Viruses

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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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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 cells 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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• 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 cells 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
  cells 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