A virion (plural, viria or virions) is an inert virus particle capable of invading a cell. Upon entering the cell, the virion disassembles and the genetic material from the virus takes control of the cell infrastructure, thus enabling the virus to replicate. The genetic material ( Viral core, either DNA or RNA, along with occasionally present virus core protein) inside the virion is usually enclosed in a protection shell, known as the capsid.
While the terms "virus" and "virion" are occasionally confused, recently "virion" is used solely to describe the virus structure outside of cells, while the terms "virus/viral" are broader and also include biological properties such as the infectivity of a virion.
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 activities) and/or
nucleoproteins.
Capsid
In the vast majority of viruses, the DNA and RNA components are packed into a
protein shell, 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 (e.g., influenza viruses), and in some viruses, the segments can also be individually packaged in their own capsids (e.g., in Nanoviridae).
Since the genome of viruses is relatively simple, the capsid architecture relies on repetition of simple structures, similar to the 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.
In many viruses, the virions have icosahedral symmetry, which can be icosahedron or elongated. Many virions also have other shapes:
From observations using
microscopy, there are indications of many more distinct shapes.
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".
The tail of the Caudoviricetes is usually divided into:
-
a neck, possibly with collar a 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.
The
Caudoviricetes tail material is also differentiated into major and minor tail proteins (MTP and mTP), as seen in the Enterobacteria phage lambda.
In addition, there may be a tail spike protein (TSP)
or tail fiber protein (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.
Spikes
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
In many
viruses, the virion also has an outer membrane, the
viral envelope.
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.
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