Editing Virion (section)

== Components ==
A virion consists of one or more [[nucleic acid]] [[genome]] molecules (single-stranded or double-stranded [[RNA]] or [[DNA]]) and coatings (a [[capsid]] and possibly a [[viral envelope]]). The virion may contain other [[proteins]] (for example with [[enzyme|enzymatic]] activities) and/or [[nucleoproteins]].<ref name="Dimmock2007"/>

=== Capsid ===
{{main article|Capsid}}

In the vast majority of viruses, the DNA and RNA components are packed into a [[protein]] shell, the [[capsid]].<ref name="Dimmock2007"/> 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 (e.g., [[influenza viruses]]), and in some viruses, the segments can also be individually packaged in their own capsids (e.g., in ''[[Nanoviridae]]'').

[[File:Capsids.jpg|thumb|upright=1.7|Multiple shapes of virions]]
Since the genome of viruses is relatively simple, the capsid architecture relies on repetition of simple structures, similar to the [[Face (geometry)|faces]] of a [[polyhedron]]. Each face in turn is formed by a repetition of simpler sub-units, with the amount of repetitions called a [[triangulation number]] (T). Similar capsid structures can be used by many different types of viruses.{{sfn|Reynolds|Theodore|2023|p=24}}

In many viruses, the virions have [[Capsid#Icosahedral symmetry|icosahedral symmetry]], which can be [[icosahedron|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
* ''[[Gammaretrovirus]]'' and other ''[[Retroviridae]]'', such as [[HIV]], roughly round to complexly multiform ([[Pleomorphism (virus)|pleomorphic]]).
From observations using [[microscopy]], there are indications of many more distinct shapes.<!--see Nucleocytoviricota, for which there is currently a preprint 2023-11-10 -->

=== Tail ===
[[File:Bacteriophage structure.png|thumb|Tailed bacteriophage structure: (1) head, (2) tail, (3) DNA, (4) capsid, (5) collar, (6) sheath, (7) tail fibres, (8) spikes, (9) base plate]]
In some groups of viruses—such as the class ''[[Caudoviricetes]]'' ("tail viruses") and the genus ''[[Tupanvirus]]''—the capsid carries an appendage called the "tail".

The tail of the ''[[Caudoviricetes]]'' is usually divided into:
* a neck, possibly with collar a <!-- What is "m.o.w."? --> long, possibly contractile tail sheath
* base plate
* possibly tail fibers/tail spikes
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.<ref name="Leprince2022"/> The ''Caudoviricetes'' tail material is also differentiated into major and minor tail proteins (MTP and mTP), as seen in the [[Escherichia virus lambda|Enterobacteria phage lambda]].<ref name="PDBe"/> In addition, there may be a tail spike protein (TSP)<ref name="Dunne2021"/> or tail fiber protein (TFP).

Even in viruses with helical [[morphology (biology)|morphology]] (such as the ''[[Rudiviridae]]'' and ''[[Ahmunviridae]]''), the terminal fiber proteins responsible for the [[receptor (virology)|receptor binding]] are called tail fiber proteins.<ref name="Laso2023"/><ref name="Zhang2023"/><ref>{{cite journal|vauthors=Lawrence CM, Menon S, Eilers BJ, Bothner B, Khayat R, Douglas T, Young MJ|date=8 May 2009|title=Structural and functional studies of archaeal viruses|journal=J Biol Chem|volume=284|issue=19|pages=12599–12603|doi=10.1074/jbc.R800078200|pmc=2675988|pmid=19158076|doi-access=free}}</ref>

=== Spikes ===
{{Main article|Peplomer}}

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 device; an extendable injection apparatus are found in virions of the family ''[[Tectiviridae]]''.

=== Viral envelope ===
{{Main article|Viral envelope}}

In many [[viruses|virus species]], the virion also has an outer membrane, the [[viral envelope]].<ref name="Dimmock2007"/> The envelope includes a [[lipid bilayer]] and surface [[proteins]], similar to the [[cell membranes]], that are usually used for the envelope construction when the virus is exiting the cell. This structure helps with attachment to the cell and also assists evading the [[immune system]] of the host organism while the virion is searching for a cell to infect.{{sfn|Reynolds|Theodore|2023|p=20}}