Viruses as Pathogens

1. Viruses as Pathogens • Viruses are obligate intracellular pathogens. • They are not living and cannot metabolize to create energy. • They cannot reproduce without the help of the host and its resources. • Read up on TMV(Tobacco mosaic virus).

2. Structure • extremely small (can fit thousands on a pinhead) • smallest 20um • nucleic acid • single or double stranded DNA or RNA • protein coat • may be a membranous envelope • derived from the membrane of the host • carry glycoproteins specific to the host cell • are usually animal viruses • may be called a capsid • made of specialized proteins called capsomeres • rod or polyhedral shaped • mostly found in bacteria that infect bacteria- bacteriophages

3. Viral infection • Host range - defines who can be infected • can be narrow or broad • West Nile Virus - broad (birds, humans, equine) • measles & poliovirus - narrow(humans • is dictated by the surface proteins present on the capsid/envelope • Entering a host - beginning of an infection • infection begins when viral nucleic acids enter a host • T-phages inject DNA/RNA through their tail • envelope may fuse with host bringing in the nucleic material via endocytosis • Viral transmission • Horizontal - infection from an external source • Vertical - infection inherited from a parent (more common in plants)

4. Types of infections • Lytic Cycle - virulent phage • phage DNA enters cell (T4 through tail) • cell's DNA is hydrolyzed (separated) • Synthesis of viral DNA and proteins by host • Assembly of complete virus • release as the cell swells and bursts releasing many new complete viruses

5. Types of infections • Lysogenic Cycle • replication without killing host • phage DNA enters cell • incorporates itself into the hosts DNA – now known as a prophage • may lie within the host dormant and create many cells carrying the prohage DNA • a trigger can switch the prophage into a lytic cycle • l phage is like the T4 but is not an obligate lysogenic virus - used in often research

6. Defense mechanisms • Host • restrictions enzymes (endonucleases) • recognize viral DNA and cut it up • evolution - favors host with different cell receptors • vaccines • made of attenuated (viral pieces which are harmless) viruses • medicines • usually work by inhibiting viral DNA/RNA replication • virus • mutation resistant to restriction enzymes • lysogenic lifecycle

7. Human Viruses

8. Virus Types • Retrovirus: HIV • ssRNA virus equipped with the enzyme reverse transcriptase • makes DNA from RNA - reverse process • DNA is inserted into host's DNA - HIV • now called a provirus • RNA pol II from the host now makes mRNA capable of making more viral ssRNA • provirus NEVER leaves the host

9. Virus Types • Viroids and Prions • Viroids - circular pieces of RNA that infect plants • smaller than viruses • do not encode proteins but take over the regulatory system of its host cell causing mass replication • Prions • small viral pieces that cause diseases in animals • cause neural diseases - mad cow (BSE), scrapie (sheep) • slow onset time • cannot be killed by heating and cooking • no known cure