Download Viruses

11/29/13
Relative Sizes of Viruses, Bacterial and Mammalian Cells
Microbiology
Lecture 5
Structure and Classification of
Viruses
Professor T.J. Foster
Viruses
•  Infectious agents
•  DNA or RNA “genomes”
•  Nucleic acid protected by protein
capsid
•  Depends on host cell for energy,
building blocks (amino acids,
nucleotides) and ribosomes.
•  Only propagates within host cell
•  Obligate intracellular parasites
•  Bacterial, plant, and animal viruses
Definitions and Terminology
• 
• 
• 
• 
Virion: complete virus particle
Capsid: protein shell encasing genome
Capsomer: protein subunit of capsid
Nucleocapsid: capsid containing
nucleic acid
•  Attachment protein: surface
component for attachment to receptor
on host cell
–  eg spike. Visible (by electron microscopy)
attachment appendage (see adenovirus, next
slide)
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Diversity of Virus Capsid Shapes
Spherical (Icosohedron)
20-faced polyhedron
eg. Adenovirus
252 identical capsomers.
Each capsomer has 6
neighbours (hexon).
Except in 12 vertices,
5 neighbours (pentons)
Diversity of Virus Capsid Shapes
Tobacco Mosaic Virus
•  Rod (rigid filament)
–  Tobacco Mosaic Virus
–  Central hollow core
–  49 capsomers every 3
turns
–  Helix
–  1 capsomer protects 3
nucleotides
X Ray crystal structure
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Filamentous Viruses
•  Nucleocapsid - flexible helix
•  eg the paramyxovirus, the Mumps virus
Complex
•  Cowpox virus (Vaccinia)
–  External surface series of parallel rows
–  Complex particle >100 proteins
–  Contains 2 membranes
–  Core nucleoprotein with at least 10 enzymes
Bullet Shaped
•  eg the rabies virus
Envelopes
•  Some animal viruses have a
membranous outer envelope
•  Derived from host cell membrane
•  Contains viral glycoproteins (proteins with
polysaccharide chains attached, synthesized in
endoplasmic reticulum)
–  Host receptor recognition, attachment
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Herpes Virus Structure
The Five Basic Structural Forms
•  Naked Icosahedron
–  Adenovirus, poliovirus,
•  Naked helical (not found in animals)
–  Tobacco mosaic virus
•  Enveloped Icosohedron
–  Herpes virus, rubella virus,
•  Enveloped helical
–  Rabies virus, influenza virus, mumps virus,
measles virus
•  Complex
–  Pox viruses
Bacteriophage
Viruses that Infect Bacteria
Bacteriophage
Icosohedral
capsid
contains
nucleic acid
Tail fibres.
Attachment to bacterial cell surface
Bacteriophage nucleocapsid does NOT enter bacterial cell.
DNA is INJECTED through cell wall.
Tail lysozyme degrades peptidoglycan.
Sheath contracts. Syringe-like mechanism injects DNA into cytoplasm.
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Classification.
The Baltimore Scheme
Based on Nucleic Acid
Composition of Genome,
Replication
and
Information Flow
“Classification imposes order out of chaos”
DNA REPLICATION
ds DNA
DNA polymerase
DNA
Double stranded
ds DNA
The Central Dogma
Information Flow
DNA > mRNA > Protein
ds DNA
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TRANSCRIPTION
TRANSCRIPTION
+
Baltimore Classification of Viruses
Group Genome
Transcription
ds DNA
Genetic Information.
Example
From genome to message
dsDNA
1
ds DNA
2
ss DNA
ssDNA
3
ds RNA
dsRNA
4
+ ss RNA
dsRNA
dsDNA
mRNA
Herpes simplex
virus
mRNA
Parvovirus
mRNA
+ssRNA (mRNA)
Reovirus
Enterovirus
Serves as mRNA
5
+
ss mRNA +
- ss RNA
dsRNA
-ssRNA
mRNA
Influenza
dsDNA
mRNA
Retrovirus
mRNA template
6
ssRNA
ssRNA
7
Nicked ds
DNA
nicked ds intact ds mRNA Hepatitis B
virus
DNA
DNA
RNA
replicate thru RNA intermediate
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Adenovirus
Baltimore 1. Double stranded DNA genome
Non-enveloped icosohedron
Common human pathogen
Self-limiting upper respiratory tract infections
Paramyxovirus
Mumps and Measles Viruses
Baltimore 5.
Negative single-stranded RNA genomes
Enveloped, filamentous
[Vaccination: Measles Mumps Rubella (MMR)]
Measles rash, raised
spots.
Middle ear infection.
Pneumonia,
Encephalitis (1:1000),
Fatal (1:3000)
Mumps
Swollen parotid gland
Influenza Virus
Baltimore 5
Negative single strand RNA
8 segmented genome
Enveloped, filamentous
Seasonal ‘flu’ vaccine
Herpes Virus
Baltimore 1. Double stranded DNA.
Enveloped icosohedron.
Herpes infections of man.
Herpes Simplex Viruses (HSV)
Cold sore on lip
Genital
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Herpes Viruses
Rubella Virus
HSV-1
Baltimore 4.
Positive stranded RNA genome
Enveloped icosohedron
Enters nerve cell
German measles, mild rash
Migrates to trigeminal ganglia
near ear.
Congenital rubella syndrome
Foetal damage
Virus remains dormant
MMR Vaccine
Re-activation
Migrates to tip of nerve cell.
Infects epithelial cells in lip.
Retrovirus
Baltimore 6
Positive single-stranded RNA plus reverse transcriptase
Diploid
Enveloped non-icosohedral elongated
Human Immunodeficiency Virus (HIV)
Acquired Immunodeficiency Diseases (AIDS)
Pox Viruses
Baltimore 1. Double stranded DNA
Complex structure
Smallpox
20% mortality. Scarring
Vaccinia (cowpox) virus
Jenner’s pioneering vaccination
Eradicated by WHO vaccination programme
No vaccine
Anti-retrovirus drugs
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