Capsid

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.

Capsids are broadly classified according to their structure. Some viruses, such as bacteriophages, have developed more complicated structures.  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 called VP1–3.

 Viral envelopes 
Most viruses (e.g. influenza and many animal viruses) have viral envelopes covering their protein capsids. 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.

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.

 References 

Category:Viruses
Category:Proteins
Category:Virology