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8/14/2019 9. Genetics of Bacteria and Viruses
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Genetics of Bacteria &
Viruses
CYTOGENETICS
AY 2008-2009
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Relevance of Bacterial & Viraln ti
Microorganisms are the most importantcomponent of environmental health
Microorganisms cause diseases
Microorganisms can help heal as well asprevent disease
Microorganisms have numerouscommercial/industrial applications
Mitochondria and chloroplasts aremicroorganisms
Microorganisms serve as model system
Microorganisms are extremely abundant
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VIRUSES (Characteristics)
Smaller than bacteria (typically, at least)
Obligate intracellular parasites (somebacteria are also)
structurally simpler than cellularorganisms
possess a relative dearth of metabolicmachinery
Many possess unusual genomes
Relative dearth of antivirals
Go through an acellular stage
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VIRUSES (Parts)
The genomes of viruses are typicallymuch smaller than the genomes ofcellular organisms
Virus genomes are also not alwayscomposed dsDNA
(i) dsDNA
(ii)
ssDNA(iii) dsRNA
(iv)
ssRNA
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VIRUSES (Parts)
Virus genomes can also take on avariety of configurations, depending onthe virus including
(i) Linear (ii) Circular
(iii) Segmented (more than one DNAmolecule, each holding a different gene
or genes) (iv) Diploid (most viruses are
haploid, though)
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VIRUSES (Parts)
Capsids and envelopes(a) Defining characteristic of viruses
is their protected extracellular state
(b) Protection is achieved via a capsi(c) In addition, an envelope may be
present, surrounding the capsid
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Polyhedral Viruses
Figure 13.2a, b
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Helical Viruses
Figure 13.4a, b
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Complex Viruses
Figure 13.5a
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VIRUSES (Host Range)
Many viruses are limited to only a singlehost species
(e.g. bacteriphage)
Other viruses have broader host ranges,being capable of successfully infectingmore than one host species
Many viruses are additionally limited in thecell types they are able to infect within ahost (i.e primary & secondary target)
One determinant of the host range of avirus is the "lock-and-key" fit between thevirus capsid or envelope proteins and virus
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Viruses (Life Cycle)
The simplified virus life cycle consists of(i) Adsorption to a host cell
(ii) Uptake of the virus genome into thecell
(iii) Transcription of virus genes
(iv) Translation of the resulting virusmRNAs
(v) Replication of the virus genome
(vi) Packaging of the new virus genomesinto capsids
(vii) Progeny-virus release from the hostcell
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Figure 13.10.1
Attachment:Phageattaches to
Penetration:Phagepnetrates hostcell and injects
Merozoitesreleased intobloodsteam fromliver may infect
new red blood
1
2
3
Bacterial cell
Bacterialchromoso
Capsi DN
Capsi
Sheat
Tail fiber
Base
Pin
Cell wall
Tail
Plasma
Sheath
Tail core
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Figure 13.10.2
Maturation:Viral componentsare assembledinto virions.
Tail
5 Release:Host cell lysesand new virionsare released.
DNA
Capsi
Tail fibers
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Viruses (Life Cycle)
A lytic life cycle requires the destructionof the host cell before progeny releasemay occur
This host-cell destruction is called lysis
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Viruses (Life Cycle)
Lysogenic life cycle (prophage,provirus, temperate virus) In a lysogenic life cycle virus progeny
are neither produced nor releasedTemperate virus = a virus capable of going
through a lysogenic cycle (e.g., phage lambda,a.k.a., )
Prophage = a bacteriophage whose genome hasintegrated into its host's genome duringlysogenic growth
Provirus = equivalent to prophage but more
generally applicable (e.g., to animal viruses)
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Beneficial Uses of Viruses inBeneficial Uses of Viruses inBiotechnologyBiotechnology
Gene therapy
Vaccines and vaccine carrier / deliveryvehicles
Antibacterial agents
Insecticides
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Bacteria
Circular chromosome
Extrachromosomal: plasmids
Constitutive and repressible genes Transposons
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Bacteria
Sex:
Transformation
Transduction
Conjugation
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