Viruses - Overview
Virus Morphology & Classification
Overview
Viruses are obligate intracellular parasites: they rely on host cellular energy and proteins for their own replication
The nucleocapsid comprises the viral genome, which consists of either DNA or RNA, surrounded by a capsid.
The capsid is a protein coat that protects the genome.
Some virions are enclosed by an envelope, which is made of lipids, proteins, and glycoproteins.
Virions come in a range of sizes, from 20-400 nanometers
Nucleic Acids
Comprise the viral genome.
Can be linear or circular.
DNA
Single- or double-stranded DNA.
RNA
Most are single-stranded.
Classified according to the polarity of their RN:
Positive-sense = the RNA is like messenger RNA
Negative-sense = they are like templates for messenger RNA that have to be transcribed into positive RNA.
Be aware that there are exceptions and complications to this classification system, which we'll learn more about in our tutorial on Virus Replication.
Common capsid shapes
Helical:
The viral genome is enclosed by the helical capsid proteins
Icosahedral capsid:
Comprises 20 equilateral triangles that that form a "shell" around the genome.
The viral genome is inside the capsid.
Viral attachment proteins extend from the corners of the capsid triangles.
Bacteriophages:
Complex capsids to facilitate invasion of bacterial cells.
Comprises a viral genome enclosed within an*icosahedral head, and a tail that facilitates delivery of the viral genome into the bacterium.
Viral Envelope
The viral envelope is linked to the viral core via matrix proteins.
The envelope is acquired from the host cell during budding (a final step of viral replication).
Envelope glycoproteins are on the surface.
These projections facilitate entry into host cells and stimulate the immune response.
Naked Capsid Virions vs. Enveloped Capsid Virions
Naked, aka, non-enveloped capsids:
Relatively stable.
Resistant to temperature changes, acids and proteases (including those found in the host's stomach), detergents, and drying.
Thus, they can survive outside the host and are spread easily, for example, via the fecal-oral route.
Enveloped capsids:
More fragile.
Labile to heat, acids, detergents, and drying.
Thus, transmission is often via respiratory or sexual routes.
KEY DNA and RNA VIRUSES
DNA Viruses
Most DNA viruses are double-stranded
– Parvoviridae is the single-stranded exception
Most DNA viruses have icosahedral symmetry
– Poxviridae, which has a complex shape, is the exception
Most DNA viruses replicate within the host nucleus
– Poxiviridae is again the exception, as it replicates in the cytoplasm
Naked DNA viruses
Enveloped DNA viruses
- Poxviridae
- Herpesviridae
- Hepadnaviridae
RNA Viruses
RNA viruses can be positive-stranded, negative-stranded, or ambisense, which means it contains both positive-sense and negative-sense RNA.
Most RNA viruses are single-stranded
– Reoviridae is the exception.
Many RNA viruses display helical symmetry
– Several exceptions:
Reoviridae
Picornaviridae
Togaviridae
Flaviviridae
Caliciviridae
Rhabdoviridae (has a distinctive bullet-shaped capsid)
- Most RNA viruses replicate in the host cytoplasm
- Picornaviridae
- Caliciviridae
- Togaviridae
- Flaviviridae
- Coronaviridae
- Retroviridae (which are positive-stranded RNA via DNA).
Summary of Virus Morphology
Naked, aka, non-enveloped capsids:
Helical arrangement: the nucleic acids are wrapped in capsid proteins to form a helical shape
– The Tobacco mosaic virus is an example of this morphology
Icosahedral: the nucleic acids in the center and viral attachment proteins projecting from the corners.
– Several of the DNA viruses have this morphology.
Enveloped capsids:
Similar to enveloped capsids
Additional outer layer of lipids and glycoproteins
– Envelope glycoproteins on their surfaces.