Transcript virus - DrMinkovskyScienceWiki

An Introduction
to the Viruses
Chapter 6
Copyright © The McGraw-Hill Companies, Inc) Permission required for reproduction or display.
Learning Objectives:
• Differentiate a virus from a bacterium.
• Define viral species. Give an example of a family, genus, and
a common name for a virus.
• Describe the structure of an enveloped and a nonenveloped
virus.
• Describe viral infectious cycle including lytic and lysogenic
cycles.
• Describe how bacteriophages and animal viruses are
cultured
• Describe the relationship between viruses and cancer.
• Provide an example of a latent viral infection
• Discuss how a protein can be infectious
Why are viruses important?
• Health—HIV is a leading cause of death in Africa
• Economics—Banana Bunchy Top Virus (BBTV)
damages banana crops in Asia and the Pacific
annually costing millions
• Important participant in the evolution of life forms35-90% of human genome came from viruses!
• Gene Therapy—Viruses are efficient at
introducing foreign DNA into a cell
The Scale of Viruses
• Too small for the light microscope
• Range from about 20 to 200 nm (0.02-0.2 µm)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
BACTERIA CELLS
Rickettsia
0.3 µm
Viruses
450nm
1. Mimivirus
2. Herpes simplex 150 nm
3. Rabies
125 nm
4. HIV
110 nm
100 nm
5. Influenza
6. Adenovirus
75 nm
7. T2 bacteriophage 65 nm
8. Poliomyelitis
30 nm
9. yellow fever
22 nm
Protein Molecule
15 nm
10. Hemoglobin
molecule
Streptococcus
1 µm
(1)
(2)
E. coli
2 µm long
(10)
(9)
(8)
(7)
(3)
(6)
(4)
(5)
Viral Classification
• Type of nucleic acid (DNA, RNA)
• Strandedness (single vs double)
• Capsid morphology
• Presence or absence of envelope
• Host range
Virus taxonomy
• International Committee on the Taxonomy of
Viruses (ICTV): 3 orders, 73 families, 287 genera
• Order names end in virales
• Family names end in viridae
• Genus names end in virus
• As an example, consider the virus from the family
Herpesviridae belonging to the genus
Simplexvirus, human herpesvirus 2
Viral Components
7
Generalized Structure of Viruses
8
Helical Capsid
• Rod-shaped
capsomeres
assembled into a
series of hollow disks
Naked: Tobacco mosaic
virus (rigid)
• Enveloped : Influenza
virus (more flexible)
9
Icosahedral Viruses
• Ring- or domeshaped capsomeres
• Spherical or cubical
capsid with nucleic
acid packed inside
10
Complex Virus
Structure
• Found in bacteriophages
11
Viral Envelope
• Found in some,
but not all viruses
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Derived from
the membranes of the
host cell
• Formed by budding
• Viral proteins (spikes)
• Functions: protection,
host penetration,
generation of immune
response
Envelope
Nucleocapsid
(c)
Viral Enzymes
• Found In nucleocapsid
• Polymerases
• Reverse transcriptase
• Found in envelope
• Hemagglutinin
• Neuraminidase
Viral Genome
• DNA or RNA, but not both
• Single or double stranded
• RNA: sense or anti-sense, segmented or nonsegmented
• RNA virus with encoded reverse transcriptase
(retrovirus)
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
Virus
Infectious
Cycle
16
Virus
Adsorption
• Receptors are
glycoproteins
necessary for cell’s
normal function
• Enveloped viruses
use envelope spikes
• Naked viruses use
capsid proteins
• Host range
Penetration/Uncoating
(a) Endocytosis
(b) Fusion with cell membrane
Replication of
DNA Viruses
Release
Lytic Bacteriophage
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Head
• Virus binds
• DNA injected
• Capsid stays outside
• Expression
• Assembly
• Cell destroyed
Bacterial
cell wall
Tube
Viral nucleic acid
Cytoplasm
Lytic and Lysogenic Cycles
E. coli host
7
Release
Bacteriophage
Lysogenic State
BacterialViral
DNA
DNA
1
Viral DNA becomes
latent as prophage.
2
Adsorption
Penetration
Lysis
6
Lytic
Cycle
DNA
splits
Viral
DNA
Spliced
viral
genome
3
5
Replication
Maturation
Bacterial
DNA molecule
The lysogenic
state in
bacteria.
Capsid
Tail
4
+
DNA
Tail fibers
Assembly
Sheath
Infection followed by integration
into the genome (prophage)
Lysogenic conversion
Bacteriophage
Growing Bactriophages in the Lab
• Bacteria cultures
• Plaque assays (used
to visualize the
presence of virus)
Growing Animal
Viruses in the
Lab
• Living animals
• Embrionated eggs
(pocks, embrio death)
• Cell cultures
(cytopathic effects)
Oncogenic Viruses
• Oncogene: cancer
causing alteration of
cellular DNA
• Transformation
• HPV-16: cervical cancer
• Epstein-Barr virus:
Burkitt’s lymphoma
• HBV: liver cancer
• HTLV-1 and 2: T-cell
leukemia
Latent Viral Infections
• Latency: the ability
of a pathogenic virus
to lie dormant (latent)
within a cell .
• Herpes simplex virus
• Chicken pox virus
Viroids
• Plant pathogens
• Naked RNA
• Single-stranded
• Circles
• Very small (100 to 300 nucleotides)
Prions
• Animal pathogens
• “Slow viruses”
• Spongiform encephalopathy
• Kuru
• Mad Cow Disease
• Creutzfeld-Jakob
Disease
• Defectively folded host protein