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Viruses
Chapter 27
Nature of Viruses
All viruses have same basic structure
-Nucleic acid core surrounded by capsid
Nucleic acid can be DNA or RNA; Circular or
linear; Single- or double-stranded
Some viruses store specialized enzymes
inside their capsids
Many animal viruses have an envelope
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Nature of Viruses
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Nature of Viruses
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Nature of Viruses
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Nature of Viruses
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Nature of Viruses
Viruses are obligate intracellular parasites
-Host range = Types of organisms infected
-Tissue tropism = Types of cells infected
Viruses can remain dormant or latent for
years
More kinds of viruses exist than organisms
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Viral Replication
Viruses can only reproduce inside cells
-Outside, they are metabolically inert virions
Virus hijacks the cell’s transcription and
translation machineries to express:
-Early genes
-Middle genes
-Late genes
End result is assembly and release of viruses
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Viral Shapes
Most viruses come in two simple shapes
-Helical
-Icosahedral
Some viruses are complex
-T-even phages = Binal symmetry
-Poxviruses = Multilayered capsid
Enveloped viruses are polymorphic
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Viral Shapes
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Viral Shapes
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Viruses vary in size, as well as in shape
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Viral Genomes
Vary greatly
Most RNA viruses are single-stranded
-Replicated in the host cell’s cytoplasm
-Retroviruses (HIV) employ reverse
transcriptase
-Most DNA viruses are double-stranded
-Replicated in nucleus of eukaryotic host
cell
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Bacteriophage
Are viruses that infect bacteria
Viruses have also been found in archaea
-Have complex symmetry
E. coli-infecting viruses are the best studied
-Include the “T” series (T1, T2, etc.)
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Bacteriophage
Exhibit two reproductive cycles
-Lytic cycle = Virus kills the host cell
-Lysogenic cycle = Virus incorporates into
the cell’s genome
Lytic phage are called virulent
Lysogenic phage are called temperate
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Bacteriophage
Lytic cycle
-Adsorption = Viral attachment
-Penetration = Viral entry
-Synthesis = Viral components are made
-Assembly = Components are put together
-Release = Viral exit
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Bacteriophage
Lysogenic cycle
-Virus integrates into cellular genome as a
prophage
-Propagates along with host genome
-Resulting cell is called a lysogen
Phage conversion occurs when the
prophage alters the bacterial phenotype
-Vibrio cholerae toxin is viral-encoded
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Induction = Switch
from the lysogenic
to the lytic cycle
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Human Immunodeficiency Virus
HIV causes acquired immune deficiency
syndrome (AIDS)
AIDS was first reported in the US in 1981
Some people are resistant to HIV infection
-Have mutation in the CCR5 gene
-Encodes a receptor for HIV
-Also for the smallpox virus
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Human Immunodeficiency Virus
HIV targets CD4+ cells, mainly helper T cells
-Without these cells, the body cannot mount
an effective immune response
-Host may ultimately die from a variety of
opportunistic infections
Tests for HIV detect anti-HIV antibodies
-Not circulating viruses
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HIV Infection Cycle
Attachment = Viral gp120 attaches to CD4
and a co-receptor on host cells
Entry = By endocytosis
Replication = Reverse transcriptase converts
RNA to double-stranded DNA
-DNA is incorporated into host genome
Assembly = Occurs after a variable period of
dormancy
Release = New viruses exit by budding
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HIV Infection Cycle
During an infection, HIV is constantly
replicating and mutating
-Initially, gp120 uses CCR5 as a co-receptor
-Later, CXCR4 is used
-Thus, the mutated virus can infect a
broader range of cells
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Treatments for HIV
Research is currently under way in the
following five areas:
-1. Combination drug therapy
-Highly active antiretroviral therapy
(HAART)
-AZT and protease inhibitors
-2. Vaccine therapy
-Using a harmless strain of HIV
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Treatments for HIV
Research is currently under way in the
following five areas:
-3. Blocking receptors
-Use chemokines to block CCR5/CXCR4
-4. Disabling receptors
-Through mutations in the genes
-5. Blocking replication
-Use of CAF (CD8+ cell antiviral factor)
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Influenza
Flu viruses are enveloped and have a
segmented RNA genome
-Type A = Serious epidemics in humans
and other animals
-Type B and C = Mild human infections
-Subtypes differ in their protein spikes
-Hemagglutinin (H) = Aids in viral entry
-Neuraminidase (N) = Aids in viral exit
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Influenza
H and N proteins are constantly changing
-Thus we have yearly flu shots, and not a
single vaccine
-Type A viruses are classified into 13 distinct
H subtypes and 9 distinct N subtypes
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Influenza
Flu viruses can also undergo genetic
recombination when 2 subtypes infect a cell
-This creates novel combinations of spikes
unrecognizable by human antibodies
-Antigenic shifts have caused pandemics
-Spanish flu of 1918, A(H1N1)
-Asian flu of 1957, A(H2N2)
-Hong Kong flu of 1968, A(H3N2)
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Influenza
New strains of flu originate in the Far East
-Virus hosts are ducks, chicken and pigs
-In 1997, avian influenza A(H5N1) was
discovered
-“Bird flu” has killed over 100 people
-However, it does not appear to spread
between humans
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Emerging Viruses
Are viruses that extend their host range
-Considerable threats in the aviation age
Hantavirus
-Causes deadly pneumonia
-Natural host is deer mice
Ebola virus
-Causes severe
hemorrhagic fever
-Host is unknown
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Emerging Viruses
SARS
-Severe acute respiratory syndrome
-Caused by a coronavirus
-Host is civets
-RNA genome contains
six main genes
-SARS vaccines currently
being developed
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Viruses and Cancer
Viruses may contribute to about 15% of all
human cancers
Viruses can cause cancer by altering the
growth properties of human cells
-1. Triggering expression of oncogenes
-2. Disrupting tumor-suppressor genes
In June 2006, the FDA approved the use of a
new HPV vaccine to prevent cervical cancer
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Prions
“Proteinaceous infectious particles”
Cause transmissible spongiform
encephalopathies (TSEs)
-Mad cow disease
-Scrapie in sheep
-Creutzfeldt-Jacob disease in humans
Animals have normal prion proteins (PrPc)
-Misfolded proteins (PrPsc) cause disease
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Viroids
Tiny naked molecules of circular RNA
Cause diseases in plants
-Coconuts
It is unclear how they cause disease
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