Viruses

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Viruses

• CP Chapter Chapter 19
• Honors Chapter 18

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Section Outline
Section 19-3

• What Is a Virus?
• particles of nucleic acid, protein, and
  occasionally lipid
• (DNA or RNA core surrounded by a protein
  coat)
• capsid a viruss outer protein coat
  (allows the virus to enter the host cell like a
  spy)
• reproduce only inside living cells
• work like a colonialistic terrorist
  using the resources of the host cell to produce
  more viruses

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3
Figure 19-13 Virus Structures
Section 19-3
Tobacco Mosaic Virus
T4 Bacteriophage
Influenza Virus
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4
Types of Viruses

• Depends upon whether it has DNA or RNA, double
  stranded or single stranded
• 1. double-stranded DNA ex Chicken Pox
• 2. single-stranded DNA ex Roseola
• 3. double-stranded RNA ex Reovirus, plant
  viruses
• 4. single-stranded RNA ex HIV, Ebola
• Called a DNA-like or RNA-like virus depending on
  the nucleic acid found

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Viral Replication

• Viruses- obligate intracellular parasites must
  have a host cell!
• Can not make own proteins
• Each type of virus can only infect within a HOST
  RANGE
• Depends on the lock-and-key fit between
  proteins on the outside of the virus and specific
  receptor molecules on host cell surface.

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5 Basic Steps of Virus Replication

• 1. Attachment Virus attaches to host
• 2. Penetration Injects DNA or RNA
• 3. Replication and Synthesis hijacks host
  machinery to make new DNA/RNA
• 4. Assembly puts viral DNA/RNA into capsids,
  envelopes
• 5. Release New Viruses Lyses (break out of)
  cell, releasing more viruses to infect new cells

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• 2 types of pathways to infection
• 1. lytic cycle
• (quick takeover, destroys some cells in
  organism)
• 2. lysogenic cycle
• (delayed takeover, but more deadly to
  organism)

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• Lytic Infection - Quick like the common cold
• 1. Virus attaches to host cell
• 2. Virus enters host cell (injects genome into
  host)
• 3. Makes copies of itself and new protein coats
  by hijacking host cells metabolic machinery
• 4. Causes the host cell to burst (lyse) and die
• 5. Host cells death releases the new virus
  particles which can infect other cells

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Figure 19-14 The Lytic Cycle
Section 19-3
Bacteriophage protein coat
Bacteriophage DNA
Bacterial chromosome
Bacteriophage attaches to bacteriums cell wall
Bacteriophage enzyme lyses the bacteriums cell
wall, releasing new bacteriophage particles
that can attack other cells.
Lytic Cycle
Bacteriophage injects DNA into bacterium
Bacteriophage proteins and nucleic acids assemble
into complete bacteriophage particles
Bacteriophage Bacteriophage DNA Bacteriophage
protein
Bacteriophage takes over bacteriums metabolism,
causing synthesis of new bacteriophage proteins
and nucleic acids
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10
Lysogenic Cycle Virus Hides

• 1. Virus attaches to host cell
• 2. Virus enters host cell
• 3. Joins its DNA to the host DNA (this viral DNA
  now embedded in hosts DNA is called a prophage)
• 4.Each time host cell DNA replicates, viral DNA
  is also replicated thus, viral DNA will be
  inside EVERY cell that was produced after the
  viral DNA was inserted
• 5. When organisms immune system is low, viral
  DNA takes over the cells machinery and makes
  copies of itself and new protein coats by
  hijacking host cells metabolic machinery
• 6. Causes the host cell to burst (lyse) and die
• 7. Host cells death releases the new virus
  particles which can infect other cells

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Figure 19-14 The Lytic vs. Lysogenic Cycle
Section 19-3
This way, viral genome is replicated with each
host cell division more viruses released
Lysogenic Cycle Slower!
Viral genome entering host incorporated as a
prophage in to host genome
Viral genome hides out until conditions are
right then lyses (exits) the cell
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12
Viruses and Disease

• Viral diseases cannot be cured once caught
  must suffer through symptoms until body can fight
  off infection
• a. Most symptoms are caused by the toxins
  released when host cell lyses (bursts)
• b. Prevention is best medicine
• vaccines to stimulate immunity

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Why not Antibiotics?

• These are powerless against viruses
• Antibiotics kill bacteria by inhibiting enzymes
  or processes specific to the bacteria since
  viruses have no metabolism of their own, the
  antibiotics do not work.
• Only drugs that have any effect are ones that
  interfere with
• nucleic acid synthesis (replication) AZT (with
  HIV), acyclovir (with herpesvirus)
• or with protein production (transcription and
  translation) (protease inhibitors with AIDS) work
• -Viruses ability to bind to host cell
• Interferon

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Are Viruses Alive?

• 
  
  NO!
• They do not have all the characteristics of life
  (cellular structure, ability to reproduce
  independently), so they are not alive.
• They can do what they do because they share the
  genetic code with living things and affect living
  things, but they are not considered to be alive.