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