Download - Ch 6 Virus

An Introduction to the Viruses

Chapter 6

The Scale of Viruses

• Too small for the light microscope

• Range from about 20 to 200 nm (0.02-0.2 µm)

1.(1)

(2)

(3)

(4)(6)

(7)

(8)

(9)

(10)

10.

2.3.4.5.6.7.8.9.

BACTERIA CELLS

Rickettsia0.3 µm

VirusesMimivirus 450nmHerpes simplex 150 nmRabies 125 nmHIV 110 nmInfluenza 100 nm

T2 bacteriophage 65 nmPoliomyelitis 30 nmyellow fever 22 nm

Protein MoleculeHemoglobin 15 nmmolecule

(5)

E. coli2 µm long

Streptococcus1 µm

Adenovirus 75 nm

Viral Classification

• International Committee on the Taxonomy of Viruses (ICTV)

• Type of nucleic acid

• Strandedness (single vs double)

• Capsid morphology (the coat)

• Presence or absence of envelope

• Host range (host specificity)

• NOT 16S rRNA

The Viral Capsid

Made of protein (Capsomeres) – auto-assemble

Nucleocapsid

Icosohedral morphology

Envelope

Spike

Capsid

Nucleic acid

Nucleic acid

Capsid

(a) Naked Nucleocapsid Virus

(b) Enveloped Virus

Spike

Other Capsid Morphologies• Helical capsid • Complex capsid

Capsomers

(a)

(b)

(c)

Discs

Nucleic acid

Nucleic acid

Capsid beginsforming helix.

Base plate

Sheath

Collar

Nucleic acid

Tailfibers

Tailpins

Capsid head

(a)

(b)a: © Harold Fisher

Concept Check

What is the protein shell that surround a viral genome called?

A. Envelope

B. Capsid

C. Core

D. Cortex

Viral Genomes

• One kind of nucleic acid (either RNA or DNA)

• DS DNA Adenovirus, Herpes virus, Poxvirus

• SS DNA Parvovirus

• DS RNA Reovirus

• SS RNA (+) Poliovirus, Rhinovirus

• SS RNA (-) Influenza, Rabies virus

• SS RNA (+) HIV (becomes DNA in cell)

• SS RNA (+) Hepatitis B (becomes DNA in virus)

Viral Membranes

• Called the envelope

• Found in some, but not all viruses

• Derived from the host cell

• Formed by budding(c)

Envelope

Nucleocapsid

Viral Enzymes• Found In nucleocapsid

• Polymerases

• Reverse transcriptase

• Found in envelope

• Hemagglutinin• Antigenic protein with a glycoprotien component,

bind receptors (sialic acid) on host respiratory cells, sialic acid receptors, very similar between strains, rapid ability to evolve (http://biology.kenyon.edu/BMB/Chime2/2005/Cerchiara-Holsberry/FRAMES/start.htm)

• Neuraminidase• cleave sialic acid (aka…sialidase) found in tissue,

glycoproteins

• Orthomyxoviridae (influenza),

• Paramyxoviridae (meales, mumps, parainfluenza viruses, and respiratory syncytial virus- children with pneumo) and bacteria

• Assist in release of viral progeny that would normal remain attach to the host cell surface

Envelope

Nucleocapsid

http://biology.kenyon.edu/BMB/Chime2/2005/Cerchiara-Holsberry/FRAMES/start.htm

http://biology.kenyon.edu/BMB/Chime2/2005/Cerchiara-Holsberry/FRAMES/start.htm

Virus Infectious Cycle

• Adsorption – specific attachment

• Penetration – entry of viral genome

• Uncoating – release of viral genome

• Synthesis – new viral products made

• Assembly – new viruses are made in the cell

• Release – often causes the host cell to lyse

Concept Check

What type of genetic material does the influenza virus contain?

A. DS DNA

B. DS RNA

C. SS (+) RNA

D. SS (–) RNA

Lytic Bacteriophage

• Virus binds

• DNA injected

• Capsid stays outside

• Expression

• Assembly

• Cell destroyed

Head

Bacterialcell wall

Tube

Viral nucleic acid

Cytoplasm

Lysogenic Viruses

• Lysogeny

• Temperate viruses

• Infection followed by integration into the genome

• Prophage are integrated viral genomes

• Lysogenic conversion

• Induction leads to lysis

Lytic and Lysogenic Cycles

Adsorption

E. coli host

Lysogenic State

Release of viruses

Penetration

1

2

3

4

5

6

7Bacteriophage

+

Sheath

Tail fibersTail

Viral DNA becomeslatent as prophage.

DNAsplits

ViralDNA

BacterialDNA molecule

Splicedviralgenome

Duplication of phagecomponents; replication of

virus genetic material

LyticCycle

BacterialDNA

ViralDNA

Lysis of weakened cell

Maturation

CapsidDNA

Assembly ofnew virions

Bacteriophage

Bacteriophage assembly line.First the capsomers aresynthesized by the host cell. Astrand of viral nucleic acid isinserted during capsid formation.In final assembly, theprefabricated components fittogether into whole parts and

The lysogenic state in bacteria.The viral DNA molecule is insertedat specific sites on the bacterialchromosome. The viral DNA isduplicated along with the regulargenome and can provide adaptivegenes for the host bacterium.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Detecting Viruses in the Lab

• Plaque assays (used to visualize the presence of virus)

• Bacteria cultures used for bacteriophage

• Tissue culture used for eukaryotes

Concept Check

Corynebacterium diphtheriae becomes more pathogenic when a bacteriophage called beta is incorporated into its genome. What is this process called?

A. Transposition

B. Transformation

C. Lysogenic conversion

D. Lytic cycle

DNA Viruses of EukaryotesCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Table 6.2Medically Relevant DNA Virus Groups

DNA Viruses

Enveloped Nonenveloped

Single-strandedgenome

Double-strandedgenome


Poxviruses-Small pox, Vaccinia, Cowpox

Herpesviruses-Simplex 1-Simplex 2

Adenoviruses(linear dsDNA)

Papovaviruses(circular dsDNA)

Parvoviruses

Source: Adapted from: Poxviridae from Buller et al., National Institute of Allergy & Infectious Disease,Department of Health & Human Services.

RNA Viruses of Eukaryotes


Nonsegmented genome

Table 6.3 Medically Relevant RNA Viruses

RNA Viruses

Enveloped


Segmentedgenome

Orthomyxoviruses

Bunyaviruses

Arenaviruses

Coronaviruses

Filoviruses

Rhabdoviruses

Paramyxoviruses

Single-stranded genomeencodes reversetranscriptase

Retroviruses

Nonenveloped



Reoviruses

Caliciviruses

Picornaviruses

Results of Viral Infections

• Death of host cell = Most viruses

• Proliferation of cells = Papillomavirus

• Fusion of host cells = Respiratory syncytial virus

• Transformation of cells = Adenovirus

• No obvious changes = Rubella

Viral Infectious Cycle

Cell membrane Spikes

Receptors

1 1

4

4

6

6

5

5

3

32

2

Host Cell Cytoplasm

(–)

(a)

Adsorption. The virusattaches to its host cell byspecific binding of its spikes to cell receptors. Penetration. The virus is

engulfed into a vesicle andits envelope isUncoated, thereby freeingthe viral RNA into the cellcytoplasm.

Synthesis: Replication and ProteinProduction. Under the control of viralgenes, the cell synthesizes the basiccomponents of new viruses: RNAmolecules, capsomers, spikes.

Assembly. Viral spikeproteins are insertedinto the cell membranefor the viral envelope;nucleocapsid is formedfrom RNA andcapsomers.Release. Envelop viruses bud

Off of the membrane, carryingAway an envelope with the spikesThis completed virus or virion is ready to infect another cell.

RNA

Newspikes

Newcapsomers

NewRNA

Nucleus

(+)

(+)

(+)

(+)

(+)

(+)


Virus Binding and Entry


(a)

(b)

Host cell membrane

EngulfmentSpecificattachment

Virus invesicle

Vesicle, envelope, andcapsid break down

FreeDNA

Uncoating step

Host cellmembrane

Receptors

Receptor-spikecomplex Entry of

nucleocapsidIrreversibleattachment

Membranefusion

Uncoating ofnucleic acid

FreeRNA

Concept Check

What is the lipid membrane that surround some viruses called?

A. Capsid

B. Envelope

C. Outer membrane

D. Capsule

RNA Virus Replication

(b)

Positive single-stranded RNA viruses (e.g., picornaviruses)

Negative single-stranded RNA viruses (e.g., paramyxoviruses and orthomyxoviruses)

Double-stranded RNA viruses (e.g., reoviruses)

Retroviruses (e.g., HIV)

+ RNAGenome – RNA

+RNA – RNA

± DNA+ RNA – DNA+RNA

– RNA

± RNA

± RNA

Virus Release

• Many viruses bud out from infected cells


Cytoplasm

Host cell membrane

(a)

Buddingvirion

Free infectiousvirion with envelope

Viralmatrixprotein

Capsid

RNA

Viral glycoprotein spikes

Viral nucleocapsid

Viroids ( affects plants)

• Plant pathogens

• Naked RNA

• Single-stranded

• Circles

• Very small (100 to 300 nucleotides)

Prions (affects animals)

• Animal pathogens

• “Slow viruses”

• Spongiform encephalopathy

• Kuru

• Mad Cow Disease

• Creutzfeld-Jakob Disease

• Defectively folded host protein

Concept Check

How are the infectious cycles of viruses different in bacterial and eukaryotic host cells?

A. The virus genome does not enter a eukaryote

B. The virus genome does not enter a bacterium

C. The capsid does not enter a eukaryote

D. The capsid does not enter a bacterium