Virus
- 1. Viruses Non-Living particle composed of nucleic acid and protein coat
- 2. Living or Non-Living? Characterisitics of viruses not shared by living cells: Simple acellular organization Absence of BOTH DNA & RNA Obligate Intracellular Parasite
- 3. 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?
- 4. Virion Structure Capsid Nucleocapsid Envelope Receptors DNA or RNA
- 5. 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
- 6. Virus Size Virion- Individual virus particle 20nm ------------------------------------ 250 nm Polio virus Smallpox
- 7. Virion Shapes Helical Resembles coiled spring Shaped by nucleic acid Icosahedron 20 triangular sides Shaped by Capsid Complex shape Bacteriophage
- 9. 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)
- 10. Mad Cow Disease
- 11. 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
- 12. 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)
- 13. Virus Replication Lytic and Lysogenic Cycles
- 14. LYTIC CYCLE 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
- 15. 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
- 16. 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
- 17. Lysogenic Cycle Attachment Entry Provirus Formation Trigger Replication & Assembly Lysis
- 18. 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
- 19. Retrovirus Cycle Reverse Transcription