Virus structure and classification
The nucleic acid is enclosed in a sort of container or lining, called the capsid and made up of the repetition of protein subunits, called capsomers, in turn formed by the repetition of one, two or more proteins. In the papilloma virus, for example, the capsid is made up of several copies of two different proteins, L1 and L2; the first, more abundant, has the ability to self-aggregate and form nucleic acid-free virions (it is very immunogenic and is therefore used as a basis for the production of vaccines).
The core guarantees the virus the ability to carry out the processes of multiplication and growth, while the capsid gives shape and stability to the viral particle, protects its genome and is important for infectivity (thanks to the presence of anti-receptor structures).
Taken together, capsid and nucleic acid form the nucleocapsid.
In nature there are viruses formed only by nucleic acids and capsid, and for this reason called "naked viruses"; bacteriophages are an example of naked viruses. There are also other viral particles that present, in addition to the capsid, an outermost membrane, called the pericapsid, peplos or cloak (envelope), which derives from the plasma membrane of the host cell and as such consists mainly of phospholipids, but also of polysaccharides and proteins (generally glycoproteins). Viruses with pericapsid are called "virus coated"and generally also have a protein structure, called matrix or integument, located between the capsid and the pericapsid.
The coated viruses - thanks to the pericapsid phospholipids - penetrate more easily into susceptible cells and reduce the host's immune response; the adenovirus, responsible for conjunctivitis and parainfluenza symptoms, is an example of a coated virus, like all other viruses that infect eukaryotes.
The viral capsid can normally assume two different conformations, being characterized by one icosahedral type structure or helical.
The icosahedron is a solid, formed by equilateral triangles, which has 12 vertices, 20 faces made up of equilateral triangles and 30 edges corresponding to the sides of the triangles, while the vertices are the meeting points of the sides. This crystallographic structure derives from the union regulate more protein; each of them is called monomer and these monomers can join five to five forming pentons, or - in larger viruses - 6 to 6 forming the so-called exons. The association of pentons and exons forms the eicosahedron, inside which the genetic material is found.
In the second structural typology of the capsid, called helical, the single monomers are arranged in a spiral around an axis (the proteins are arranged like the steps of a spiral staircase), forming a hollow cylinder that encloses the nucleic acid.
The Tobacco Mosaic virus is a typical example of a helical structure virus, while the papilloma virus has an icosahedral structure. There are also viruses in which the capsid lacks symmetry and has a mixed morphology (such as the HIV virus, which consists of an icosahedral capsid and a filamentous core of nucleic acid).
Each virus is identified by a species, a genus and a family. For example, if we take the HIV virus, the species indicates the common name of the microgranism, that is acquired immunodeficiency virus or, precisely, HIV. This virus belongs to the genus lentivirus (ie with a slow growth cycle) and to the family retroviridae (retroviruses are viruses with a replicative cycle characterized by two forms of nucleic acid, which present an enzyme called reverse transcriptase, capable of operating a reverse transcription transforming the viral RNA into DNA).
Each viral species is then described on the basis of the structure (size, morphology and type of nucleic acid), the type of replication (nuclear or cytoplasmic), the host cell (animals, fungi, plants or bacteria), the tropism of tissue or organ and the type of transmission (fecal-oral, parenteral, etc.).
Antiviral vaccines can consist of killed (inactivated), attenuated viruses (they have maintained the ability to replicate but have lost much of their pathogenic potential) or of viral subunits (new generation vaccines are formed, for example, from viral pericapsid proteins , capable of stimulating passive immunity). A new generation preparation is the hepatitis B vaccine, based exclusively on a capsid glycoproein of the HBV virus.
Other articles on "Virus Structure and Classification"
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