Ch. 25 Virus

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Ch. 25 Virus
LIVING OR DEAD?
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Characteristics

• Smallest particle capable of causing diseases
• Similar to living cells EXCEPT
• No nucleus, cytoplasm, organelles or cell
  membrane, and unable to carry out cellular
  functions.
• ONLY able to replicate by infecting cells and
  using organelles within living cells.

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Structure

• Two main features
• 1) Nucleic acid
• -DNA or RNA
• -Helical, closed loop or a long strand
• 2) Protein coat
• -Capsid
• Envelope a membrane-like lipid structure outside
  the capsid.
• ex influenza, chicken pox, herpes simplex and
  HIV
• Glycoprotein located on the surface of the
  envelope used to attach to the host cell.

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Viral Shape ( Icosahedron and Helix)
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Virus Type
Grouping is based on 1) the presence of a
capsid structure and an envelope 2) RNA or
DNA 3) Single stranded or Double stranded

• DNA Virus

• RNA Virus

• Picornavirus
• Polio and Hepatitis
• Myxovirus
• Influenza A, B C
• Retrovirus
• AIDS

• Papoviruses
• Warts and cancer
• Herpesviruses
• Herpes and chicken pox
• Poxviruses
• Smallpox

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DNA VS. RNA

• 1) Enter host cell
• 2) Produce RNA and makes viral proteins
• OR
• 2) Join to hosts DNA and synthesize new viruses
• 3) Replicates

• 1) Enter host cell
• 2) Viral RNA is released into the host cells
  cytoplasm
• 3) Uses host cells ribosomes to produce viral
  proteins
• Retroviruses contain enzyme called reverse
  transcriptase which is uses RNA as a template to
  make DNA

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• Viroids

• Prions

• Simple viruses
• Smallest known particle that is capable of
  replication
• Cause disease especially in plants
• Ex potatoes, cucumbers, avocados and oranges
• Single-strand of RNA and no capsid
• Disrupt plant cell metabolism

• Abnormal forms of proteins which clump together
  in a cell
• Clumping death of cell
• Composed of 250 amino acids but lack nucleic acid
• Cause disease especially in the brains of humans
  and animals
• Ex Scrapie and Mad cow disease

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Bacteriophage

• Viruses that infect bacteria
• Also called T phages
• Used to study viral replication due to their
  similarity to viruses that cause colds and
  measles.
• HEAD contains nucleic acids
• TAIL helps to inject nucleic acids into host
  cell.
• TAIL FIBERS used to attach to host cell

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Viral Replication

• Lytic Cycle
• Virulent kills host cell causes disease
• Virus invades a host cell
• Produces new viruses
• Destroys host cell
• Releases new viruses
• Lysogenic Cycle
• Temperate slowly kills host cell
• DNA integrates itself into the host cell
• Enters cells genome called Prophage
• Host cell replicates and virus replicates
• In the presence of chemicals or stress, virus
  enters into the lytic cycle

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Lytic Cycle
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Lysogenic Cycle
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Infectious Diseases

• Chicken Pox
• Shingles
• Rabies
• H1N1
• Common Cold

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Can you cure a virus?

• Vaccinations inject patient with a genetically
  altered live or dead virus which stimulates an
  immune response.
• Inactivated do not replicate in host cell
• Attenuated genetically altered (typically last
  longer)

• Antiviral drugs drugs that inhibit viral nucleic
  acid synthesis
• Antibiotics are not effective to treat or prevent
  viral infections.
• Ex Tamiflu