Viruses and Bacteria

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Viruses and Bacteria
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Size comparisons
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Viruses

• Much smaller and simpler in organization than
  bacteria
• Lack most of structures found in bacteria
• Lack most of metabolic machinery, so must hijack
  another cell to reproduce
• Are little more than aggregates of nucleic acid
  and protein genes in protein coats

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History of discovery of viruses

• 1883 Mayer tobacco mosaic disease, spread by
  spraying sap from infected plant onto healthy
  plant was it a tiny bacteria?
• 1893 Ivanowsky filtered sap from tobacco
  mosaic disease, still caused disease
• 1897 Beijerinck found that infectious agent in
  filtered sap could reproduce sprayed plants
  with filtered sap, when developed disease, used
  their sap to infect more plants ability to
  infect was UNDILUTED over several generations, so
  had to be reproducing

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• Found that tobacco mosaic disease could only
  reproduce within the infected host could not be
  cultivated in culture
• Not inactivated by alcohol (which kills bacteria)
• 1935 Stanley crystallized the infectious
  particle Tobacco Mosaic Virus (TMV)
• no cells aggregate into crystals, so what ARE
  viruses?????
• Infectious particles consisting of a nucleic acid
  inside of a protein coat, and occasionally in
  some cases, a membraneous envelope.

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Viral Genomes

• May be double-stranded DNA, single-stranded DNA,
  double-stranded RNA, or single-stranded RNA
• Called a DNA-like or RNA-like virus depending on
  the nucleic acid found
• Organized as a single linear or circular model of
  nucleic acid

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Capsids and Envelopes

• Capsid protein shell that encloses the nucleic
  acid built from large numbers of protein
  subunits called capsomeres (but the number of
  proteins of different kinds in one virus is
  small)
• Shapes helical, polyhedral, very complex
• Some have accessory structures that aid in host
  infection.
• Ex. viral envelopes cloak viral capsid,
  derived from hosts membrane, have
  glycoproteins that can act as anchoring
  spikes
• Ex. Head and tail, act as hypodermic syringe

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Figure 18.2 Viral structure
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Virus Replication

• Viruses are obligate intracellular parasites
  must have a host cell
• Viruses lack the enzymes for metabolism and have
  not ribosomes, so cant make own proteins
• Each type of virus can only infect and parasitize
  within a HOST RANGE this specificity depends on
  the lock-and-key fit between proteins on the
  outside of the virus and specific receptor
  molecules on cell surface. Some ranges are
  specific, some are broad.

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Figure 18.3 A simplified viral reproductive cycle
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Viral reproductive cycle

• Virus uses hosts nucleotides and enzymes to
  replicate itself.
• At the same time, other host resources are used
  to make new capsid proteins by transcription and
  translation.
• The new viral genomes and capsids are assembled
  into new virus particles when the number exceeds
  the cells surface area limitations, the cell
  bursts open and new viruses are released to
  infect other cells exponential increase

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5 basic steps of virus replication

• 1. Attachment
• 2. Penetration
• 3. Replication and Synthesis
• 4. Assembly
• 5. Release

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Speed of takeover

• Lytic vs. Lysogenic cycles
• In the lytic cycle, the virus takes over the host
  cell immediately and reproduces quickly the
  host cell can lyse within a few minutes (Ex.
  Cold virus)
• Viruses that reproduce by lytic cycles are called
  virulent viruses
• In the lysogenic cycle, the virus hides in the
  original host cells DNA until optimal conditions
  for viral survival are present (provirus) then,
  because the host cell has reproduced, the virus
  will reproduce and emerge from MULTIPLE cells at
  once, causing much more severe cellular damage
  (Ex. AIDS, ebola)
• Viruses that reproduce by both lytic and
  lysogenic cycles are called temperate viruses

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Figure 18.02x2 Phages
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Figure 18.4 The lytic cycle of phage T4
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Figure 18.02x1 Adenovirus
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Figure 18.5 The lysogenic and lytic reproductive
cycles of phage ?, a temperate phage
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So why arent all cells wiped out?

• Natural Selection favors mutations with receptor
  sites that viruses cannot attach to.
• Cellular enzymes of cell may break down virus
  before it can replicate and take over
  (restriction nucleases act as DNA/RNA scissors
  for foreign DNA).
• BUT, natural selection also favors viral mutants
  that are resistant to destruction, so virus-host
  relationship is in constant evolutionary flux.

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Table 18.1 Classes of Animal Viruses, Grouped by
Type of Nucleic Acid
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Animal Viruses

• Many animal viruses have a membranous envelope
  present.
• This envelope is a lipid bilayer with
  glycoproteins for attachment.
• Viral envelope is derived from the host cells
  plasma membrane, so host cell may not be killed.

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Figure 18.6 The reproductive cycle of an
enveloped virus
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Nuclear membrane-derived envelopes

• Herpesviruses genomes of double-stranded DNA
• Reproduce within the cell nucleus
• Can become a provirus (like the prophages
  earlier)
• Infections tend to recur throughout life in times
  of stress as the proviruses exit the cell

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RNA viruses

• Found mostly in animals
• Classified according to the strandedness of their
  RNA and how functions in host
• single stranded RNA that serves as mRNA
• single stranded RNA that is mRNA
  template
• single stranded RNA that is template for
  DNA synthesis

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Retroviruses

• Have most complicated reproductive cycles
• Have reverse transcriptase which transcribes DNA
  from and RNA template (backwards from normal)
• Newly made DNA integrates as a provirus into a
  chromosome in animal cell nucleus
• Hosts RNA polymerase transcribes the viral DNA
  into RNA
• Ex. HIV human immunodeficiency virus

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Figure 18.7 HIV, a retrovirus
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Figure 18.7x1 HIV infection
See page 336 for color pictures
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Viral Infection and symptoms

• No clear link
• some viruses damage or kill cells by
  causing lysosomes to release killing enzymes
• some cause infected cells to produce toxins
  that lead to disease symptoms
• some have toxic molecular components

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How much damage?

• Depends on regeneration ability of tissue
• Ex. Colds pass b/c respiratory epithelium
  efficiently repairs
• Ex. Polio attacks nerve cells, so no
  healing
• Temporary symptoms (fever, aches, inflammaiton)
  are result of bodys own attempt to defend itself
  -- immune response

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Controlling Viruses

• Diseases causes by viruses are difficult to treat
• Drugs are only used to treat SYMPTOMS, not cure
  the disease (just make patient feel better for
  short duration)
• Only methods have to control are to PREVENT
  illness vaccines and antibody production, use
  of interferon in body

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Antibodies

• Are made by hosts immune system after infection
  occurs (if host survives the infection)
• Help inactivate viruses and destroy harmful
  bacteria
• Are specific for viruses or bacteria
• Once an antibody is produced that recognizes a
  specific virus or bacteria, then that strain will
  be ineffective on that individual organism

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Vaccines

• Harmless variants or derivatives of pathogenic
  microbes
• Stimulate the immune system to mount defenses
  against a specific pathogen
• Developed by Edward Jenner cowpox used to
  develop smallpox vaccination
• Vaccinated or immunized again disease
• Ex. MMR, DPT, polio, smallpox, influenze,
  rabies, hepatitis C

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Figure 18.x1 Smallpox
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Figure 18.x2 Measles
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Figure 18.x3 Polio
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Interferon

• Chemical in body that is activated when cells are
  attacked
• Cell under seige produces interferon which binds
  to neighboring cells cell membranes to warn them
  of the dangerous pathogen

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Why not Antibiotics?

• These are powerless against viruses
• Antibiotics kill bacteria by inhibiting enzymes
  or processes specific to the pathogens 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 AZT (with
  HIV), acyclovir (with herpesvirus)or with
  protein production (protease inhibitors with AIDS)

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Emerging Viruses

• AIDS
• Hantavirus
• Ebola (hemorrhagic fever)
• Nipah virus
• Influenza
• What contributes?
• 1. Mutation
• 2. Spread from one species to another
• 3. Dissemination from small, isolated population
• so, 2 and 3 require close contact travel
  allows

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Plant Viruses

• Most are RNA viruses
• Two major routes plant viruses spread
• 1. horizontal transmission plant is
  infected from an external source
• 2. vertical transmission plant inherits a
  viral infection from parent plant
• Once infected, crops must be burned to prevent
  transmission

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Viroids

• Viroids tiny molecules of naked circular DNA
  that infect plants
• -do not encode proteins
• -can replicate in host plant cells using
  cellular enzymes
• -disrupt the metabolism of cell and stunt
  growth of whole plant
• Point is that MOLECULES can be an infectious
  agent.

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Prions

• Prions Infectious proteinmisfolded form of a
  protein normally found in brain cells
• Cause degenerative brain diseases
• Ex. Scrapie, mad cow disease,
• Creutzfeldt-Jakob disease
• When prion gets into a cell containing the normal
  form of the brain cell protein, prion converts
  the normal protein to the prion version

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Figure 18.10 A hypothesis to explain how prions
propagate