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Virus Virus Presentation Transcript

  • Viruses Non-Living particle composed of nucleic acid and protein coat
  • Living or Non-Living?
    • Characterisitics of viruses not shared by living cells:
      • Simple acellular organization
      • Absence of BOTH DNA & RNA
      • Obligate Intracellular Parasite
  • History of Virology
    • 3 rd Century- Epidemic weakens Roman Empire
    • 16 th Century- Smallpox wipes out 1/3 of Aztec Tripe
    • 18 th Century-Edward Jener vaccinates patients for smallpox using cowpox
    • 1884- Charles Cumberland develops a bacterial filter- viral discovery possible
    • 1935: First structural image of virus
    • 1980- Smallpox eradicated
    • 1981: Aids virus first described
    • 2005- Avian flu epidemic?
  • Virion Structure Capsid Nucleocapsid Envelope Receptors DNA or RNA
  • Virion Components
    • Nucleocapsid: hollow core of virus
      • Contains either RNA or DNA; never both
      • Genes are programmed for viruses to make copies of self
    • Capsid: protein coat surrounding virus
    • Envelope: Membrane-like coating on the outside surface of capsids of some viruses
      • Made of lipids taken from host cell membrane
      • Surface contains glycoprotein that act as receptor sites for attachment to host cells
      • Influenza, chickenpox, herpes, HIV
  • Virus Size
    • Virion- Individual virus particle
    • 20nm ------------------------------------ 250 nm
    • Polio virus Smallpox
  • Virion Shapes
    • Helical
      • Resembles coiled spring
      • Shaped by nucleic acid
    • Icosahedron
      • 20 triangular sides
      • Shaped by Capsid
    • Complex shape
      • Bacteriophage
  • Virion Shape Images
  • Subviral Particles
    • Viroids
      • Smallest particles able to replicate
      • Short single stranded circular RNA w/o capsid
      • Disrupts plant cell metabolism
    • Prions
      • Abnormal proteins that clump in the cell
      • Disrupt molecular transport of cell
      • Made of only proteins
      • Bovine Spongioform Encephalopathy (Mad Cow Disease)
  • Mad Cow Disease
  • Evolution
    • Likely evolved from early cells
    • Naked strands of nucleic acids mutated and were able to travel from one cell to another
    • Constant mutations allowed viruses to escape being destroyed by host’s immune system
  • Classification
    • Phylogenic classification limited due to lack of knowledge of evolutionary history
      • No fossil record
    • Divided into taxonomic groups based on:
      • Nature of host- plant, animal, bacteria
      • Nucleic Acid Characteristic: DNA, RNA, single or double stranded
      • Capsid type: icosahedral, helical, complex
      • Presence of envelope
      • Size
    • Named according to:
      • Tissue they infect
      • Diseases they infect
      • Code #s (T1)
  • Virus Replication Lytic and Lysogenic Cycles
    • Step 1: Attachment
      • Virus attaches to receptors of host cell
      • The virus must have the correct receptors in order to attach
    • Step 2: Entry
      • DNA or RNA is injected into the cell
      • Protein coating remains outside the cell
    • Step 3: Replication
      • Viral DNA is replicated and programs cell to make viral proteins
  • Lytic Cycle (cont)
    • STEP 4: Assembly
      • New virus particles are assembled
    • STEP 5: Lysis & Release
      • Enzymes from virus break the membrane of the host cell
      • Virus emerges and the cycle repeats itself
    • Viruses in lytic cycle are called virulent
    • Lytic cycle only lasts 15min -1 hr
  • Lysogenic Cycle
    • Step 1: Attachment
    • Step 2: Entry
    • Step 3: Provirus formation- Virus DNA becomes part of host cells DNA
      • Host cell does not recognize this as a problem
    • Step 4: Cell multiplication
      • Host cell multiplies as if nothing is wrong
    • Step 5: Trigger
      • Some environment factor triggers the virus to enter the remaining stages of the lytic cycle
    • VIRUS in Lysogenic cycle called temperate virus
    • Virus may remain in Lysogenic cycle for days to years
  • Lysogenic Cycle Attachment Entry Provirus Formation Trigger Replication & Assembly Lysis
  • Retrovirus
    • Has RNA instead of DNA
    • After RNA is injected into cell it must be converted into DNA before it can be integrated into the host cell’s DNA
    • An enzyme, Reverse Transcriptase, converts RNA into DNA
    • HIV virus
  • Retrovirus Cycle Reverse Transcription