capsid

Examples of capsid in the following topics:

• Virus Classification

  • Viruses can also be classified by the design of their capsids .
  • Enveloped viruses have membranes surrounding capsids.
  • For example, the tobacco mosaic virus has a naked helical capsid .
  • The adenovirus has an icosahedral capsid .
  • The capsid of the (a) polio virus is naked icosahedral; (b) the Epstein-Barr virus capsid is enveloped icosahedral; (c) the mumps virus capsid is an enveloped helix; (d) the tobacco mosaic virus capsid is naked helical; and (e) the herpesvirus capsid is complex.

• Viral Morphology

  • Viruses of all shapes and sizes consist of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope.
  • 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.
  • The capsid is made up of protein subunits called capsomeres.
  • Enveloped viruses have membranes surrounding capsids.
  • Enveloped virions like HIV consist of nucleic acid and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins.

• Steps of Virus Infections

  • A virus attaches to a specific receptor site on the host cell membrane through attachment proteins in the capsid or via glycoproteins embedded in the viral envelope.
  • The nucleic acid of bacteriophages enters the host cell naked, leaving the capsid outside the cell.
  • Once inside the cell, the viral capsid is degraded and the viral nucleic acid is released, which then becomes available for replication and transcription.
  • The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and to assemble new virions.

• Animal Viruses

  • When a protein in the viral capsid binds to its receptor on the host cell, the virus may be taken inside the cell via a vesicle during the normal cell process of receptor-mediated endocytosis.
  • An alternative method of cell penetration used by non-enveloped viruses is for capsid proteins to undergo shape changes after binding to the receptor, creating channels in the host cell membrane.
  • These viruses, which include HIV among others, use special fusion proteins in their envelopes to cause the envelope to fuse with the plasma membrane of the cell, thus releasing the genome and capsid of the virus into the cell cytoplasm.
  • (a) Varicella-zoster, the virus that causes chickenpox, has an enveloped icosahedral capsid visible in this transmission electron micrograph.

• Prions and Viroids

  • They do not have a capsid or outer envelope, but, as with viruses, can reproduce only within a host cell.