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A virion (plural, ''viria'' or ''virions'') is an inert
virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...
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 (''
core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (laboratory), a highly specialized shared research resource * Core (manufacturing), used in casting and molding * Core (optical fiber ...
'', either
DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
or
RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...
, along with occasionally present virus core protein) inside the virion is usually enclosed in a protection shell, known as the
capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or m ...
. While the terms "
virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...
" 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 In epidemiology, infectivity is the ability of a pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be refer ...
of a virion.


Components

A virion consists of one or more
nucleic acid Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
molecules (single-stranded or double-stranded
RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...
or
DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
) and coatings (a
capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or m ...
and possibly a
viral envelope A viral envelope is the outermost layer of many types of viruses. It protects the genetic material in their life cycle when traveling between host cells. Not all viruses have envelopes. A viral envelope protein or E protein is a protein in the en ...
). The virion may contain other
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...
(for example with enzymatic activities) and/or nucleoproteins.


Capsid

In the vast majority of viruses, the DNA and RNA components are packed into a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
shell, the
capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or m ...
. 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 Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...
viruses of the '' Narnaviridae'' and the
viroids Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective econo ...
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 ''Orthomyxoviridae'' () is a family of negative-sense RNA viruses. It includes nine genera: '' Alphainfluenzavirus'', '' Betainfluenzavirus'', '' Gammainfluenzavirus'', '' Deltainfluenzavirus'', '' Isavirus'', '' Mykissvirus'', '' Quaranjavir ...
), 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 In geometry, a polyhedron (: polyhedra or polyhedrons; ) is a three-dimensional figure with flat polygonal Face (geometry), faces, straight Edge (geometry), edges and sharp corners or Vertex (geometry), vertices. The term "polyhedron" may refer ...
. 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 In mathematics, and especially in geometry, an object has icosahedral symmetry if it has the same symmetries as a regular icosahedron. Examples of other polyhedra with icosahedral symmetry include the regular dodecahedron (the dual polyhedr ...
, which can be ideally isometric or elongated. Many virions also have other shapes: * '' Inoviridae'' and ''
Filoviridae ''Filoviridae'' () is a International Committee on Taxonomy of Viruses, family of RNA virus#Group_V_–_negative-sense_ssRNA_viruses, single-stranded negative-sense RNA viruses in the order (biology), order ''Mononegavirales''. Two members of the ...
'': thread-like/filamentous/helical * '' Ampullaviridae'': bottle-shaped * '' Bicaudaviridae'', ''
Fuselloviridae ''Fuselloviridae'' is a family of viruses. Sulfolobus species, specifically shibatae, solfataricus, and islandicus, serve as natural hosts. There are two genera in the family. Viruses in ''Fuselloviridae'' are ubiquitous in high-temperature (≥ ...
'', ''
Halspiviridae ''Halspiviridae'' is a family of viruses that consists of a single genus, ''Salterprovirus'', which consists of a single recognised species; Halovirus His1 (His1, ''Salterprovirus australiense''). This virus was isolated from hypersaline water ...
'' and '' Thaspiviridae'': spindle- to lemon-shaped * ''
Poxviridae ''Poxviridae'' is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. The family contains 22 genera that are assigned to two subfamilies: ''Chordopoxvirinae'' and ''Entomopoxvirinae''. ''Entomopoxvirinae'' ...
'' and '' Ovaliviridae'': ovoid to ellipsoid * ''
Gammaretrovirus ''Gammaretrovirus'' is a genus in the ''Retroviridae'' family. Example species are the murine leukemia virus and the feline leukemia virus. They cause various sarcomas, leukemias and immune deficiencies in mammals, reptiles and birds. Introdu ...
'' and other ''
Retroviridae A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. After invading a host cell's cytoplasm, the virus uses its own reverse transcriptase ...
'', such as
HIV The human immunodeficiency viruses (HIV) are two species of '' Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the im ...
, roughly round to complexly multiform ( pleomorphic). From observations using
microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...
, 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 Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
(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 ''Coronaviridae'' is a family (biology), family of Viral envelope, enveloped, positive-strand RNA viruses which infect fish, amphibians, birds, and mammals. The group includes the subfamilies ''Letovirinae,'' ''Orthocoronavirinae'', and ''Pitovir ...
'', 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 virus species, the virion also has an outer membrane, the
viral envelope A viral envelope is the outermost layer of many types of viruses. It protects the genetic material in their life cycle when traveling between host cells. Not all viruses have envelopes. A viral envelope protein or E protein is a protein in the en ...
. The envelope includes a
lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cell (biology), cells. The cell membranes of almost all organisms and many viruses a ...
and surface
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...
, similar to the
cell membranes The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extra ...
, 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 The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
of the host organism while the virion is searching for a cell to infect.


References

Protein Data Bank in Europe
NMR structure of the gpu tail protein from lambda bacteriophage
On: ''ebi.ac.uk''
N. J. Dimmock, Andrew J. Easton, Keith Leppard: ''Introduction to Modern Virology.'' 6th edition, Wiley & Blackwell, Malden 2007, ISBN 978-1-4051-3645-7, p. 49, Chapter 4: ''Classification of Viruses.''. Matthew Dunne, Nikolai S. Prokhorov, Martin J. Loessner, Petr G. Leiman: ''Reprogramming bacteriophage host range: design principles and strategies for engineering receptor binding proteins.'' In: ''Current Opinion in Biotechnology.'' Volume 68, April 2021, pp. 272–281, doi:10.1016/j.copbio.2021.02.006. Audrey Leprince, Jacques Mahillon: ''Phage Adsorption to Gram-Positive Bacteria.'' In: ''MDPI'': ''Viruses.'' Volume 15, No. 1, October 29, 2022, p. 196, doi:10.3390/v15010196. Yu Zhang, Zhongjie Zhu, Yuchan Ma, Zhifeng Fu: ''Paper-based analytical device integrated with bacteriophage tail fiber protein for bacteria detection and antimicrobial susceptibility test.'' In: ''Biosensors and Bioelectronics'', volume 239, November 1, 2023, p. 115629; doi:10.1016/j.bios.2023.115629.


Sources

* {{cite book, last1=Reynolds, first1=M.M., last2=Theodore, first2=L., title=A Guide to Virology for Engineers and Applied Scientists: Epidemiology, Emergency Management, and Optimization, publisher=Wiley, year=2023, isbn=978-1-119-85313-8, chapter-url=https://books.google.com/books?id=ILukEAAAQBAJ&pg=PA24, chapter=Basics of Virology, access-date=2024-11-30, pages=19–32 Virology