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MICB 202 Lecture Notes - Rna-Dependent Rna Polymerase, Dna Polymerase Iii Holoenzyme, Rna Virus

Course Code
MICB 202

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Viruses Lecture 1
Viruses=obligate intracellular parasites
oExtracellular: inert virion
oIntracellular: nucleic acid replication (gene expression, viral protein synthesis)
Viruses vs Organisms
oNo plasma membrane
oRequire ATP, nucleotides from host
oNo independent translation
oNo metabolism
Origin of viruses?—uncertain
oWere cells that became parasitic and lost lots of cellular components
oMobile genetic agents that could enter cells
oWere self replicating units then evolved to live in other cells
oDon’t really know for sure since rapid mutation/evolution and no fossils
Rapid evolution due to: short generation time, lots of progeny, high
mutation rate
Mutation rate: 10^-6 to 10^-7 for viral DNA POL, vs 10^-4,-5 for
Compare to humans of 10^-11
New viruses from mutations of pre-existing strains
oEg. Smallpox from camelpox
oEg. HIV from SIV
oH1N1 from recombination(combine with host genome) and rearrangement of
segmented genome of bird swine flu located in 1 host cell
Viruses diverse:
oReplication strategy, genetics, structure, type of host

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Viral Structure
oNaked: genome surrounded by protein coat=capsid
Capsid made of repeating capsomers
If more than 1 capside, inner capsid=nucleocapsid
oEnveloped: has a phospholipid bilayer covering capsid, thru budding
Can have GPs embedded to attach to host
Some have matrix proteins to link capsid with envelope
Some enveloped have CORE instead of capsid. Is capsid that doesn’t
immediately break apart upon entering host cytoplasm. Instead, docks to
nucleus before releasing genetics into nucleus
oGenome: linear, circular, segmented
oSome viruses contain own enzymes necessary for replication
If they replicate in cytoplasm can’t use DNA pol from host’s nucleus or
host pol not appropriate eg. retrovirus
5 structural forms of viruses
oNaked, icosahedral: eg. Polio
oNaked, helical
oEnveloped icosahedral: eg. HIV
oEnveloped, helical. Eg flu
oComplex eg. Pox
Enveloped vs naked viruses
oEnveloped viruses use budding for prolonged viral synthesis in cell “(doesn’t kill
host right away)
But envelope sensitive, and dies quickly in outside environment thru
dessication or harsh chemicals
oNaked viruses=hardier
Viral Replication Cycle:

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oAdsorption, attach to host
By chance. Host has receptors that virus takes advantage of. Virus has
anti-receptors. If fit, enter.
oGenome (and enzymes) go into cell
Dif for dif types of viruses
A: only genome enters
B: enter via endosome, then genome released
C: envelope fuse, capsid moves to nucleus eg. HIV
oSynthesis of viral genome, synthesis of viral proteins, synthesis of viral mRNA
Doesn’t always occur in this order. Depends on the virus
Viral components have affinity for each other
Can be spontaneous or require host mechanism for assembly
oRelease: thru budding or lysis =egress
oViruses must infect a cell that is: permissive, susceptible
Susceptible=has virus receptor and virus has antireceptor=ability for virus
to infect cell
Note: receptor is for normal host function but exploited by virus
Permissible=ability for virus to replicate inside cell
Central dogma=DNARNAproteins
oOnly applies to cells, not to viruses
DNA viruses=use central dogma
RNA viruses=RNAproteins aka bypass DNA part
Retrovirus=RNADNARNAproteinsgenome replication
Eukaryotics :
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