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A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic material of the virus.
Capsids are broadly classified according to their structure. The majority of viruses have capsids with either helical or icosahedral  structure. Some viruses, such as bacteriophages, have developed more complicated structures due to constraints of elasticity and electrostatics. The icosahedral shape, which has 20 equilateral triangular faces, approximates a sphere, while the helical shape is cylindrical. 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 named VP1–3.
Some viruses are enveloped, meaning that the capsid is coated with a lipid membrane known as the viral envelope. The envelope is acquired by the capsid from an intracellular membrane in the virus' host; some examples would include the inner nuclear membrane, the golgi membrane, and the cell's outer membrane.
Once the virus has infected the cell, it will start replicating itself, using the mechanisms of the infected host cell. During this process, new capsid subunits are synthesized according to the genetic material of the virus, using the protein biosynthesis mechanism of the cell. During the assembly process, a portal subunit is assembled at one vertex of the capsid. Through this portal, viral DNA or RNA is transported into the capsid. The structure and assembly of the herpes virus capsid portal protein has been imaged via cryo-electron microscopy.
It is this capsid or protein shell that makes protective vaccines a possibility. Structural analyses of major capsid protein (MCP) architectures have been used to categorise viruses into families. For example, the bacteriophage PRD1, Paramecium bursaria Chlorella algal virus, and mammalian adenovirus have been placed in the same family.
Icosahedral virus capsids are typically assigned a T-number to describe the relation between the number of pentagons and hexagons i.e. their quasi-symmetry in the capsid shell. The T-number idea was originally developed to explain the quasi-symmetry by Caspar and Klug in 1962.
For example, a purely icosahedral virus has a T-number of 1 (usually written, T=1) and a truncated icosahedron is assigned T=3. The T-number is calculated by (1) applying a grid to the surface of the virus with coordinates h and k, (2) counting the number of steps between successive pentagons on the virus surface, (3) applying the formula:
where and h and k are the distances between the successive pentagons on the virus surface for each axis (see figure on right). The larger the T-number the more hexagons are present relative to the pentagons.
|capsid parameters||hexagon/pentagon system||triangle system|
|(h,k)||T||# hex||Conway notation||image||geometric name||# tri||Conway notation||image||geometric name|
|Truncated icosahedron||60||kD||Pentakis dodecahedron|
|(2,0)||4||30||cD=t5daD||Truncated rhombic triacontahedron||80||k5aD||Pentakis icosidodecahedron|
|(2,1)||7||60||dk5sD||140||k5sD||Pentakis snub dodecahedron|
|(3,0)||9||80||dktI||180||ktI||Hexapentakis truncated icosahedron|
|(2,2)||12||110||dkt5daD||240||kt5daD||Hexapentakis truncated rhombic triacontahedron|
|Hexapentakis snub truncated icosahedron|
T-numbers can be represented in different ways, for example T=1 can only be represented as a Icosahedron or a Dodecahedron and, depending on the type of quasi-symmetry, T=3 can be presented as a Truncated_dodecahedron, an Icosidodecahedron, or a Truncated icosahedron and their respective duals a Triakis icosahedron, a Rhombic triacontahedron, or a Pentakis dodecahedron. 
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