IN:

1
IN

• Discuss the following two questions with your
  group.
• What is a virus?
• (Come up with a definition.)
• Are viruses alive?
• (Be prepared to defend your answer.)

2
Characteristics of Life

• Are made of one or more cells
• Reproduce
• Grow and develop
• Obtain and use energy
• Respond to their environment

3
Viruses poison (Latin) infectious particles
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Viruses

• Not living (???) organisms not in 6 kingdoms
• Lack cell structures for growth and reproduction
• Must replicate in a living host
• Smaller than bacteria need electron microscope
  to see

5
Structure

• Nucleic Acid DNA or RNA, single or double
  stranded
• Wrapped in a protein coat called a capsid
• Some have an envelope (taken from host cell
  membrane)

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Symmetry

• Icosahedral 20 sides
• Helical (Spiral)
• Complex (e.g. head tail or
• enveloped)

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Classification

• Based on
• 1. RNA or DNA
• 2. Capsid
• (size, symmetry, presence of envelope)

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DNA Viruses

1. DNA RNA proteins
2. Join to host DNA RNA proteins

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RNA Viruses

• 1.make proteins directly from RNA
• 2.RNA (reverse transcriptase) DNA RNA
  proteins
• - seen in retroviruses

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Reproduction

• Obligate intracellular parasites, gain control of
  host cells machinery

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1.Lytic Cycle (all viruses)

• -virus attaches to host cell, injects nucleic
  acid
• -DNA/RNA replicates utilizing host cells
  enzymes
• -host ruptures, releasing 100s of viruses.

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The Lytic cycle
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Lysogenic Cycle (some viruses HIV, herpes)

• Virus attaches to host cell, injects nucleic acid
• Viral nucleic acid inserts into hosts DNA
  (provirus)
• Provirus replicates whenever the host cell
  reproduces
• No new viruses are produced at this time
• The provirus does not harm the host cell
• Upon activation (e.g. radiation, weakened
  state, chemicals), provirus takes over the cell
  and resumes lytic cycle.

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The Lysogenic Cycle
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Diseases

• Spread by air, human contact, insects, animals,
  and food
• Common diseases flu, cold, chickenpox, measles,
  herpes, hepatitis A
• Serious Diseases Smallpox, polio, AIDS,
  hepatitis C
• Some Viruses (e.g. hepatitis B, human
  papillomavirus)
• 1. Carry oncogenes (genes that cause cancer) or
• 2. Stimulate host oncogenes
• Few antiviral drugs
• Immune system provides protection e.g. white
  blood cells, lymohocytes, fever, interferons
  (interfere with synthesis of viral proteins)
• Vaccines are made from weakened form of virus
  stimulate immune response

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Viral Diseases

• Warts
• Herpes
• Chicken pox
• Smallpox
• The Common Cold
• Influenza (flu)
• Rabies
• AIDS
• Cancer

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Viroids

• Recently isolated
• Viroids naked RNA segments with no protein coat
  or envelope, smallest known particles to
  replicate infect plants

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Prions

• Prions infectious proteins with no nucleic
  acid, clump together inside of cell and
  eventually destroy it
• Cause slow, progressive disease e.g. kuru, mad
  cow disease, Creutzfelt-Jakob disease

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Remember Viruses !!!!

• Not considered living things by many
  scientists.
• 1. No Growth
• 2. No Homeostasis
• 3. No Metabolism
• 4. Reproduce only in a host cell
• 5. Only Protein and Nucleic Acid

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Studying Viruses

• Virus poison Latin) infectious particles
• Wendell Stanley was the first to isolate a virus
  in 1935 by crystallization