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