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.