Microbiology: Structure of Viruses

The Structure of Viruses

• 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.

Viral Genome

• 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: An Overview

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

Size and Shapes of Viruses

• 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.

Viral Structure

• 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.