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Chapter 13
Viruses, Viroids, and Prions
Part 2
Viral Multiplication
• Viral nucleic acid contains only few genes
for the virion’s structural components
– Capsid proteins
– Genes for a few of the enzymes used in the
viral life cycle (replication or processing viral
nucleic acid)
• Host cells supply enzymes needed for
protein synthesis, ribosomes, tRNA, and
energy production
Viral Multiplication (phages)
• Lytic cycle: Phage causes lysis and death of
host cell (e.g. T-even phages in E. coli)
• Lysogenic cycle: Prophage DNA incorporated
in host DNA (e.g. bacteriophage  in E. coli)
– phage remains latent (inactive) in lysogeny
Multiplication of Bacteriophages
(Lytic Cycle)
• Attachment
• Penetration
• Biosynthesis
• Maturation
• Release
Phage attaches by tail fibers to
host cell
Phage lysozyme opens cell wall,
tail sheath contracts to force tail
core and DNA into cell
Production of phage DNA
and proteins
Assembly of phage particles
Phage lysozyme breaks cell wall
Lysogenic cycle
• Prophage: phage DNA inserted into the
host cell’s DNA
• During lysogenic cycle most of the
prophage genes are repressed (turned off)
by two repressor proteins (phage genes
products)
• Excision of the phage DNA induced by a
rare spontaneous event, or action of UV
light or certain chemicals
lytic cycle
Lysogenic cycle
Lysogenic cycle
• Three important results of lysogeny
– Lysogenic cells are immune to reinfection by
the same phage
– Phage conversion: host cell may exhibit new
properties (e.g. many of bacterial toxins are
encoded by prophage genes)
– Specialized transduction: only certain bacterial
genes (usually the host genes adjacent to the
prophage) can be transferred
Specialized
Transduction
Multiplication of Animal viruses
•
•
•
•
Attachment
Penetration
Uncoating
Biosynthesis
• Maturation
• Release
Viruses attaches to cell membrane
By endocytosis or fusion
By viral or host enzymes
Production of nucleic acid and
proteins
Nucleic acid and capsid proteins
assemble
By budding (enveloped viruses) or
rupture
Uncoating: separation of the viral
nucleic acid from its protein coat.
Biosynthesis of DNA viruses
• Generally, DNA replicates in the nucleus of
the host cell using viral enzymes
• Capsid and other protein are synthesized in
cytoplasm using host cell enzymes
– Proteins migrate into the nucleus
• Virions formed in nucleus
• Virions transported along the endoplasmic
reticulum to the host cell’s membrane for
release
DNA virus
DNA viruses
• Examples
– Adenoviridae: adenoviruses (cause common
cold)
– Poxviridae: poxviruses (cause smallpox, and
cowpox)
– Herpesviridae: cold sores, chicken pox,
infectious mononucleosis
– Papovaviridae: Papillomavirus (cause warts,
some can cause cancer)
– Hepadnaviridae: hepatitis B virus
• Hepadnaviruses synthesize DNA by copying RNA
(use viral reverse transcriptase)
Biosynthesis of RNA viruses
• Multiply in the host cell’s cytoplasm
• Several different mechanisms of mRNA
formation occur among different groups of
RNA viruses
– Single-stranded (ss) RNA virus  Use RNAdependent-RNA polymerase to synthesize a
double-stranded RNA
• RNA is sense strand (+ strand)  acts as mRNA to
synthesize antisense strand (- strand) from sense (+)
strand RNA
Biosynthesis of RNA viruses
• RNA is antisense strand (- strand)  serve as
template for RNA-dependent RNA polymerase to
synthesize + strand (serve as mRNA)
– Double-stranded (ds) RNA virus
– Retrovirus: carries reverse transcriptase (RNAdependent DNA polymerase = RNA  cDNA)
• Many infect vertebrates; some can cause cancer
• Viral DNA synthesized must be integrated into the
DNA of a host cell  becomes provirus (never
comes out of the chromosome)
Retrovirus
RNA Viruses
• Examples
– ss RNA viruses (+ strand): Picornaviridae &
Togaviridae (enveloped)
– ss RNA virus (- strand): Rhabdoviridae
(rabiesvirus)
– ds RNA virus: Reoviridae (+ and – strands
RNA)
• inhabitants of human respiratory and
digestive/enteric systems
– ds RNA virus: Retroviridae (two + stands
RNA)
Maturation and Release
• Assembly of the protein capsid
– Spontaneous process
• Envelope develops around the capsid by
budding
– Budding does not immediately kill the host; in
some cases the host survives
• Nonenveloped viruses released through
ruptures in the host plasma membrane
– Host is killed
Release of an enveloped virus by
budding
Figure 13.20
Viruses and Cancer
• Viral cause of cancer often not recognized
– Infection by virus do not always induce cancer
– Cancer takes long time to develop after the initial
viral infection
– Unlike viral disease, cancers are not contagious
(hard to prove the cause-effect relationship)
• About 10% of cancers known to be caused by
viruses
Viruses and Cancer
• Oncogene: a gene that can bring about
malignant transformation; cancer-inducing
genes
– Carried by viruses, but actually derived from
animal cells
• Oncogenic virus (oncovirus): a virus
capable of inducing tumors in animals
Viruses and 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
– Adenoviridae
– Poxviridiae
– Herpesviridae
• EB virus (infectious mononucleosis, Burkitt’s
lymphoma, & nasopharyngeal carcinoma )
– Papovaviridae
• papillomaviruses (uterine/cervical cancer)
– Hepadnaviridae
• hepatitis B virus (liver cancer)
Oncogenic Viruses
• Oncogenic RNA viruses
– Retroviridae: viral RNA is transcribed to DNA
which can integrate into host DNA
• human T-cell leukemia viruses (HTLV 1 & HTLV 2)
• feline leukemia virus (FeLV)
transmissible
among cats
– Some retroviruses contain oncogenes; some
carry promotors to turn on oncogenes or other
cancer-causing factors
Latent and Persistent Viral
Infections
• Latent Viral Infections
– Virus remains in asymptomatic host cell for
long periods
• Cold sores (herpes virus), shingles (chicken pox
virus)
• Persistent Viral Infections
– Disease processes occurs over a long period,
generally fatal
• Subacute sclerosing panencephalitis (measles virus)
Prions
• Infectious proteins
• Inherited and transmissible by ingestion,
transplant, & surgical instruments
• Spongiform encephalopathies: Sheep scrapie,
Creutzfeldt-Jakob disease, Gerstmann-SträusslerScheinker syndrome, fatal familial insomnia,
mad cow disease
• PrPC, normal cellular prion protein, on cell
surface (PrPSc reacts with PrPC  convert to
PrPSc)
• PrPSc, scrapie protein, accumulate in brain cells
forming plaques (taken in by endocytosis)
Plant Viruses and Viroids
• Plant Viruses
– Resemble animal viruses (morphology & nucleic
acid types)
– Plant viruses enter through wounds or via insects
– Some can multiply inside insect cells
– Protoplasts and insect cell cultures to grow in lab
• Viroids (plant pathogens)
– Viroids are infectious RNA (short pieces of naked
RNA); similar to base sequences of introns
• potato spindle tuber disease
Chapter Review
• General Characteristics of Viruses
– Very small in size (20 – 100 nm range)
– Inert outside living host cells
– Obligatory intracellular parasites (Take over
host’s metabolic machinery to multiply itself)
– Carry either DNA or RNA (single-stranded or
double-stranded); never both
– Nucleic acid enclosed by capsid (protein coat);
some may have envelope outside capsid
General Characteristics
– Hard to treat
• most antiviral drugs would interfere with the
function of the host cell since viral multiplication
done using host cell machinery
– Have host range (spectrum of host cells that
virus can infect) or specific host
• Host range determined by specific host attachment
sites and the availability of cellular factors within
the (potential) host
• Host receptor site for bacteriophage = cell wall, part
of fimbriae or flagella
• Host receptor for anima viruses = plasma membrane
Viral Structures
• Virion: a complete, fully developed,
infectious viral particle composed of nucleic
acid and surrounded by a protein coat
• Viral nucleic acid
– ssDNA, dsDNA, ssRNA, ds RNA
– Linear or circular (can be in several separate
segments)
• Capsid (protein coat)
– Composed of capsomeres; enclose viral nucleic
acid; protect nucleic acid from digestion by
nuclease enzymes in nonenveloped virus
Viral Structures
• Envelope: an outer covering surrounding
the capsid of some viruses
– Some animal virus derive envelope from host
plasma membrane by budding
– Some may be covered by spikes (used for
attachment to host)
– Mutation of viral surface proteins allows
viruses to escape from antibodies (made in
infected host) and reinfect the same host
General Morphology
• Helical viruses: resemble long rods (e.g.
Rabies an Ebola viruses)
• Polyhedral (icosahedron) viruses: manysided (e.g. Adenovirus and poliovirus)
• Helical and polyhedral viruses may be
enveloped
– Enveloped helical virus: influenzavirus
– Enveloped polyherdral virus: herpes simplex
virus
• Complex viruses (e.g. T-even
bacteriophages & poxviruses)
Taxonomy
• ICTV group viruses into families based on:
– Nucleic acid type
– Strategy for replication
– Morphology
• Virus species: a group of viruses sharing the same
genetic information and ecological niche (host
range)
• Order (-ales)>Family (-viridae)>Genus (-virus)>use
descriptive common names for specific epithets
(species)
Isolation, Cultivation, and
Identification of Viruses
• Virus must be grown in living cells
– Bacteriophages  use bacteria cultures in
liquid, or on solid media (plaque formation)
• Plaque: a clearing in a bacterial lawn resulting from
lysis by phages
• Plaque-forming units (pfu): the concentration of
viral suspensions measured by the number of
plaques
– Animal viruses  living animals, embryonated
eggs, or cell cultures
Isolation, Cultivation, and
Identification of Viruses
• Some animal viruses can only be cultured in live animals
• Embryonated eggs can be used to grow viruses for some
vaccine
• Cell cultures more commonly used than embryonated
eggs now; look for cytopathic effect (CPE)
• Viral Identification
– CPE in cell culture; Serological tests (detect
antibodies, viral neutralization tests; viral
hemagglutination, and Western blot); Nucleic acids
(RFLPs; PCR)
Viral Multiplication (Phages)
• Lytic cycle: Phage causes lysis and death of
host cell
– Attachment: phage attaches by tail fibers to host
cell
– Penetration: phage lysozyme opens cell wall, tail
sheath contracts to force tail core and DNA into
cell
– Biosynthesis: production of phage DNA and
proteins
– Maturation: assembly of phage particles
– Release: phage lysozyme breaks cell wall
Viral Multiplication (Phages)
• Lysogenic cycle: Prophage DNA incorporated
in host DNA
– Prophage: phage DNA inserted into the host cell’s
DNA
– Most of the prophage genes are repressed during
lysogenic cycle
– Can enter lytic cycle when phage DNA is excised
from host cell’s DNA (induced spontaneously, by
UV light, or certain chemicals)
Lysogenic cycle
• Three important results of lysogeny
– Host is immune to reinfection by the same phage
– Phage conversion: host may exhibit new
properties
– Specialized transduction: only certain bacterial
genes adjacent to the prophage are transferred to a
new host
Multiplication of Animal Viruses
• Attachment: viruses attaches to cell
membrane
• Penetration: endocytosis or fusion
• Uncoating: separation of the viral nucleic acid
from its protein coat by viral or host enzymes
• Biosynthesis: production of nucleic acid and
proteins
• Maturation: nucleic acid and capsid proteins
assemble
Multiplication of Animal Viruses
• Release: budding (enveloped viruses) or
rupture plasma membrane (nonenveloped
viruses)
• Biosynthesis of DNA viruses: DNA
replicates in the nucleus; capsid & other
proteins synthesized in cytoplasm; virions
formed in nucleus; released from a host
– e.g. Adenoviruses, Papillomavirus, hepatitis B
virus
Multiplication of Animal Viruses
• Biosynthesis of RNA viruses: occurs in the
host cell’s cytoplasm
– ssRNA viruses --> use RNA-dependent-RNA
polymerase to synthesize a double-stranded
RNA
– Retrovirus --> use RNA-dependent DNA
polymerase to synthesize cDNA from RNA -->
a double-stranded DNA that integrates into the
host DNA (provirus)
• Provirus do not jump out of host DNA like prophage
Viruses and Cancer
• About 10% of cancers caused by viruses
• Oncogene: a cancer-inducing gene
– Carried by viruses, but actually derived from
animal cells
• Oncogenic virus (oncovirus): a virus capable
of inducing tumors in animals
• Activation of oncogenes transform normal
cells into cancerous cells
– Transformed cells have increased growth, loss of
contact inhibition, tumor specific transplant
antigen (cell surface) and T antigens (in nucleus)
Oncogenic Viruses
• DNA viruses
– EB virus (Herpesviridae): cause infectious
mononucleosis, Burkitt’s lymphoma, & nasopharyngeal
carcinoma
– Papillomaviruses (Papovaviridae): cause
uterine/cervical cancer
– Hepatitis B virus (Hepadnaviridae): cause liver cancer
• RNA viruses
– Retroviridae  human t-cell leukemia viruses (HTLV 1
& 2); feline leukemia viruses
• Some contain oncogene; some carry promotors to turn on
oncogenes and other cancer-causing factors
Latent & Persistent Viral Infections
• Latent viral infections
– Virus remains in asymptomatic host cell for
long periods
– E.g. Cold sore (herpes virus), shingles (chicken
pox virus)
• Persistent viral infections
– Disease processes occurs over a long period;
usually fatal
– E.g. Subacute sclerosing panencephalitis
(measles virus)
Prions, Plant Viruses, & Viroids
• Prions infectious proteins
– Inherited and transmissible by ingestion,
transplant, & surgical instruments
– e.g. Sheep scrapie, Creutzfeldt-Jakob disease,
mad cow disease
• Plant viruses  morphologically and
nucleic acid types similar to animal viruses
– Enter through wounds or via insects
– Some can multiply inside insect cells
– Cultivate using protoplasts and insect cell
cultures
Viroids
• Viroids (plant pathogen)  infectious RNA
(short pieces of naked RNA)
– Similar to base sequences of introns in
eukaryotic cells
– E.g. Potato spindle tuber disease