Since viruses do not belong to the cellular form of life, the term "virion" is used to denote a discrete viral particle. This concept was introduced in 1962 by the Frenchman Andre Lvov.
The virus does not exist in this form permanently, but only at a certain stage of its life cycle.
What is virion
Virion is the final phase of the development of the virus, including a complete set of structural and functional elements packaged in a single particle. This form is characteristic of the extracellular stage of the life cycle of the virus, however, for some time after assembly, the virion may exist inside the infected cell.
Since the virion is just a designation of a morphological unit, it should not be identified with the concept of "virus". The latter includes the entire set of biological properties characterizing this taxon, and not just structural features.
The structure of the virion
A viral particle consists of a nucleic acid (RNA or DNA) surrounded by a protein layer (capsid), which performs protective functions and provides interaction with the host cell. Some virions have an additional shell in the form of a bilipid membrane penetrated by spikes of viral proteins sticking out. This structure is of cellular origin and is called supercapsid. Sizes of viral particles range from 20 to 200 nm.
Protein subunits of the virion membrane can be folded into various spatial configurations, on the basis of which the morphological classification of viruses is built. According to the type of structural organization, virions are distinguished:
- with spiral symmetry - protein units are arranged in a spiral, in the center of which lies a similarly structured nucleic acid;
- with cubic symmetry - equilateral triangles (capsomeres) formed from protein molecules form various forms of polyhedra (tetrahedrons, octahedrons, icosahedrons, etc.);
- with binary (mixed) symmetry - a combination of both types of organization in one viral particle (typical for bacteriophages);
- intricately organized, covered with supercapsid.
In addition to the structural subunits of the envelope, some virions contain enzymes necessary for the transcription of genetic material.
The spatial structure, protein composition and type of nucleic acid of the virion are the main taxonomic signs of the biological differentiation of viruses. Additional criteria are life cycle features and host spectrum.
Genetic material of viral particles
Unlike the genetic material of other organisms, virus virions contain only one type of nucleic acid: DNA or RNA. These molecules can be ring or linear, fragmented or whole, with closed (fully or partially) or free ends, contain both two chains, and one. Such a variety of nucleic acid organization is characteristic of viruses only.
The viral genome also has a functional characteristic. So, virion RNA can be positive, that is, translated into the host cell with the formation of viral proteins, and negative, not possessing matrix activity (in this case, translation is preceded by the synthesis of positive RNA by the transcriptase enzyme that is part of the virus).
Depending on the combination of these characteristics, 6 types of RNA are distinguished in the composition of virions:
- single-chain unfragmented positive;
- single-chain unfragmented negative;
- single chain fragmented negative;
- negative double-stranded fragmented;
- single chain double positive;
- single chain ring defective.
In the DNA genome, "+" and "-" chains are distinguished and the following types of molecular organization are distinguished:
- partially single chain ring;
- supercoiled closed annular;
- single chain linear;
- linear duplex;
- linear duplex with covalently stitched ends;
- single chain linear;
Among all types of genomes, groups are distinguished, each of which is characterized by a certain replication mechanism in the infected cell.
Assembly of the virion inside the host cell
The formation of viral particles is carried out due to enzymes and biosynthesis mechanisms of the infected cell, which the virus makes work for itself. This process involves several steps.
Initially, the virion’s genetic material enters the host cell. Moreover, in simple viruses, the protein shell remains outside, while in complex viruses it penetrates through the fusion of supercapsid with the plasma membrane (receptor endocytosis). In the latter case, the capsid in the cytoplasm is destroyed by the action of lytic enzymes of the phagosome.
On the basis of nucleic acid, 2 processes proceed in parallel: genome replication (creating multiple copies of genetic DNA or RNA molecules) and translation of virion proteins in the ribosomal apparatus of the host cell.
The synthesized protein and genetic elements combine into a nucleocapsid - a complete virion of simple viruses. In complex ones, the assembly is completed at the moment the particle exits the cell, during which the capsid is enveloped with a plasma membrane containing receptor proteins built into it in advance.
Scientific and Production Association "Virion"
The research enterprise "Virion" is the largest pharmaceutical complex for the creation and production of immunobiological preparations in Russia. In 1906, it was founded as the Tomsk Bacteriological Institute named after Ivan and Zinaida Churins, and in 1953 it received the status of the Research Institute of Vaccines and Serums. In 1988, the institute was renamed into the Virion Scientific and Production Association (NPO) , which later became a branch of the Moscow Federal State Unitary Enterprise NPO Microgen.
The main activities of the company include the creation and production of immunoglobulins, antiviral vaccines, probiotics, psychotropic drugs and various diagnostic products. The company is located at Tomsk, Ivanovsky street 8.
Currently, the Virion production complex is a well-known large company with a high-tech production base and a professional staff of 600 people.