Transcript Chapter 18: The Genetics of Viruses & Bacteria

Viral structure
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Nucleic acid in a protein coat (capsid)
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sometimes viral envelope (host cell
membrane + viral proteins + glycoproteins)
Depending on the virus the nucleic acid
may be:
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double or single stranded DNA or RNA
linear or circular
few or many genes
Viral Reproductive Cycles
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reproduce in host cell
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Three patterns of replication
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Host range dependent on proteins
DNA-->DNA
RNA -->RNA
RNA--> DNA-->RNA
Injects genome into host cell
Uses host’s nucleic acids, enzymes, ribosomes,
amino acids, ATP, etc.
Reproduction of bacteriophages
-Bacteriophage: virus which infects bacteria
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Lytic cycle
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Death of host cell
Bacterium lyses upon virus exit
Exclusively lytic cycle = virulent phages
Bacteria use restriction enzymes to cut viral
DNA (pseudo- immune response)
Reproduction of bacteriophages
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Lysogenic Cycle
Phage DNA incorporated into host cell’s
chromosome (Prophage)
 Phage DNA replicated when bacteria
replicates its genome.
**Temperate phages- uses both lytic &
lysogenic cycles; environmental trigger causes
switch to lytic cycle
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Reproduction of Animal Cell Viruses
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Nearly all have envelopes
Helps virus go undetected
Virus uses host ER or nucleus to construct
envelope
Reproduction of Animal Cell Viruses
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Retroviruses
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Reverse transcriptase transcribes RNA
template into DNA within host
Ex. HIV
Viral DNA integrates into host genome
(provirus) becoming a permanent fixture
Effects of viral infection
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Host cell damaged by:
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releasing hydrolytic enzymes from lysosome
Some viral proteins are toxic
Symptoms usually related to body’s attempt to defend
itself
Vaccines stimulate immune system to mount defenses
before actual infection
Effective anti-viral drugs interfere with viral nucleic acid
synthesis
Viruses mutate rapidly new outbreaks & new host
organisms (emergent viruses)
Plant cell viruses
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Symptoms: bleached or brown spots on
leaves & fruits, stunted growth, damaged
flowers or roots
RNA genome
rod-shaped or polyhedral capsid
Spread
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Horizontal transmission- infection from an
external source i.e. insect, farmer
Vertical transmission- inherit virus from parent
plant
Viroids & Prions
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Viroids
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Small circular RNA molecules that infect plants
NO proteins made  use host cell to replicate its RNA
Cause abnormal development & stunted growth
Prions
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Very small infectious proteins in animals
Ex. mad cow
Slow acting with long incubation periods
Indestructible; thought to be transmitted through
food
Bacteria
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Double stranded circular DNA
concentrated in nucleoid
Contain plasmids- small self-replicating
circles of DNA
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not a part of the main genome
Divide rapidly
Genetic Recombination in Bacteria
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Transformation
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Transduction
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Bacteria uptakes foreign DNA
Phages carry bacterial genes from one host to
another
Conjugation
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Temporary joining of 2 bacterial cells allows
for one-way direct transfer of genes using
sex-pili
Requires F factor
Bacteria: Transposition of Genes
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Transposable elements
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DNA in a single bacteria moves within the
genome
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Gene can be:
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“copy and paste”
“cut and paste”
Transposons- carry additional genes to new
site
Bacteria: Regulation of gene
expression
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Feedback inhibition
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Environmental stimuli determine enzyme
activity (negative feedback)
Operons
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Negative feedback through gene expression
Operons
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Operon= promoter, operator, & the genes they
control
Operator is between promoter and genes being
controlled
Operator controls the access of RNA polymerase
to the genes
Inducible operons: transcription normally off
stimulated by a regulatory protein
Repressible operons: transcription normally on
inhibited with binding of regulatory protein
Repressible operon: trp operon
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RNA polymerase normally can access
genes
Regulator gene located away from operon
& has own promoter produces inactive
repressor protein
Repressor protein activated by tryptophan
Trp repressor protein binds to operator
preventing RNA polymerase attachment
Inducible operons: Lac operon
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Beta-galactosidase: lactoseglucose +
galactose
Lac operon genes for Beta-galactosidase
Regulator gene (lacI) produces active repressor
protein blocks transcription
Allolactose (Inducer) binds to repressor
protein inactivation; lac operon is then turned
on
Lac operon also affected by [glucose]; high
operon remains off