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).
• extremely small (can fit thousands on a
pinhead)
– smallest 20nm (RBC 100mm, cardiac cell
1500mm, fat cell 6000mm)
• 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
Structure
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
• 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
infection
• Viral transmission
– Horizontal - infection from an external source
– Vertical - infection inherited from a parent (more
common in plants)
• 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 –
Types of
infections
– 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
• Host
Defense
• recognize viral DNA and cut it up
– evolution - favors host with different cell mechanisms
receptors
– restrictions enzymes (endonucleases)
– 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
• Viroids and Prions
Virus Types
– 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