

In many virus species the virion also has an outer lipid membrane, the viral envelope.