virion

Examples of virion in the following topics:

• Viral Morphology

  • A virion consists of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope made of protein and phospholipid membranes derived from the host cell.
  • An interesting feature of viral complexity is that host and virion complexity are uncorrelated.
  • Some of the most intricate virion structures are observed in bacteriophages, viruses that infect the simplest living organisms: bacteria.
  • Enveloped virions like HIV consist of nucleic acid and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins.
  • Other envelope proteins include the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions.

• The Lytic and Lysogenic Cycles of Bacteriophages

  • When infection of a cell by a bacteriophage results in the production of new virions, the infection is said to be productive.
  • With lytic phages, bacterial cells are broken open (lysed) and destroyed after immediate replication of the virion.
  • Viruses that infect plant or animal cells may also undergo infections where they are not producing virions for long periods.
  • In a process called latency, these viruses can exist in nervous tissue for long periods of time without producing new virions, only to leave latency periodically and cause lesions in the skin where the virus replicates.

• Steps of Virus Infections

  • Some infected cells, such as those infected by the common cold virus known as rhinovirus, die through lysis (bursting) or apoptosis (programmed cell death or "cell suicide"), releasing all progeny virions at once.
  • Many animal viruses, such as HIV (Human Immunodeficiency Virus), leave the infected cells of the immune system by a process known as budding, where virions leave the cell individually.
  • The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and to assemble new virions.
  • The last stage of viral replication is the release of the new virions produced in the host organism.
  • RNA and proteins are made and assembled into new virions.

• Animal Viruses

  • On the other hand, fusion only occurs with enveloped virions.
  • After making their proteins and copying their genomes, animal viruses complete the assembly of new virions and exit the cell.
  • On the other hand, non-enveloped viral progeny, such as rhinoviruses, accumulate in infected cells until there is a signal for lysis or apoptosis, and all virions are released together.
  • This allows the virus to escape elimination by the immune system and persist in individuals for years, while continuing to produce low levels of progeny virions in what is known as a chronic viral disease.
  • Once virions are produced in the skin and viral proteins are synthesized, the immune response is again stimulated and resolves the skin lesions in a few days by destroying viruses in the skin.

• Discovery and Detection of Viruses

  • Virions, single virus particles, are very small, about 20–250 nanometers in diameter.
  • Unlike bacteria (which are about 100 times larger), we cannot see viruses with a light microscope, with the exception of some large virions of the poxvirus family.
  • The surface structure of virions can be observed by both scanning and transmission electron microscopy, whereas the internal structures of the virus can only be observed in images from a transmission electron microscope.

• Vaccines and Anti-Viral Drugs for Treatment

  • Tamiflu works by inhibiting an enzyme (viral neuraminidase) that allows new virions to leave their infected cells.

• Virus Classification

  • The surface structure of virions can be observed by both scanning and transmission electron microscopy, whereas the internal structures of the virus can only be observed in images from a transmission electron microscope.