protein coat that surrounds the infective nucleic acid in some virus particles

A capsid is the protein shell of a virus. It is made of several protein subunits. The observable 3-dimensional subunits are called 'capsomeres'. The capsid encloses the nucleic acid of the virus.

A cytomegalovirus also has an outer "envelope"

Capsids are broadly classified according to their structure.[1][2] Some viruses, such as bacteriophages, have developed more complicated structures.[3] The icosahedral shape, which has 20 equilateral triangular faces, approximates a sphere, while the helical shape is cylindrical.[4] The capsid faces may consist of one or more proteins. For example, the foot-and-mouth disease virus capsid has faces consisting of three proteins called VP1–3.[5]

Viral envelopesEdit

Most viruses (e.g. influenza and many animal viruses) have viral envelopes covering their protein capsids.[6] This means the capsid is coated with a lipid membrane.

The envelope is got by the capsid from an intracellular membrane in the virus' host; such as the inner nuclear membrane, the golgi membrane, or the cell's outer membrane.[7]

The envelope may help viruses avoid the host immune system. Glycoproteins on the surface of the envelope bind to receptor sites on the host's membrane. The viral envelope then fuses with the host's membrane This allows the capsid and viral genome to enter and infect the host.


  1. Lidmar J, Mirny L, Nelson DR (2003). "Virus shapes and buckling transitions in spherical shells". Phys. Rev. E. 68 (5): 051910. doi:10.1103/PhysRevE.68.051910.CS1 maint: multiple names: authors list (link)
  2. Vernizzi G, Olvera de la Cruz M (2007). "Faceting ionic shells into icosahedra via electrostatics". Proc. Natl. Acad. Sci. USA. 104 (47): 18382–18386. doi:10.1073/pnas.0703431104.
  3. Vernizzi G, Sknepnek R, Olvera de la Cruz M (2011). "Platonic and Archimedean geometries in multicomponent elastic membranes". Proc. Natl. Acad. Sci. USA. 108 (11): 4292–4296. doi:10.1073/pnas.1012872108. PMC 3060260. PMID 21368184.CS1 maint: multiple names: authors list (link)
  4. Branden, Carl and Tooze, John (1991). Introduction to Protein Structure. New York: Garland. pp. 161–162. ISBN 0-8153-0270-3.CS1 maint: multiple names: authors list (link)
  5. "Virus Structure (".
  6. "CHAPTER #11: VIRUSES". Archived from the original on 2008-11-10. Retrieved 2008-11-07.
  7. Alberts, Bruce; et al. (1994). Molecular biology of the cell (4th ed.). pp. 280.