Virus, or

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Virus, or Poison

• Is it alive? Not really
• Depend on cells to reproduce ? obligate
  intracellular parasites
• Inert organic particles when outside cells
• Viruses for all - infect bacteria, protists, and
  all higher plants and animals
• Infect specific cell types in host

Virus size comparison at Cellsalive.com
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Edward Jenner 1700s, Smallpox vaccine Vaca
Cow, noticed milkmaids immune to smallpox
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Louis Pasteur rabies vaccine, 1800s
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Wendell Stanley crystallized TMV (Tobacco
Mosaic Virus), 1900s, showed that viruses are
NOT small bacteria
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Viruses - Structure

• nucleic acid (RNA or DNA) instructions for
  making proteins
• protein capsid
• may have plasma membrane, or envelope
• Surface proteins for attachment to host cell
• Little or no metabolism
• Who will read the DNA/RNA and make viral proteins
  that assemble into a viral capsid?
• If a virus has an envelope (membrane), where does
  it come from?

The host cell
The host cells plasma membrane
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Influenza Virus
(membrane from host cell)
(nucleic acid,instructions formaking proteins)
(protein)
Surface proteins
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• Criteria used to Classify Viruses
• Type of nucleic acid
• Presence or absence of an envelope
• Shape/morphology of the capsid
• Specific surface proteins ? different strains

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Viral Replication by Host Cell
2. Entry
1. Adsorption
Lytic Cycle Cells lyses and releasesnew viral
particles
3. Replication
4. Assembly
5. Lysis / Cytolysis
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Lysogenic Cycle Period of dormancyuntil
stimulus causeslytic cycle
Viral Replication by Host Cell
Integration of viral DNA into hosts DNA ?
Provirus (in this case a prophage)
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Viral Replication by Host Cell(compare
contrast based on lecture diagrams)
Lytic Cycle Lysogenic Cycle
Viral DNA/RNA transcribed and translated by host cell Cells lyse and releasesnew viral particles Period of dormancyuntil stimulus causes cell to transcribe and translate viral DNA (viral DNA is incorporated into cells DNA) Cells lyse release new viral particles
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Retroviruses

• RNA core
• Replication cells make viral DNA from RNA
  (reverse order)
• Enzyme reverse transcriptase virus contains it
• Ex. HIV and tumor producing viruses

HIV inside human T4 lymphocyte
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Retrovirus injects RNA and reverse transcriptase
enzyme into host cell viral DNA is integrated
into cells DNA (provirus forms) and may remain
dormant for long various periods of time
(lysogenic).
Reverse transcriptase
New retroviruses
Provirus (viral DNA in cells chromosome)
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Viruses can be used to insert genetic material
into cells. Ex. Gene therapy to cure genetic
diseases
(Cystic Fibrosis)
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Gene Therapy for CF
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Possible Evidence of Virus Evolution
Chunks of DNA that can move around and be
independent Plasmids and Transposons

• Today Plasmid in Bacteria

Today jumping genes in Corn
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Viral Disease Examples

• Rubella
• Ebola (90 lethality)
• Lassa fever
• Dengue fever
• Many other hemorrhagic fevers
• The list goes on and on and on.

• Common cold
• Influenza
• Rhinovirus
• Smallpox (eradicated)
• Chicken pox
• Rabies
• Polio (still out there)
• Mumps

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Are Viruses Considered to be Living or Non-Living? Are Viruses Considered to be Living or Non-Living? Are Viruses Considered to be Living or Non-Living?
Characteristic No, Viruses do not display this trait Yes, Viruses do display this trait
Nucleic Acid
Growth
Mutate
Production of ATP
Cell Organelles
Homeostasis
Metabolism
Independent Reproduction
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Other Infectious Particles

• Viroids single strand of RNA, no capsidsEx.
  plant diseases

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• Prions infectious protein moleculesEx. CJD,
  scrapie, mad cow disease (BSE)

Prion causes proper proteins to fold incorrectly
? cannot do their job in the cell, collect
inside cell ? cell death
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Prion Disease holes in brain tissue