ch. 6 viruses elc
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Georgia briefly tops chart for flureports
Monday, October 26th, 2015
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Chapter 6 Introduction
All life-forms can be infected by viruses.
Some viruses generate seriousepidemics, from dengue feverto influenza to AIDS.
- thers fill essential nichesin the environment,
particularly inmarine ecosystems.
In research, viruses haveprovided both tools andmodel systems inmolecular biology.
Figure 6.1
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6.! "hat Is a #irus$
A virusis a noncellular particle that must infect ahost cell, %here it reproduces.
- It typically subverts the cell&s machinery and directs it
to produce viral particles.
'he virus particle, or virion, consists of a singlenucleic acid (D)A or *)A+ contained %ithin a
protective protein capsid.
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#iruses are ubiuitous in all environments
#iruses are part of our daily lives
- ost freuent infections of college students
- *espiratory pathogens such as rhinovirus (the commoncold+ and /pstein-0arr virus (infectious mononucleosis+
- Se1ually transmitted viruses such as herpes simple1virus (2S#+ and papillomavirus (genital %arts+
Different viruses infect every group of organisms- /ach species of virus infects a particular group of
host species, or host range
#iruses Infect All 3orms of 4ife
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Figure 6.3
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Figure 6.3Why are there
holes on the
growth medium
where bacteria
mixed with lytic
phage was spread?
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'he propagation of viruses is mimic5ed by thespread of computer viruses,7 %hose informationinfects7 computer memory.
#iral 8enomes
Figure 6.4
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Small viruses commonlyhave a small genome,encoding under ten genes
- 'he genes may actuallyoverlap in seuence
Figure 6.5
4arge viruses may havemore than !99 genes
- Indeed, mimivirus encodesover !,999 genes
Figure 6.6
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Small viruses commonlyhave a small genome,encoding under ten genes
- 'he genes may actuallyoverlap in seuence
Figure 6.5
4arge viruses may havemore than !99 genes
- Indeed, mimivirus encodesover !,999 genes
Figure 6.6
What type
o# proteins
are
encoded
by the
cauli#lower
$irus $s% amimi$irus?
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Viroidsare *)A molecules that infect plants. 'hey have no protein capsid.
- Are replicated by host *)A polymerase.
- Some have catalytic ability.
#iroids
Figure 6.7
'hrysanthemum in#ected by
'hrysanthemum stunt $iroid (')*d+
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Prionsare proteins that infect animals. 'hey have no nucleic acid component.
- 2ave an abnormal structure that alters theconformation of other normal proteins
:rions
Figure 6.8
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Prion History
18thcentury scrapie
affecting sheep is documented
1920s neurologists Creutzfeldt andJakob discover similar disease in man 190s !a"dusek studies kuru spread
by cannibalistic rituals in ne# !uinea $on %obel &rize in 19'
1990s mad co# disease in !reat(ritain causing 1'0 deaths
199' %obel &rize a#arded to )tanley&ursiner for identifying prions
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Transmissible Spongiform
Encephalophathy )ymptoms
*ost associated #ith Creutzfeldt+Jakobdisease ,CJ-.
(ehavior changes/ aniety/ insomnia *uscle "erks/ memory loss/ demenia
gent &rions + &r&
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Transmissible SpongiformEncephalophathy
&athogenesis &rions promotes ,.
misfolding of normalproteins on neuralsurface
3ventually causetissue damage
4ransmission Corneal transplants/
contaminatedinstruments/hormone in"ections
Can be transmittedfrom cattle #ithcontaminated meat
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Effects of Prion Infection
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Prion Generation
%& or &r&c
%ormal proteins
&& or &r&)c
*isfolded &r&cproteins 5arious theories remain about ho# a
&r&Cis formed 6esistant to7
-isinfectants eat and 5 6adiation
-eleaulta/ %: 6: et. al., ;solation of phosphatidylethanolamine as a solitary cofactor for prionformation in the absence of nucleic acids/ &%)/ 2012/ 10978
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Prion Diseases
$hat other disease may beassociated #ith prions that #e haveyet to confirm
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Prion Diseases
$hat other disease may beassociated #ith prions that #e haveyet to confirm
lzheimer>s &arkinson>s *ultiple system atrophy ,*).
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6.; #irus Structure
'he viral capsidis composed of repeated proteinsubunits.
- 'his ma1imizes the capacity %hile minimizing the
reuired number of genes.
'he capsid pac5ages the viral genome anddelivers it into the host cell.
Different viruses ma5e different capsid forms.- 'hese can be divided into symmetricaland
asymmetrical types.
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Icosahedral viruses- Are polyhedral %ith ;9 identical triangular faces
Symmetrical #iruses
Figure 6.9
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In some icosahedral viruses, the capsid isenclosed in an envelope, formed from the cell
membrane.- 'he envelope contains glycoprotein spikes, %hich are encoded
by the virus.
- 0et%een the envelope and capsid, teguentproteins may be
found.
Figure 6.1!
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Filaentous viruses
- 'he capsid consists of a long tube of protein, %ith the
genome coiled inside- #ary in length, depending on genome size
- Include bacteriophages as %ell as animal viruses
Figure 6.11
A
B
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3ilamentous viruses sho% helical symmetry
- 'he pattern of capsid monomers forms a helical tube
around the genome, %hich usually %inds helically%ithin the tube.
Figure 6.1"
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'hese have comple1 multipart structures
#4 $acteriophages- 2ave an icosahedral head7 and helical nec57
Asymmetrical #iruses
Figure 6.13
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Po% viruses- 'heir genome is surrounded by several layers
-
A core envelope studded %ith spi5e proteins- An outer membrane
- Also contain a large number of accessory proteins
Figure 6.14
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6.< #iral 8enomes and Classification
#iral genomes can be
- D)A or *)A
- Single- or double-stranded (ss or ds+
- 4inear or circular
Include genes encoding viral proteins
- Capsid
- /nvelope proteins (if need be+
- Any polymerase not found in host cell
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6.< #iral 8enomes and Classification
#iral genomes can be
- D)A or *)A
- Single- or double-stranded (ss or ds+
- 4inear or circular
Include genes encoding viral proteins
- Capsid
- /nvelope proteins (if need be+
- Any polymerase not found in host cell
Whatcellularcomponentis missingfrom
viruses?
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,o ,hin- .bout
Why would a $irus want to bring to its host a
uni/ue polymerase?
0int ,hin- about $iral genomes
bola $irus
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'he International Committee on 'a1onomy of#iruses (IC'#+ has devised a classificationsystem, based on several criteria
- &enoe coposition
- 'apsid s(etr(
- )nvelope
- *i+e o, the virion
- -ost range
'he International Committee on
'a1onomy of #iruses
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So far, the genome composition and mechanismsof replication and m*)A e1pression define
seven fundamental groups of viral species- &roup I dou$le/stranded 02
- &roup II single/stranded 02
-&roup III dou$le/stranded 2
- &roup IV single/stranded 2
- &roup V single/stranded 2
-&roup VI 2 retroviruses
- &roup VII 02 pararetroviruses
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Figure 6.152
*irions do notcontain
ribosomes
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Figure 6.152
*iral
proteins
*irions do notcontain
ribosomes
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,hin- this out some more
What parts o# a $irus must be reproduced to
ma-e more $iruses?
What in the host cell ma-es .? m.?
proteins?
What about single stranded .? r double
stranded . genomes?
Fi 6 152
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Figure 6.152
Fi 6 152
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Figure 6.152
6 = #i l * li ti ith
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6.= #iral *eplication %ith
/mphasis on 0acteriophage
All viruses reuire a host cell for reproduction.
- 'hus, they all face the same needs for host infection
- -ost recognition and attachent
- &enoe entr(
- 2sse$l( o, virions
- )%it and transission
0acteriophage are viruses that infect bacterialcells
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Contact and attachment are mediated by cell/sur,ace receptors.
- :roteins that are specific to the host species and%hich bind to a specific viral component.
- 0acterial cell receptorsare normally used forimportant functions forthe host cell.
- /1ample sugar upta5e
0acteriophages Attach to 2ost Cells
Figure 6.18
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ost bacteriophages (phages+ in>ect only theirgenome into a cell through the cell envelope.
- 'he phage capsid remains outside, attached to the cellsurface.
:hage *eproduction %ithin 2ost Cells
Figure 6.192
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emember?
What are two ways bacterial cells can de#end
themsel$es #rom a phage in#ection?
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0acteriophages can undergo t%o different types oflife cycles
1. (tic c(cle
0acteriophage uic5ly replicates, 5illing host cell
". (sogenic c(cle
0acteriophage is uiescent.
Integrates into cell chromosome, as a prophage
Can reactivate to become lytic
'he decision7 bet%een the t%o cycles is dictated
by environmental cues In general, events that threaten host cell survival trigger a
lytic burst
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Current pplications of
!ytic Phage !ab
Phage "isplay for protein engineering
#e"ical Phage therapy
Treatment of antibiotic resistant infections
$ith specific lytic phage
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Phage Therapy "vances
Phage therapy centers in Georgia %formerly ofthe Soviet &nion' Creates lytic phage coc(tails that target
specific antibiotic resistant bacteria
Phagoburn European Commission "e"icates )*+,
million in ,-./ Phage treatment of burn victims suffering
from infecte" $oun"s
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Phage Therapy
.+ What $oul" be the a"vantage of using phage therapy forbacterial infections?
,+ What $oul" be the "isa"vantage of using phage therapy in
humans?/+ What other applications coul" be use" for phage therapy asi"e
from human infections?
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0ther Potential pplications
1oo" applications1resh cut fruit
#eat an" poultry pro"ucts
Currently available2 !istShiel"3 a phage coc(tail that targets Listeriamonocytogenes
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nimation2lysis and lysogeny
7ysis and 7ysogeny
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:hages use host cell machinery to synthesize capsids
'hey then assemble progeny phages
/1it from cell- (sis
- a5e enzyme that brea5s do%n cell %all
- 2ost cell bursts to release progeny phage- *lo release
- 3ilamentous phages can e1trude individual progenythrough cell envelope
- 2ost cells gro% slo%ly but do not die
Figure 6 "!
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Figure 6."!
0 t i l 2 t D f
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0acteria have evolved several forms of defense
against bacteriophage infection- &enetic resistance
- Altered receptor proteins
- estriction endonucleases- Cleave viral D)A lac5ing methylation
- 'I*Pintegration of phage D)A seuences
-Clustered regularly interspaced short palindromic repeats
- A bacterial immune system of sorts
0acterial 2ost Defenses
6 ? Animal and
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6.? Animal and
:lant #irus *eplication
Animal and plant viruses solve problems similar tothose faced by bacteriophages
- 2ost attachment, genome entry and gene e1pression,
virion assembly, and virion release 2o%ever, eu5aryotic cells have a more comple1
structure than pro5aryotic cells
-'herefore, animal and plant viruses have greatercomple1ity and diversity of viral replication cyclesthan %e see in bacteriophages
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A i l #i Sh 'i ' i
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Animal viruses bind specific receptor proteins on
their host cell.- *eceptors determine the viral tropis.
- /bola virus e1hibits broad tropism, infecting many 5indsof host tissues.
- :apillomavirus sho%s tropism for only epithelial tissues.
ost animal viruses enter host as virions.
- Internalized virions undergo uncoating, %heregenome is released from its capsid.
- Can occur in several different %ays.
Animal #iruses Sho% 'issue 'ropism
A i l #i * li ti C l
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'he primary factor determining the life cycle of an animal
virus is the form of its genome. 02 viruses
- Can utilize the host replication machinery
2 viruses- @se an *)A-dependent *)A-polymerase to transcribe their
m*)A
etroviruses
- @se a reverse transcriptase to copy their genomic seuence intoD)A for insertion in the host chromosome
Animal #irus *eplication Cycles
A i l #i * li ti C l
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All animal viruses ma5e proteins %ith hostribosomes
- 'ranslation occurs in the cytoplasm.
Assembly of ne% virions
- Capsid and genome
- ay occur in the cytoplasm or nucleus
- /nvelope proteins are inserted into a membrane
- Cell membrane or organelle membrane
Animal #irus *eplication Cycles
Animal #ir s *eplication C cles
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*elease of progeny viruses from host cell
- 4ysis of cell
- 0udding
- #irus passes through membrane
- embrane lipids surround capsid to form envelope
- All enveloped viruses bud from a membrane
- /ither plasma membrane or organelle membrane
Animal #irus *eplication Cycles