Chapter 13: Viruses, Viroids, and Prions - Dr. Jill O'Malley

Chapter 13: Viruses, Viroids, and Prions
VIRUSES:
• are obligate intracellular parasites
o must live in host cells to multiply (also true of bacterial rickettsias)
o multiply inside of cells by using the cell’s own machinery
• Contain DNA or RNA
• Contain a protein coat that surrounds nucleic acid
• Some are enclosed by an envelope
• Some viruses have spikes
• Most viruses infect only specific types of cells in one host
• Host range is determined by specific host attachment sites and cellular factors
o must attach to specific receptor sites on surface of cell
can be cell membrane, cell wall, flagellum, etc.
phage therapy: use of bacteriophages to kill bacteria
some have tumor destroying or oncolytic properties
• Viral size
o must be viewed with an electron microscope
o range in size: 20 – 1000nm in length
BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

Viral Structure:
• virion = complete, fully developed, infectious viral particle
o has nucleic acid surrounded by protein coat
• nucleic acid:
o have DNA or RNA, not both
can be double or single stranded
o can be linear or circular
• capsid and envelope
o capsid = protein coat
composed of protein subunits: capsomeres
o capsid may be covered by an envelope
combination of lipids, proteins and carbohydrates
• spikes
o carbohydrate-protein complexes
o can mutate easily (ex. influenza virus)
GENERAL MORPHOLOGY:
A. Helical Viruses
• resemble long rods; may be rigid or flexible
B. Polyhedral Viruses
• polyhedral = many sided
o icosahedron: 20 triangular faces and 12 corners
BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

C. Enveloped Viruses
• roughly spherical
D. Complex Viruses
• ex. bacteriophages
Viral Taxonomy
• Family names end in -viridae.
• Genus names end in -virus.
• Order names end in -ales
• Viral species: A group of viruses sharing the same genetic information and
ecological niche (host).
o Common names are used for species.
• Subspecies are designated by a number (ex. HIV-1)
• Viral Taxonomy ex:
BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

o Herpesviridae
Herpesvirus
• Human herpes virus (HHV) 1, HHV 2, HHV 3
o Retroviridae
Lentivirus
• Human immunodeficiency virus (HIV) 1, HIV 2
Growing Viruses
• Viruses must be grown in living cells.
o Bacteriophages form plaques on a lawn of bacteria
PFU: plaque forming units
• Animal viruses may be grown in
o living animals
o embryonated eggs
o cell cultures
• monolayer
• cytopathic
effect (CPE)
• Virus is injected in
membrane most appropriate
for its growth
• viral growth: death of
embryo, cell damage or by
lesion formation on egg
membranes
• method used for culturing
vaccinations
BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

• viruses can be grown in primary or continuous cell lines
o primary cell lines: derived from tissue slices….die out after a few
generations
o diploid cell lines: from human embryos; maintained for about 100
generations
o continuous cell lines: are transformed (cancerous) cells
HeLa
Virus Identification
• Cytopathic effects
• Serological tests
o Detect antibodies against viruses in a patient.
o Use antibodies to identify viruses in neutralization tests, viral
hemagglutination, and Western blot.
• Nucleic acids
o RFLPs
o PCR
• 1 virus may give rise to several or
thousands of new viruses
o cell may die or produce viruses indefinitely
“Life Cycle”
A. Lytic cycle: Phage causes lysis and death of host cell.
B. Lysogenic cycle: Prophage DNA incorporated in host DNA.
A. Multiplication of Bacteriophages (Lytic Cycle)
1. Attachment: Phage attaches by tail fibers to host cell
2. Penetration: Phage lysozyme (enzyme) opens cell wall, tail sheath contracts to
force tail core and DNA into cell.
• capsid remains outside of cell wall…..action similar to hypodermic needle
3. Biosynthesis: Production of phage DNA and proteins.
• eclipse period: period during which viral multiplication is complete buf
virions are not yet present
4. Maturation: Assembly of phage particles into complete virions.
5. Release: Phage lysozyme breaks cell wall.
• This is lysis
BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

B. The Lysogenic Cycle
• lysogenic phages (also called temperate phages) may proceed through
a lytic cycle
• can incorporate their own DNA into host cell DNA
• lysogeny: phage remains inactive or latent
1. Penetration
2. linear DNA forms a circle
3. circle can multiply and be transcribed (3A) or can recombine and
become part of the cell (called a prophage)
Results of Lysogeny:
• lysogenic cells are immune to reinfection by same phage
• phage conversion
o cell exhibits new properties
• specialized transduction is possible
o phage DNA and some bacterial DNA enter
same capsid
BI 230 Chapter 13: Viruses, Viroids and Prions
O’Malley

Multiplication of Animal viruses
1. Attachment: Viruses attach to cell membrane
o animal viruses do not possess appendages
o receptors and presence of receptor vary from host to host
2. Penetration by endocytosis (pinocytosis) or fusion
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3. Uncoating by viral or host enzymes.
• is the separation of viral nucleic acid from its protein coat
4. Biosynthesis: Production of nucleic acid and proteins.
5. Maturation: Nucleic acid and capsid proteins assemble.
6. Release by budding (enveloped viruses) or rupture.
Chapter 13: Viruses, Viroids and Prions O’Malley

Pathways of Multiplication for RNA-Containing Viruses
BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

Multiplication of a Retrovirus
DNA and RNA Viruses Compared
BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

Cancer
• Activated oncogenes transform normal cells into cancerous cells.
• Transformed cells have increased growth, loss of contact inhibition, tumor specific
transplant and T antigens.
• The genetic material of oncogenic viruses becomes integrated into the host cell's
DNA.
Oncogenic Viruses
• Oncogenic DNA viruses
o Adenoviridae
o Herpesviridae
causes cervical cancer (HPV-16 accounts for nearly 1/2 of all
cervical cancers)
o Poxviridae
o Papovaviridae
o Hepadnaviridae
• Oncogenic RNA viruses
o Retroviridae
Viral RNA is transcribed to DNA which can integrate into host DNA
HTLV 1
HTLV 2
Latent viral infections
o Virus remains in asymptomatic host cell for long periods.
Cold sores, shingles
Persistent viral infections
o Disease processes occurs over a long period; generally is fatal.
Subacute sclerosing panencephalitis (measles virus)
BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

PRIONS
• proteinascous infections particle
• Infectious proteins
• Inherited and transmissible by ingestion, transplant, and surgical instruments
• Spongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease,
Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow
disease
• PrP C : Normal cellular prion protein, on cell surface
o gene is located on chromosome 20 in humans
• PrP Sc : Scrapie protein; accumulates in brain cells forming plaques
• Plant viruses: Enter through wounds or via insects.
• Viroids: Infectious RNA; e.g., potato spindle tuber disease.
Some Plant Viruses
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Chapter 13: Viruses, Viroids and Prions O’Malley

BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley

BI 230 Chapter 13: Viruses, Viroids and Prions O’Malley