Viruses

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Viruses
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Viruses

• Nonliving particles
• Very small (1/2 to 1/100 of a bacterial cell)
• Do not perform respiration, grow, or develop
• Are able to replicate (only with the help of
  living cells)
• Host cella cell where a virus replicates
• Bacteriophage (phage)virus that infects a
  bacterium

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T4 bacteriophage infecting an E. coli cell
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Comparing the size of a virus, a bacterium, and
an animal cell
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Viral Structure

• 2 main parts
• inner core of nucleic acid (DNA or RNA)
• instructions for making copies of the virus
• outer coat of protein (capsid)
• determines shape of virus (which cells how
  cells are infected)
• polyhedral
• helical
• envelope with projections
• classic phage shape

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Viral structure
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Infection by tobacco mosaic virus (TMV)
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Attachment to Host Cell

• Order of events
• virus recognizes host cell
• virus attaches to receptor site on membrane of
  host cell
• Receptor site on host matches with viral proteins
  (like a puzzle)
• virus enters host cell
• virus replicates inside host cell

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Attachment is Specific

• viruses have specifically shaped attachment
  proteins
• each virus infects only certain types of cells
• most are species specific
• Smallpox, polio, measlesaffects only humans
• although some are not
• West Nile virusmosquitoes, birds, humans, horses
• some are cell-type specific
• polioaffects intestine nerve cells

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Simplified viral reproductive cycle
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Lytic cycle of phage T4, a virulent phage
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Lytic vs Lysogenic

• Lytic cycle (virulent phage)
• Release of virus bursts and kills host cell
  (lysis)
• Lysogenic cycle (temperate phage)
• Viral DNA integrates into host genome (provirus)
• Can be transmitted to daughter cells
• Can initiate lytic cycle in response to
  environmental signal (stress)

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The lytic and lysogenic cycles of phage ?, a
temperate phage
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 The structure of HIV, the retrovirus that
causes AIDS
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The reproductive cycle of HIV, a retrovirus
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Complete the Following Venn Diagram. Describe in
detail similarities and differences, give
examples.