Post on 28-Oct-2014
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
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.
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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
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nonsegmented genome
Table 6.3 Medically Relevant RNA Viruses
RNA Viruses
Enveloped
Single-strandedgenome
Segmentedgenome
Orthomyxoviruses
Bunyaviruses
Arenaviruses
Coronaviruses
Filoviruses
Rhabdoviruses
Paramyxoviruses
Single-stranded genomeencodes reversetranscriptase
Retroviruses
Nonenveloped
Double-strandedgenome
Single-strandedgenome
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
(+)
(+)
(+)
(+)
(+)
(+)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Virus Binding and Entry
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(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
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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