Virion structure

The virion contains only a single type of nucleic acid, which can be either ds or ss DNA or RNA. This genome is encased by a protein coat called a capsid. This capsid protects the genome by forming an impermeable barrier. It is composed of multiple copies of a single protein (two kinds in more complex viruses). Some animal viruses have a lipid membrane around the capsid, and these are called enveloped viruses. Viruses without this lipid membrane are called naked viruses. There are two capsid shapes:

• Icosahedrons
• Helix

To summarize:

• Nucleic acid is either RNA or DNA.
• Multiple proteins form a capsid to protect nucleic acid.
• Capsids either icosohedral or helical in structure.
• Some viruses are naked; others have an envelope.

There have been 2 important advances in visualization of viruses:

• Electron microscopy
• Plaque assay
  • Add dilute suspension containing virus to lawn of susceptible host cells.
  • After infection, cover layer of cells with agar.
  • Incubate.