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Chapter18 Microbial Models
The genetics of Virus and Bacteria
The Genetics of Virus
Researchers discovered virus by studying a plant
disease
A virus is a genome enclosed in a protective coat
 Phage reproduces using lytic or lysogenic cycle
Animal virus are diverse in their modes of infection
and reproduction
 Plant virus are serious agriculture pests
Viroid and prion are infectious agent even simpler
than virus
 Viruses may have evolved from other mobile genetics
elements
Figure 18.1 Comparing the size of a virus, a bacterium, and
a eukaryotic cell
1883 Adolph Mayer
Tobacco Mosaic Virus-- contagious
1890 Dimitri Ivanowsky
Bacteria makes filterable toxins
1897 Martinus Beijerinck
Infectious agent in the filtered sap could
reproduce and cannot inactivate by alcohol
1933 Wendall Stanley
Crystallized the TMV particle
Figure 18.9x Tobacco mosaic virus
Rod shape
> 1000
molecules
Figure 18.02x1 Adenovirus
Infect respiratory
tract
252 identical
protein
 polyhedral
Capsid
Protein shell that encloses the viral
genome
Capsomere
Capsid build from a large number of
protein subunit
Viral envelope
Membrane cloaking the capsid, derived
from host cell
Figure 18.2 Viral structure
Figure 18.3 A simplified viral reproductive cycle
 Limited host range
Identify host by lock-and-key
Virus of eukaryotic are
tissue specific
Uses host DNA polymerase to
synthesize genome
Lytic cycle
A phage reproductive cycle that culminate in
death of host cell, bacteria lyse, phages release
Figure 18.4 The lytic cycle of phage T4
Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a
temperate phage
Figure 18.02x2 Phages
Table 18.1 Classes of Animal Viruses, Grouped by Type of Nucleic Acid
Figure 18.6 The reproductive cycle of an enveloped virus
Figure 18.7 HIV, a retrovirus
Three process for emergence of viral disease:
1. Mutation of existing virus
i.e.. High mutation of RNA virus
flu virus
2. Spreading existing virus from one host to another
i.e.. SARS, Hanta virus
3. Dissemination of viral disease from a small isolated
population
I.e. AIDS
Figure 18.7x1 HIV infection
Figure 18.7x2 Couple at AIDS quilt
Figure 18.x1 Smallpox
Figure 18.x2 Measles
Figure 18.x3 Polio
Figure 18.x4 Hepatitis
Figure 18.x5 Influenza epidemic
Figure 18.8 Emerging viruses
Ebola virus
Hemorrhagic fever
Figure 18.8x Deer Mouse
Hanta virus
Figure 18.x6 Herpes
Plant virus
mostly are RNA virus
Two major route to spread virus:
1. Horizintal transmission
a plant infect from external source of the virus
I.e wind, chilling, injury, insects bite………
2. Vertical transmission
inherit the viral infection from a parent
Figure 18.9 Viral infection of plants
Viroid
Naked circular RNA
Replicate by using host enzyme
Cause error in regulatory system and control
plant growth
Figure 18.10 A hypothesis to explain how prions propagate
1997 Stanley Prusiner
Prion
Infectious protein
Mad cow disease; degenerative in brain
Virus may have evolved from mobile genetic elements
1. Plasmids
Circular DNA separate from genome
2. Transposon
DNA fragments that move from one location to
another
The Genetics of Bacteria
The short generation span of bacteria
helps them adapt to changing environments
Genetic recombination produces new
bacterial strain
The control of gene expression enables
individual bacterial to adjust their metabolism
to environmental change
Bacterial genome
d.s circular DNA
DNA localized in the nucleoid region
Divide by binary fission
Figure 18.11 Replication of the bacterial chromosome
Figure 18.x7 E. coli
Figure 18.x8 E. coli dividing
Figure 18.x9 Bacterium releasing DNA with plasmids
Plasmids
Small circular, self replicating DNA
Figure 18.x10 Plasmids
Figure 18.12 Detecting genetic recombination in bacteria
Different process bring bacterial DNA from different
individuals:
1. Transformation
uptake of naked, foreign DNA from surrounding
i.e. uptake of pathogenic pneumonia DNA from
broken bacteria pieces
2. Transduction
DNA transfer process by bacterial phage
Figure 18.13 Transduction (Layer 1)
Figure 18.13 Transduction (Layer 2)
Figure 18.13 Transduction (Layer 3)
Figure 18.13 Transduction (Layer 4)
3. Conjugation
Direct transfre of genetic materials between
two bacterial
donar: male
receiver: female
Figure 18.14 Bacterial mating
Plasmids
Small circular, self replicating DNA
 Incorporate reversible into bacterial genome
 Episome exist as plasmids or in bacteria genome
F plasmid
Required for sex pili
Hfr cells( high frequency of recombination)
F factor integrate into bacterial chromosome
R plasmid
Plasmids carrying antibiotic resistance gene
Figure 18.15 Conjugation and recombination in E. coli
Figure 18.15 Conjugation and recombination in E. coli
Figure 18.15 Conjugation and recombination in E. coli
Figure 18.15 Conjugation and recombination in E. coli
Transposon( jumping gene)
A transposable genetic element
Movement occur only when recombination of
transposon and target site occur
Figure 18.16 Insertion sequences, the simplest transposons
Figure 18.17 Insertion of a transposon and creation of
direct repeats
Figure 18.18 Anatomy of a composite transposon
Include extra genes beside insertion
sequence
 Helps bacterial adapt to the new environment
Figure 18.19 Regulation of a metabolic pathway
Figure 18.20a The trp operon: regulated synthesis of repressible
enzymes
Figure 18.20b The trp operon: regulated synthesis of
repressible enzymes (Layer 1)
Figure 18.21a The lac operon: regulated synthesis of
inducible enzymes
Figure 18.21b The lac operon: regulated synthesis of
inducible enzymes
Figure 18.22a Positive control: cAMP receptor protein
Figure 18.22b Positive control: cAMP receptor protein
Figure 18-22x cAMP
Bacterial and viral growth curves
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