Viruses

Chapter 27

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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

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Viruses vary in size, as well as in shape

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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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Induction = Switch from the lysogenic to the lytic cycle

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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

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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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