Introduction to viruses

General structure of a virion

• A virion is a complete virus particle. Always contains protein and nucleic acid, sometimes lipids.
• The most basic virion is the nucleocapsid and is merely a genome and protein coat, or capsid.
• Capsids can be enveloped by lipid.
• The capsids are made from multiple monomer units called protomers.
• Protomers can have one of 3 symmetries:
• helical (self explanatory)
• icosahedral (20 face polyhedron - each face an equilateral triangle. Each triangle has 3 subunits called capsomers and each capsomer is comprised of 5-6 protomers [could be pentamer or hexamer)
• complex (a composite of icosahedral and helical symmetry e.g. a head and tail. This is called binal symmetry. However some virus do not fit either symmetry.

Viral genome

• Comprise of nucleic acid (DNA or RNA)
• Can be linear, circular or segmented and can vary in size from 4kb to 1Mb.
• There are 4 possible forms of RNA/DNA found in viruses:
• ds DNA
• ss DNA
• ss RNA (positive or negative sense)
• ds RNA