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7/31/2019 Viruses and Bacteriophages
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27/09/20
4. Overview: Viruses & Bacteriophages
What is a Virus / Bacteriophage?
What agents are even smaller than a virus?
The structure of viruses
How viruses are classified
The life cycles of bacteriophages
How viruses are cultured
The ecology of viruses Readings – Ch 6 (BUT omit pgs. 192-196;203-205; 212-213 )
Viral trivia: the CroV virus (Mimivirus family) at ~600nm diameter, is the
largest virus identified; CroV virus is itself parasitized by a virophage
(~50nm) called mavirus!
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What Is a Virus / Bacteriophage?
An Acellular microbe; most common l ife-
form on the biosphere
~1032 viruses; Particularly important in
marine ecosystems (Virosphere concept)
Size range: ~30nm – 600 nm diameter
Depends on a host’s metabolism
Exhibit host preference or host range
Most have a small genome
DNA or RNA, single- or double-stranded
Genome is protected by a ‘capsid’
Capsid proteins
DNA or RNA coiled
inside tube
Fig. 6.11
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Figure 6.2
All cellular life-forms can
be infected by viruses
Some viruses have caused
frightening epidemics &
pandemicse.g…….
Are used as both
tools/vectors & modelsystems in molecular
biology;
e.g……
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Viriods & Prions are even SmallerThan a Virus (figs. 6.6; 6.7)
Viroids Infectious RNA only, no capsid / proteins
Capable of infecting plants
Hair-pin RNA
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Figure 6.7
Prions Protein only, no nucleic acid
Cause of “mad cow” disease; Creutzfeldt-Jakob disease
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Discovery of Viruses
Martinus Beijerinck (1899) proposed the concept of a
‘viruses’ as sub-cellular particles
d’Herelle & Twort (1917) discovered bacteriophages
Martha Chase (1952),discovered that only the phage DNA
(not capsid) enters the host prokaryote cell
Peyton Rous – sdiscovered RNA retroviruses (Roussarcoma virus) in the causation of cancers
Question: What scientist(s) discovered the virus that causes AIDS in
humans? When was it discovered/identified & what is the name of the
virus??6
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Virus Structure
Capsids
composed of repeated protein subunits packages & protects the genome; target / delivers virus
to the host cell
Capsids can be divided into two main types
Symmetrical capsids
Asymmetrical capsids & structures
Viral envelopeOnly in some viruses; external to / covers the capsid
Composed of ‘host’- derived membranes
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Symmetrical (capsid) Viruses
2 types: Icosahedral & Filamentous viruses
Icosahedral viruses
- Are polyhedral with 20 identical triangular faces
- Have a structure that exhibits rotational symmetry
- Includes many animals viruses e.g.
Fig. 6.8
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Symmetrical Viruses
Filamentous viruses
- Long capsid ‘tube’, with the genome coiled inside.- Vary in length, depending on genome size
- Include bacteriophages as well as animal viruses
Fig 6.11 (A)
Tobacco
mosaic virus
Figure 6.10Bacteriophage M13 by
TEM
Ebola virus
filaments by SEM
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Asymmetrical Viruses
Complex capsids, multipart structures
Fig. 6.13
T4 bacteriophages
- have icosahedral “head” &
filamentous / helical “neck”
Poxviruses
- irregular shapes & have
several layers
Fig 6.12
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Enveloped viruses
-the capsid is enclosed in a lipid envelope
- the envelope contains glycoprotein spikes
- between the envelope and capsid, tegument proteins maybe found
- Envelope has role in infection of host
Fig. 6.9 HIV
Envelope
Envelope
Proteins
Capsid
Herpes virus (TEM)
Envelope
spikes
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Viral GenomesCan be,
DNA or RNA
Single- or double-stranded (ss or ds) [& ssRNA can be (+) or (-); ssDNA is (+) ……]
Linear or circular
Include genes encoding viral proteins
Capsid
Envelope proteins (if need be)
Virus/phage-specific nucleic acid polymerase not foundin host cell,
Eg. RNA-dependent RNA polymerase
Reverse transcriptase (ie. RNA to DNA)
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Classification of Viruses
The International Committee on Taxonomy of Viruses (ICTV)has devised a classification system, based on several criteria:
Genome composition
Capsid symmetry
Envelope
Size of the virion
Host range
► The Baltimore classification is based on genome
form / type & the route for generating mRNA
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Viral / Phage Life Cycles
All viruses must infect a living host cell to replicate &
reproduce All face the same challenges (ie. Have the same to-do-list..)
Host recognition and attachment (i.e. targeting/specificity )
Get genome into the host cell (i.e. infection )
Replicate the genome
Make viral proteins (i.e. reproduce ;evolve?; progeny ) Assemble capsids/virions
Exit from host and be transmitted (i.e. perpetuate / survive )
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Bacteriophage Life Cycles
Attachment to specific host cell receptors
Receptors are normally used for important cell functions
Phage genome is injected through the cell wall Capsid is shed
Fig. 6.18A
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Bacteriophages can undergo two differenttypes of life cycles,
1. Lytic cycle- rapid phage replication & lyses / kills host
- Lytic phages
2. Lyso genic cycle- phage infects & becomes quiescent
- may establish lysogeny- ..but can reactivate to become lytic
►What are the factors / conditions that dictate the type of life cycle & trigger a lytic burst?
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Figure 6.18B
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Animal Virus Life Cycles
Animal viruses / attach to specific receptors proteins on
host cell
Most (not all) viruses enter host cells as virions
Internalized virions undergo uncoating, to release the
genome
Uncoating can occur in several different ways
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AnimalVirus Life
Cycles
Fig. 6.20 19
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The genome type is the primary factor whichdetermines the life cycle of an animal virus
DNA viruses
- utilize the host replication machinery
RNA viruses
- Use a viral RNA-dependent RNA-polymerase totranscribe their mRNA
Retroviruses
- Use viral reverse transcriptase to copy their genomic sequence into DNA for insertion in the host chromosome
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Plant Virus Life Cycles
Most plant viruses enter by one of 3 routes
- Contact with damaged tissues
- Transmission by an vector
- Transmission through seed
Entry of plant viruses into host cells usually
requires mechanical transmission.
Fig 6.25Plum pox virus disease is caused by potyvirus.
Aphid
vector Potyvirus is a filamentous (+)
strand RNA virus (see (a),
TEM); infection causes
streaking on flowers, rings on
fruit & pitting on the stone
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Plant cell – to – plant cell transmission
Plants have thick complex cell walls,
Prevent lytic burst or budding out of virions Virion or viral proteins are transmission from
infected to unifected cells is via Plasmodesmata(membrane & ER ) channels
Fig 6.26
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Viral /Phage Ecology
Viruses exist naturally within hosts in terrestrial and
marine ecosystems (Virosphere)
In oceans they are the most genetically diverse and
numerous life form
Viruses significantly affect host cells,
limiting population host density
increase host diversity Have roles in ‘ evolution ’ of genomes
Role in adaptation
Are the cause of many recent emerging infections
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Viral Ecology
On a global scale, viruses play a significant role in thecarbon balance (carbon cycle).
Figure 6.37 24
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Culturing Viruses
requires a host cell (eukaryote/prokaryote)
Two types of Bacteriophages culture systems
batch culture (in liquid)
isolated plaques on a bacterial lawn (on a plate).
Batch culture of viruses generates a step curve.
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Viruses: infectious cycle takes hrs not mins
- delayed release of virions (i.e. virions are observed inside host
cells several hrs before extracellular virions are detected
Fig 6.28
One-step growth
curve for a phage
in batch culture
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Plaque Assay of Bacteriophages (Fig 6.31)
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Culturing of Animal Viruses
within whole animals by serial inoculation.
in human cell tissue culture.
Figure 6.30
Fig. 6.30- Poliovirus replication in human tissue culture cells
“smooth’ monolayer
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Plate Culture of Animal Viruses
Figure 6.33