Viruses Chapter 27

2 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

3 Nature of Viruses

4 Viruses vary in size, as well as in shape

5 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

6 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

7 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

8 Induction = Switch from the lysogenic to the lytic cycle

9 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

10 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

11 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

12 HIV Infection Cycle

13 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

14 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

15 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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17 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

18 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)

19 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)

20 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

21 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 (PrP c ) -Misfolded proteins (PrP sc ) cause disease

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