ch. 6 viruses elc

7/26/2019 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 #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


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