Topic 10 Viruses

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Topic 10Viruses

• October 21, 2005
• Biology 1001

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What Are Viruses?

• Viruses are infectious particles consisting of
  nucleic acid enclosed in a protein coat, and
  sometimes a membranous envelope
• As obligate intracellular parasites, viruses are
  either the most complex macromolecules or the
  tiniest, simplest forms of life

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The Discovery of Viruses1883-1935

• Discovered as the causative agent of tobacco
  mosaic disease
• Sap from diseased leaves causes the disease in
  new plants
• Ruled out a toxin, as the causative agent
  reproduced in the new plants
• Could not isolate or culture a bacterium
• Concluded that the particle could only reproduce
  within a host
• The infectious particle was subsequently shown to
  crystallize and was observed with the electron
  microscope

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The Structure of Viruses

• The viral genome
• The nucleic acid component of the virus is either
  dsDNA, ssDNA, dsRNA, or ssRNA depending on the
  virus
• A single linear or circular molecule containing a
  few to several hundred genes
• (Details of table for interest only)

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The Structure of Viruses

• The protein coat of viruses is called the capsid
• The capsid is rod-shaped, polyhedral or complex
  in shape
• Capsids are built from a large number of one or a
  few protein subunits called capsomeres
• TMV is a helical arrangement of gt1000 molecules
  of a single protein
• Adenovirus is a polyhedral capsid of 252
  identical protein subunits
• The most complex capsids belong to the phages,
  such as T4 or T2
• The viral envelope, if it exists, is a
  host-derived phospholipid membrane
• Functions to help infect the host
• May contain viral proteins and glycoproteins
• Encases or cloaks the capsid

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Figure 18.4
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Viral Reproductive Cycles

• Viruses can reproduce only within a host cell
  because they lack the enzymes and ribosomes
  necessary to make proteins
• The range of host cells that a particular virus
  can infect is called its host range
• The host range can be broad (eg. rabies virus can
  infect humans, bats, dogs, raccoons) or narrow
  (eg. poliovirus only affects humans)
• Host specificity results from the evolution of
  recognition systems by the virus to receptors on
  the host cell
• In multicellular eukaryotes viruses infect
  specific tissues or cell types (eg. rhinovirus
  human cold virus - infects the upper respiratory
  tract, poliovirus infects nerve cells, HIV
  attacks certain white blood cells only)

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Viral Reproductive Cycles

• Overview of a simplified viral reproductive
  cycle, Fig. 18.5
• A viral infection begins when the genome of a
  virus enters the host cell
• Once inside, the viral genome takes over the
  cells machinery enzymes, precursors, ribosomes
  etc.
• New viral nucleic acid and new viral proteins
  are synthesized
• New viral particles assemble and exit the host
  cell, spreading the infection to new host cells

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Reproductive Cycles of Phage Viruses

• Bacteriophages are dsDNA viruses of bacteria
• They have two alternate reproductive mechanisms
  the lytic cycle and the lysogenic cycle
• The lytic cycle culminates in the death of the
  host and release of hundreds of new phage
  particles
• Phages which can only reproduce lytically are
  called virulent
• During a lysogenic cycle, the phage DNA
  integrates into the hosts genome where it is
  called a prophage, and reproduces whenever the
  bacteria reproduces
• A virus such as ? that can enter a lysogenic
  cycle is called temperate
• Ultimately the prophage exits the bacteria genome
  and initiates a lytic cycle

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Figure 18.6 The lytic cycle of phage T4, a
virulent phage
Animation
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Reproductive Cycles of Animal Viruses

• Animal viruses are varied in terms of
  reproductive cycle one key feature is type of
  genome, and another is the presence or absence of
  a viral envelope (see Table 18.1)
• RNA as the genetic material
• Retroviruses are a type of ssRNA virus where the
  RNA acts as a template for DNA synthesis (reverse
  flow of information)
• Retroviruses are packaged with the enzyme reverse
  transcriptase
• The DNA produced by a retrovirus is incorporated
  as a provirus into the host genome it is then
  transcribed into RNA that serves as the mRNA as
  well as the genome for the next viral generation
• Role of the viral envelope
• Nearly all animal viruses with RNA genomes also
  have viral envelopes
• During infection, the viral envelope fuses with
  the host plasma membrane to allow the virus to
  enter the cell
• Viral envelope glycoproteins are made in the host
  and transported to the cell surface for packaging
  of new viruses

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Figures 18.9 18.10 - The structure and
reproductive cycle of HIV, the retrovirus that
causes AIDS
New human immunodeficiency viruses exiting a
helper T cell
Animation
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Viruses and Disease

• Viruses cause disease in animals, plants,
  bacteria, and fungi
• Symptoms caused by damaging or killing cells,
  producing toxins, or stimulating the host immune
  system
• Range from mild to severe, depending on
  reproductive cycle, type of tissue or cell, etc.
• Major medical intervention is the vaccine, a
  harmless variant that stimulates the host immune
  system to mount a pre-emptive defense
• Certain drugs such as acyclovir for herpes
  resemble nucleosides and interfere with viral
  nucleic acid synthesis

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Emerging Viruses

• Viruses that appear, or come to the attention of
  scientists, suddenly
• HIV a retrovirus
• SARS a coronavirus
• Ebola a filovirus
• bird flu1 an orthomyxovirus
• Three factors contribute to emergence mutation
  of existing viruses, spread from one host species
  to another, spread from a small isolated
  population

SARS
Hantavirus a group V ssRNA virus
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The Origin of Viruses

• Evolution of viruses
• Because viruses depend on cells for their own
  propagation, it is reasonable to assume that they
  evolved after the first cells
• Most molecular biologists favor the hypothesis
  that viruses originated from fragments of
  cellular nucleic acids that could move from one
  cell to another
• Candidates for the original sources of viral
  genomes include plasmids and transposons
• Plasmids are small circular DNA molecules, found
  in bacteria and yeast, that replicate
  independently from the main chromosome(s) and can
  be transferred between cells
• Transposons are DNA segments that can move from
  one location to another within a cells genome
• Viruses, plasmids, and transposons are all mobile
  genetic elements

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Are Viruses Alive?

• Cell as basic unit of structure?
• DNA as hereditary material?
• Growth and development?
• Reproduction?
• Regulation/homeostasis?
• Emergent properties?
• Evolutionary adaptations?
• Order?
• Energy processing?
• Response to environment?