viruses and bacteriophages

7/31/2019 Viruses and Bacteriophages

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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

viruses and bacteriophages

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