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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