Viruses, Viroids, and Prions
General Characteristics of Viruses: Objectives
4. Explain what determines viral host range.
8. Differentiate between an enveloped and a naked
Definition: Obligate intracellular parasite composed of:
Nucleic acid - either DNA or RNA
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
The specific types of cells a virus can infect in its host species represent the host range of the virus.
Usually species specific
Bacterial virus (bacteriophage)
Host range is determined by attachment sites (receptors).
Anti-bacterial therapy - phage therapy
Anti-tumor therapy - oncolytic viruses
Determined by electron microscopy.
Ranges from 20 to 14,000 nm in length.
Virions are complete, fully developed viral particles composed of nucleic acid surrounded by a coat.
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.
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.
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: