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Microbial Models:
The Genetics of Viruses
Chapter 18
p. 334-346
Discovery of Viruses

Tobacco Mosaic Virus: stunts
growth & changes leaf
coloration of plants
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Contagious, but not caused by
bacteria (too small)
Able to reproduce (in host)
Did not dilute when passed
plant → plant
A Virus is a Genome in a Protective
Coat

Virus: infectious particles consisting of
nucleic acid enclosed in protein coat
Some also have membrane envelope
 Nucleic Acid: DNA or RNA; single- or
double-stranded
 Protein Coat: capsid; composed of protein
subunits (capsomeres)
 Envelope: viral envelope; composed of
host cell membrane, proteins, glycoproteins

Viral Structures
Viral Reproduction: an overview

Viruses can ONLY reproduce inside a host
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Lack ability to make own proteins or metabolize
Can only infect certain types of hosts (“Host
Range”)

May also be tissue-specific in eukaryotes
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i.e.: cold: upper respiratory; AIDS: WBC’s
Viruses use host resources to replicate their
genomes
Viral Reproduction: an overview

Use replication to
make copies of viral
DNA
 Use transcription, &
translation to
produce capsids
 Genomes & capsids
spontaneously
recombine to
produce several new
viral particles
The Lytic Cycle

Phage reproductive cycle that kills host cells
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Virulent phage: infects ONLY by Lytic cycle
1) Phage binds to receptor site on bacteria
2) Phage injects DNA into host cell
3) Bacterial DNA hydrolyzed
4) Phage genome & capsid components copied
5) 100-200 phages reassembled, bacteria cell wall
destroyed
6) Cell lyses, releasing phages to infect more cells
Both bacteria & phages have defenses against
each other (constantly evolving)
The Lytic Cycle
The Lysogenic Cycle

Phage reproductive cycle that replicates
genome but does not kill host
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Temperate Phage: uses Lytic & Lysogenic cycles
1) Phage binds to bacteria cell & injects DNA
2) Phage DNA incorporates into bacteria DNA at
specific site through crossing over (“Prophage”)
3) Prophage DNA replicated as bacteria replicates
4) Prophage genome “dormant” inside host until
triggered to detach & initiate lytic cycle

By certain chemicals or radiation
The Lysogenic Cycle
Animal Virus
Reproduction

Have many different
modes of infection &
reproduction
 Depends on:
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Presence of viral
envelope
Type nucleic acid
Viral Envelopes
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Made of lipid bilayer that
fuses w/ host cell
membrane
Once inside cell, viral
genome replicates &
directs synthesis of new
viral envelopes
New viruses “bud off” host
cell membrane & spread
to infect more cells
RNA as Viral Genetic Material

1) Double-stranded
 2) ss mRNA: directly translated into viral
protein
 3) ss mRNA template: viral genome used to
make compliment; many copies made
 4) ss DNA template (“Retroviruses”): uses
reverse transcriptase to make DNA from
RNA template

DNA incorporates into host animal cell (“provirus”)
& remains permanently
Cause & Prevention of Animal Viral
Diseases
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Viral symptoms may be caused by:
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Hydrolytic enzymes (lysosomes)
Production of toxins
Toxic components (envelope proteins)
Degree of damage depends on host cell’s
ability to repair/replace themselves
 Vaccine: harmless form of virus that triggers
body to defend itself against actual virus
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i.e.: smallpox – cowpox virus vaccine
Emerging Viruses
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New viral diseases may emerge by:
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1) Mutation of existing virus (esp. RNA)
2) Spread of virus to new host species
3) Spread of virus from small, isolated
population to large one
Ebola
Viruses and Cancer
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Tumor Viruses: cause cancer in
animals
i.e.: retroviruses, papovirus, adenovirus
 Transform healthy cells → cancerous by
incorporating viral genome into host DNA
 Usually require other mutagens
 i.e.: Hepatitis & liver cancer
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Plant Viruses are Agricultural Pests
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Mostly RNA; may stunt
plant growth & decrease
yields
Horizontal
Transmission: virus is
from external source
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i.e.: other plants,
insects, tools
Vertical Transmission:
virus is inherited from
parent plant
 Viral infections spread
throughout plant through
plasmodesmata
Microbial Models:
The Genetics of Bacteria
Chapter 18
p. 346-358
Bacteria Have Short Generation
Span
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Prokaryotes: contain
small, ds circular DNA in
nucleoid region
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Divide by Binary
Fission: DNA
replicates, cell grows,
divides, produces 2
new identical cells
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May mutate to form
new strains
Can reproduce very
quickly (E. coli every
20 minutes!)
Genetic Recombination
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DNA from 2 different bacterial strains can
recombine to form new strains
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By crossing-over
Leads to genetic diversity
Uses transformation, transduction, or
conjugation
Transformation
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The alteration of bacterial DNA by
incorporating environmental DNA
“Genetic Recombination”
 New alleles replace native alleles
 May code for pathogens, resistance, new
proteins, etc.
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Transduction
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Phages carry bacterial genes
from one cell to another
Generalized Transduction:
bacteria host cell DNA is
packaged inside capsid;
“infects” new bacteria cell,
replacing homologous
section
Specialized Transduction: a
temperate phage will take
with it small sections of
bacteria host cell DNA
 Only genes near
prophage site
Conjugation
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The direct transfer of
genetic material between 2
joined bacterial cells
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“Male” cell extends sex
pilus to pull cells together
“Maleness” determined by
F factor (DNA segment)
Once joined, “male”
donates portion of DNA to
“female” through
cytoplasmic bridge
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Aids in genetic
recombination
R Plasmids & Conjugation
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R plasmids carry genes for resistance to
antibiotics
May code for enzymes that destroy
antibiotic
 Resistant population tends to grow
 Resistant bacteria may spread resistance
through conjugation
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Transposons
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Movement within a cell’s genome is
result of recombination
May “cut-and-paste” or “copy-and-paste”
 Brings genes for resistance to R plasmid
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Insertion Sequences
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Simple, containing only the sequence to be transposed
(“Transposase Gene”)
Capped at each end by inverted repeats
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Signal removal of transposase & guides new placement
DNA polymerase fills in gaps
Results in direct repeats at new location
Composite Transposons
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Include extra genes sandwiched between
insertion sequences
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i.e.: for resistance
May help bacteria adapt to new environment
by ↑ resistance
Metabolic Control of Bacteria
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Bacteria are able to
adjust their metabolism
in response to
environment
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1) Adjusting # enzymes
made (gene
expression)
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During transcription
2) Adjust activity of
present enzymes
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By “Feedback
Inhibition”
Operons: the basic concept
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Genes involved in same metabolic processes are
often grouped together as 1 transcription unit
 Single “on/off” switch (“Operator”) controls
group
 Operon = operator + promoter + transc. unit
 “On” unless repressor present, which blocks
RNA polymerase (specific!!!)
 Produced by regulatory gene at separate
location
 May require corepressor (may be molecule
itself = Negative Feedback)
 i.e. trp Operon
trp
Operon
lac
Operon
Some repressors are
always “on” & require an
inducer to inactivate
-Genes of operon are
silenced
Repressible vs. Inducible Operons
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Repressible Operons: trp Operon
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Usually “On”
Anabolic pathways (raw material → product)
“Repress” end product when already present
Inducible Operons: lac Operon
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Usually “Off”
Catabolic pathways (nutrients → simple
molecules)
Produce enzymes (“induce”) only when nutrient
present
Positive Gene Regulation
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Promotes gene
expression when
molecule (cAMP)
binds to protein
(CRP) & activates it
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Facilitates binding of
RNA polymerase to
promoter (activates
transcription)