Download Viruses Lecture 1

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Small Pox
Ebola – filamentous form – 60-80nm in diameter, on average 1000 nm long (but highly variable)
Chickenpox virus
Polio Virus
Influenza virus
HIV virus
Hepatitis – this one actually Hepatits B magnified 120,000X
Foot and Mouth disease – 1967 outbreak in the UK saw the destruction of 400,000 animals
Viruses are most numerous “microorganisms” on the planet. Infect ALL types of cellular organisms
All viruses are the same (strategically)!
Viruses package their genome into a particle which is then infectious
The viral genome has all of the information that it needs to attach and enter the host, decode its
genome, allow for its proteins to be made, replicate its genome and then assemble and release virus
particles.
Viruses establish themselves in a host community to survive
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Viruses
Extracellular state
virus particle or virion
Intracellular state
viral replication
infection
Extracellular
minute particle containing nucleic acid surrounded by protein and
sometimes other macromolecular components
METABOLICALLY INERT – does not carry out respiratory or
biosynthetic functions
The virion is a way for the virus genome to get from cell to cell
Intracellular
infections
HOST – cell in which a virus can infect and once inside the virus can
replicate
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Viruses
Characteristics
Small
Obligate intracellular parasite
Crystallizable
Nucleic Acid
DNA or RNA
Protein coat
Capsid
Envelope – Not all viruses
lipid, protein, carbohydrate
NOT susceptible to antibacterial drugs
The discovery of viruses
Ivanowsky, 1892, the agent which causes tobacco mosaic disease is not retained by filters
Beijerinck, 1898, same observation, proposed small agent
1898, foot and mouth disease similar
Could not be isolated in pure culture!
Required hosts for replication
Contagium vividum fluvium -> (ultra)filterable virus -> virus
Bacteriophage, 1915 Twort and d'Herelle, 1917
Small -- Range from 20 to 300nm in diameter. 200nm is just smaller than the smallest
bacterium, while smallpox virus at 28nm is about the size of a ribosome!
Obligate intracellular parasite
Crystallizable
Nucleic Acid
DNA or RNA
Protein coat
Capsid
Envelope – Not all viruses
lipid, protein, carbohydrate
NOT susceptible to antibacterial drugs
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TABLE 9.1 Some types of viral genomes
Viral Genome
Virus
Host
Type of nucleic
acid in virion
structure
Number of
molecules
Size
H-l parvovirus
Animals
ssDNA
Linear
1
5.1kb
φX174
Bacteria
ssDNA
Circular
1
5.4kb
Simian virus 40
(SV40)
Animals
dsDNA
Circular
1
5.2kb
Poliovirus
Animals
ssRNA
Linear
1
7.4kb
Cauliflower mosaic
virus
Plants
dsDNA
Circular
1
8.0kb
Cowpea mosaic
virus
Plants
ssRNA
Linear
2 different
9.4kb total
Reovirus type 3
Animals
dsRNA
Linear
10 different
23.5kb total
Bacteriophage
lambda
Bacteria
dsDNA
Linear
1
48.5kb
Herpes simplex
virus type I
Animals
dsDNA
Linear
1
152.3kb
Bacteriophagc T4
Bacteria
dsDNA
Linear
1
168.9kb
Human
cytomegalovirus
Animals
dsDNA
Linear
1
229.4kb
Despite the diversity of viral genomes they all follow the CENTRAL DOGMA of
molecular biology
nucleic acid => protein
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Viruses
Capsid –
Nucleic acid enclosed inside
Structural subunits
arranged in highly precise and
repetitive pattern
Few viruses single protein as capsid
Most viruses have several
are chemically distinct
are associated with each other in
specific ways to form Capsomers..
Capsomer – the morphological unit that can be seen with the electron microscope.
Nucleocapsid == complete complex of nucleic acid and protein, packaged in the
virion.
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Viruses
Viral Symmetry
Helical Symmetry
Icosahedral Symmetry
Helical == rod shapped TMV
Icosahedral = Spherical
rougly spherical
20 faces
simplest is 3 morphological units per face for a total of 60 units per
virus particle
3 units could be identical or different
HERPES VIRUS
The HSV-1 capsid is an icosahedral shell 15 nm thick and 125 nm in diameter. It is
composed of 162 capsomers (12 pentons and 150 hexons) which can be seen in
the capsid reconstruction shown at the left. Representative pentons are shown in
orange and hexons in red.The capsomers lie on a T=16 icosahedral lattice. They
are connected in groups of three by trivalent structures called triplexes (green) that
lie above the capsid floor and connect the capsomers in groups of three. There are
a total of 320 triplexes in the capsid. The capsid mass is approximately 200 MDa
without the DNA and 300 MDa with DNA.
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Viruses
Enveloped Viruses
Complex membranous structure
surrounds the nucleocapsid
lipid bilayer
from host
proteins
viral
Why might a viral envelope consisting of the hosts cell membrane be
advantageous?
What part of an enveloped virus would first contact the cell?
Host immune system
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Viruses
Virus Host Range
Main determinant of host recognition
cell surface receptors
Restriction modification enzymes
Restriction endonuclease
Viral DNA
glucosylate or methylate
Cell surface receptors
specific proteins, or glycoprotein or polysaccharides
e. Coli bacteriophage would not attach to s. aureus
no recognizable receptor on s. aureus
CAT virus would not infect Human
Monkey virus might with
modification/mutation? Simian Virus 40
SV40 and Mesothelioma
NOT a factor
SV40 was a
contaminant in polio vaccines
T-even bacteriophage 5’hydroxymethylcytosine == modified cytosine not
recognized by RE in this form
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Viruses
Viral multiplication
Best understood in Bacteriophages –
T even phage of E. coli
dsDNA - linear
100 genes
Complex Architecture
both icosahedrol and helical
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Viruses
Adsorption
Penetration
Biosynthesis of
viral components
Maturation
Release
Adsorption – tail fibers to receptor on E. coli cell surface -- CHANCE COLLISION
Penetration – inject DNA, empty phage (ghost) stays on the outside.
Contraction, lysozyme degrades peptidoglycan
energy ATP from tail fibers
Biosynthesis of Viral Components
Viral DNA hijacks host cells metabolic machinery
Host DNA is degradded == stops transcriptiion of host DNA
DNA replicated
mRNA
Proteins synthesized
DNA=> mRNA=> protein very much time dependent
Early-early enzymes
Mid Nucleic acid replication and transcription
late synthesis and assembly of virions and viral proteins.
Maturation
components assembled to mature viruses USE WILKINSON”S DIAGRAM -- SEE NOTES
Release
lysozyme produced, degrades peptidoglycan, cell lyses (bursts) phage are liberated.
Approx 100 phage per cell
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