Outside the cell the virus stays as inert particle known as virion. It can’t propagate in the environment.
Virion is the extracellular form of virus existence. Virions are liberated from the cells after virus maturation.
The inner (or core) part of virion harbors viral genome – the molecule of nucleic acid.
Viral nucleic acid is enwrapped with protein coat, termed a capsid.
Capsid is composed of identical protein subunits called capsomeres.
Viral capsid proteins are bound tightly to genomic nucleic acid. This complex is termed nucleocapsid. Great number of viruses contains the capsid as the only external coat. These viruses are determined as the naked (or simplex) viruses. More composite viruses carry additional outer lipid membrane or envelope, which is usually obtained from the cytoplasmic membrane of the host cell. These viruses are designated as enveloped viruses.
Lipid coat usually contains the protein termed the matrix protein. Viral glycoprotein receptors form spikes that jut out of the viral envelope. They are responsible for the viral attachment to the susceptible cells.
Viral structure is organized according to different types of symmetry. Widespread is icosahedral (or cubic) symmetry, because it is based on the icosahedral pattern – polyhedron with 20 faces. This assemblage provides the most efficient fit of capsomers into the tightly packed coat. Virions of icosahedral symmetry are usually spherical in shape.
Another type is helical symmetry, where protein subunits with core nucleic acid are wound into the common helix with periodic coils. This complex is packed within the external lipid envelope.
And some viruses with composite architectonics possess complex structure (or mixed symmetry). For instance, bacteriophages resemble spermatozoids in their shapes, where phage head is of icosahedral symmetry and the tail is of helical symmetry. Poxvirus resembles brick in shape with lateral bodies within the envelope.
Naked viruses are usually resistant to the ether or other organic solvents, whereas enveloped lipid-containing viruses are sensitive. Impairment of external lipid shell leads to the loss of viral infectivity.
There is a very special group of viruses, composed solely of nucleic acid, termed viroids.
Viroids are the smallest viral agents infecting plants. Known viroids contain only the small single-stranded circular RNA. They are devoid of capsid and viroid RNA doesn’t encode protein molecules.