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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
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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Bacteriophage
Are viruses that infect bacteria
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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Lysogenic Bacteriophage
Lysogenic cycle
-Virus integrates into cellular genome as a prophage
-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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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
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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
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 (different mutants at these genes)
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Influenza
Flu viruses can also undergo genetic recombination (this is not mutation) 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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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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