<P> 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; examples include the inner nuclear membrane, the golgi membrane, and the cell's outer membrane . </P> <P> Once the virus has infected a cell and begins replicating itself, 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 . </P> <P> 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 . </P> <P> The icosahedral structure is extremely common among viruses . The number and arrangement of capsomeres in an icosahedral capsid can be classified using the "quasi-equivalence principle" proposed by Donald Caspar and Aaron Klug . Like the Goldberg polyhedra, an icosahedral structure can be regarded as being constructed from pentamers and hexamers . The structures can be indexed by two integers h and k, with h ≥ 1 (\ displaystyle h \ geq 1) and k ≥ 0 (\ displaystyle k \ geq 0); the structure can be thought of as taking h steps from the edge of a pentamer, turning 60 degrees counterclockwise, then taking k steps to get to the next pentamer . The triangulation number T for the capsid is defined as: </P>

What is the primary function of the viral capsid and envelope