Viruses, Viroids, and Prions

General Characteristics of Viruses

Definition: Obligate intracellular parasite composed of:

Nucleic acid - either DNA or RNA

Protein coat


Single type of nucleic acid - DNA or RNA

Protein coat, or capsid, some have envelopes

Multiply inside of living cells using the host cell machinery

Direct the synthesis of structures to transfer viral nucleic acid to other cells

Host Range

The specific types of cells a virus can infect in its host species represent the host range of the virus.

Usually species specific


Animal virus

Plant virus

Bacterial virus (bacteriophage)

Host range is determined by attachment sites (receptors).

Anti-bacterial therapy - phage therapy

Anti-tumor therapy - oncolytic viruses


Viral Size

Determined by electron microscopy.

Ranges from 20 to 14,000 nm in length.

There is also a group of giant viruses, including the giant mimivirus, which is something like 800 nm in diameter and has a genome with 1.2Mbp base pairs carrying somewhere in the neighborhood of 1000 genes, 911 of which code for proteins.



Viral Structure

Virions are complete, fully developed viral particles composed of nucleic acid surrounded by a protein coat. Some viruses have an envelope composed of a phospholipid bilayer with viral glycoproteins.

1. Nucleic acid

Viral genomes are either DNA or RNA (not both).

Nucleic acid may be single- or double-stranded

Nucleic acid may be circular or linear or separate molecules.

Nucleic acid:protein ranges from about 1% - 50%.

2. Capsid

Capsid - protein coat

Capsomeres are subunits of the capsid

Protomeres are capsomere subunits.


3. Envelope – the outer covering of some viruses, the envelope is derived from the host cell plasma membrane when the virus buds out. Some enveloped viruses have spikes, which are viral glycoproteins that project from the envelope.

Influenzavirus has two kinds of spikes, H (hemagglutinin) and N (neuraminidase). The H spike allows the virus to attach to host cells (and red blood cells), the N spike is an enzyme that allows the mature viral particles to escape from the host cell

Non-enveloped or naked viruses are protected by their capsid alone.

General Morphology

Based on capsid architecture, although enveloped viruses end up being approximately spherical.

1. Helical, non-enveloped

2. Helical, enveloped

3. Polyhedral, non-enveloped

4. Polyhedral, enveloped

Polyhedral means many sides (most are icosahedral - 20 triangular faces and 12 corners)

5. Complex viruses are, well, complex.

See bacteriophages.

Taxonomy of Viruses

Classification of viruses is based on type of nucleic acid, strategy for replication, and morphology.

Virus family names end in –viridae; genus names end in –virus, order names end in -ales.

A viral species is a group of viruses sharing the same genetic information and ecological niche. There is no specific epithet used, common names that are descriptive are used; subspecies are designated with a number.

Families of viruses that affect humans:

DNA viruses

RNA viruses