Viruses, Viroids, and Prions

General Characteristics of Viruses
The Isolation, Cultivation
and Identification of Viruses
Viral Multiplication
Viruses and Cancer
Latent Viral Infections

General Characteristics of Viruses: Objectives

1. Define virus.
2. List the characteristics of viruses.
3. Describe the:

· Composition of a virus
· Location of viral multiplication
· Host range viruses

4. Explain what determines viral host range.
5. List the three kinds of viruses according to type of hosts.

Viral Structure
6. Define:

· Virion
· Capsid
· Capsomere
· Protomere

7. Describe

· The nucleic acid content of a virus
· Viral envelopes
· Viral spikes

8. Differentiate between an enveloped and a naked virus.
9. List the:

· Two kinds of viral spikes
· Five morphological types of viruses

DNA Viruses
10. List the six families of DNA viruses and give medically important examples of each family.

RNA Viruses
11. List the fourteen families of RNA viruses and give medically important examples of each family.

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.



Viral Structure

Virions are complete, fully developed viral particles composed of nucleic acid surrounded by a coat.


Nucleic acid

Viruses contain 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%.


Capsid and Envelope

Capsid - protein coat

Capsomeres are subunits of the capsid

Protomeres are capsomere subunits.


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.

1. Helical viruses



2. Polyhedral viruses, non-enveloped

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

3. Enveloped helical

4. Enveloped icosahedral

Enveloped viruses end up being approximately spherical.

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.

A viral species is a group of viruses sharing the same genetic information and ecological niche.

Families of viruses that affect humans:

DNA viruses

RNA viruses