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).

Structure extremely small (can fit thousands on a pinhead) – smallest 20nm (RBC 100 m m, cardiac cell 1500 m m, fat cell 6000 m m) 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

Viral infection 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 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

Viral transmission – Horizontal - infection from an external source – Vertical - infection inherited from a parent (more common in plants)

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

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

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

Human Viruses dsDNA – Adenoviris (common cold) – Herpesviris (herpes, chicken pox) – Poxvirus (smallpox, cowpox) ssRNA template for mRNA – Orthomyxovirus (influenza) ssRNA template for mRNA – Retrovirus (HIV)

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

Virus Types Viroids and Prions – Viroids - circular pieces of RNA that infect plants smaller than viruses Caused by the misfolding of proteins 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