Microbiology: Structure of Viruses

Study Notes

The capsid contains enzymes to assist cell penetration during replication.
The capsid is composed of individual protein subunits called capsomeres, with a specific number characteristic for a particular virus (e.g., 162 capsomeres in herpesviruses and 252 capsomeres in adenoviruses).
Viruses can be either enveloped (covered by a lipid membrane) or non-enveloped (lacking a membrane).
Non-enveloped viruses construct a capsid from many copies of one protein and a few minor proteins, with each subunit termed a protomer.
Enveloped viruses require both nucleocapsid proteins and additional proteins to anchor the membrane.
Some viruses use non-capsid proteins as scaffolding for capsid assembly.

Viruses have four possible nucleic acid types: dsDNA, single-stranded (ss) DNA, ssRNA, and dsRNA.
All four types are used by animal viruses, while most plant viruses have ssRNA genomes and most bacterial and archaeal viruses have dsDNA.
The size of viral genomes varies greatly.

Viruses are major causes of diseases, despite their simplicity.
Examples of viral diseases include H1N1 influenza, H7N9 avian influenza, MERS-CoV, and COVID-19.

Viruses vary in size and shape, ranging from 20 nm (parvoviruses) to 250 nm (poxviruses).
The smallest viruses are slightly larger than ribosomes.
Mimiviruses (giant viruses) can be seen in the light microscope.
Viral shapes include simple helical and icosahedral forms, as well as more complex structures.
Helical capsids are shaped like hollow tubes with protein walls.
Icosahedral capsids have 20 triangular faces and 12 corners.
Certain viruses have a combination of helical and icosahedral symmetry, described as complex.
Poxviruses are brick-shaped, with submicroscopic filaments occurring in a swirling pattern at the periphery of the virus.

Viruses lack cellular structure and organelles, unlike most other microbes.
A complete virus particle is called a virion.
All viruses consist of two basic components: a nucleic acid (genome) and a surrounding protein coat called a capsid.
The genome is usually folded and condensed in icosahedral viruses and coiled in helical fashion in helical viruses.
The capsid plus the genome is called the nucleocapsid.