Virology
Viruses
• Cause many infections of humans, animals,
plants, and bacteria
• Cannot carry out any metabolic pathway
• Neither grow nor respond to the environment
• Cannot reproduce independently
• Obligate intracellular parasites
• Are however considered “alive” by most
scientists
• Cannot be seen by light microscopy
Characteristics of Viruses
• Virus – miniscule, acellular, infectious agent
having one or several pieces of either DNA or
RNA
• No cytoplasmic membrane, cytosol, organelles
• Have extracellular and intracellular state
Characteristics of Viruses
• Extracellular state
– Called virion
– Protein coat (capsid) surrounding nucleic acid
– Nucleic acid and capsid also called nucleocapsid
– Some have phospholipid envelope
– Outermost layer (capsid or envelope) provides
protection and recognition sites for host cells
• Intracellular state
– Capsid removed
– Virus exists as nucleic acid
How Viruses Are Distinguished
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•
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•
•
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*Type of genetic material they contain
Kinds of cells they attack
Size of virus
Nature of capsid coat
Shape of virus
Presence or absence of envelope
Genetic Material of Viruses
• May be DNA or RNA; never both
• May be linear and composed of several
segments or single and circular
• Much smaller than genomes of cells
Hosts of Viruses
• Most only infect particular kinds of host’s cells
– Due to affinity of viral surface proteins or
glycoproteins for complementary proteins or
glycoproteins on host cell surface
• Generalists
• Bacteriophage (phage)
HIV
Tobacco mosaic
virus
bacteriophages
Sizes of Viruses
Figure 13.4
Capsid Morphology
• Capsids – protein coats that provide protection for viral
nucleic acid and means of attachment to host’s cells
– capsomeres
Shapes of viruses
Also bullet and thread shapes
Complex Viruses
Figure 13.6a
The Viral Envelope
• Acquired from host cell during viral replication
or release
• Composed of phospholipid bilayer and proteins;
some proteins are virally-coded glycoproteins
(spikes)
– play role in host recognition
Nucleic acid
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•
•
•
Genome
Used to replicate within the host
DNA or RNA
Can be ss, ds, linear, circular, segmented (pieces)
• DNA – ss or ds
• RNA – positive (+) sense, negative (-) sense, ds
Viral classification
• Originally by type of host or host structures
• Biochemical and molecular now
• Don’t use kingdom, phylum…
Viral Replication
• Dependent on host’s organelles and enzymes to produce new
virions
• Cycles vary between bacteriophages and animal viruses
• Replication cycle usually results in death and lysis of host cell
(hence lytic cycle, lytic phages, virulent phages)
• Stages of lytic replication cycle
– Attachment
– Entry/Penetration
– Synthesis
– Assembly/Maturation
– Release
Lytic Replication of Bacteriophages
Lysozyme – phage
tails
Figure 13.8
Lytic Replication of Bacteriophages
Lysozyme
Figure 13.8
Lytic Phage Replication Cycle
Figure 13.9
Lysogeny (Temperate phages)
Figure 13.11
• Lysogenic conversion – prevents entry of another virus into a
bacteria that is already infected by another phage of the same
type
– Can be infected however by virus of different type
– Medical importance - toxins
Bacterium
Phage
Gene Product
Phenotype
Vibrio cholerae
CTX phage
cholerae toxin
cholera
Escherichia coli
lambda phage
shigalike toxin
hemorrhagic
diarrhea
Clostridium botulinum
clostridial
phages
botulinum toxin
botulism (food
poisoning)
Corynebacterium
diphtheriae
corynephage
betadiphtheria
toxin
diphtheria
Streptococcus pyogenes
T12
erythrogenic toxins scarlet fever
Replication of Animal Viruses
• Same basic replication pathway as
bacteriophages
• Differences result from
– Presence of envelope around some viruses
– Eukaryotic nature of animal cells
– Lack of cell wall in animal cells
Attachment of Animal Viruses
• Chemical attraction
• Animal viruses do not have tails or tail fibers
• Have glycoprotein spikes or other attachment
molecules that mediate attachment
Entry and Uncoating of Animal Viruses
Figure 13.12a, b
Synthesis of Animal Viruses
• Each type of animal virus requires different
strategy depending on its nucleic acid
Synthesis in DNA viruses
• Replicate their DNA in the host cell nucleus with
the aid of viral enzymes
• Synthesize new viral parts in cytoplasm with
host enzymes
• Parts then move to nucleus to combine with
new DNA to make complete virions
– Exception Poxviruses assemble in cytoplasm
• ssDNA viruses – must make complement strand
before proceeding to replicate
RNA animal viruses
• (+) sense RNA viruses – (+) strand acts as mRNA and viral
proteins made immediately after penetration and uncoating
– Nucleus not involved
– RNA-dependent RNA polymerase makes (-) strand RNA
(antisense strand) to act as template for more (+) strand RNA
– Assembly in cytoplasm
• (-) sense RNA viruses
– RNA-dependent RNA polymerase uses (-) sense RNA to make
(+) sense RNA which acts as mRNA to make viral parts
– New (-) sense RNA made to package into new virions
– Assembly in cytoplasm
• Retroviruses – two copies of (+) sense RNA don’t act as mRNA
– Reverse transcriptase – turn the RNA into ssDNA, which is
then replicated to dsDNA
– dsDNA moves to nucleus and turns into provirus in host
cell chromosome
– Can remain for indefinite amount of time, passed to
subsequent generations
– Genome cannot be excised like prophages
– Once activated, makes viral mRNA, and full length (+)
sense RNA to package into virion (2 per new virion)
• dsRNA viruses
– (+) strand acts as mRNA template
– Make more (-) strands (RNA-dependent RNA
polymerase)
– Assembly in cytoplasm, (+) and (-) strands come
together to form viral genome, polymerase
packaged with genome
Synthesis of Animal Viruses
Table 13.3
Assembly (maturation) and Release of
Animal Viruses
• Number of viruses produced and released depends
on type of virus and size and initial health of host
cell
• Site of assembly varies by virus type
• Enveloped viruses cause persistent infections
• Naked viruses released by exocytosis or may cause
lysis and death of host cell
Release of Enveloped Viruses by
Budding
Budding may/may not kill cell
Naked virions exocytosed
Figure 13.13
Latency of Animal Viruses
• When animal viruses remain dormant in host
cells
• May be prolonged for years with no viral activity,
signs, or symptoms
• Some latent viruses do not become
incorporated into host chromosome
• When provirus is incorporated into host DNA,
condition is permanent; becomes permanent
physical part of host’s chromosome
– Site of integration can affect host cell
Summary of Bacteriophage and Animal
Virus Replication
Table 13.4
Damage to host cells
• Cytopathic effect
– Virus-induced damage to
cell, alters cell appearance
• Shape, changes
intracellularly (inclusion
bodies), damage to
organelles
• Single cells merge into large
giant cells (syncytia)
The Role of Viruses in Cancer
• Normally, animal’s genes dictate that some cells can
no longer divide and those that can divide are
prevented from unlimited division
• Genes for cell division “turned off” or genes that
inhibit division “turned on”
• Neoplasia – uncontrolled cell division in
multicellular animal; mass of neoplastic cells is
tumor
• Benign vs. malignant tumors
– Metastasis
– Cancers
• Oncogenes – genes that promote neoplasia
– In DNA viruses, part of viral genome needed to
synthesize viral proteins is oncogene
– In RNA viruses, viruses pick up “extra” genes from
normal host cell during replication which are similar
to oncogenes (proto-oncogenes)
How Viruses Cause Cancer
• Some carry copies of oncogenes as part of their
genomes
• Some stimulate oncogenes already present in
host
• Some interfere with tumor repression when
they insert into host’s repressor gene
• Several DNA and RNA viruses are known to
cause ~15% of human cancers
– Burkitt’s lymphoma
– Hodgkin’s disease
– Kaposi’s sarcoma
– Cervical cancer
Viruses and pregnancy
• Teratogenesis – induction of defects during
embryonic development (caused by teratogen)
– Can cause more severe abnormalities if early in
pregnancy
– Can pass disease to unborn fetus
Culturing Viruses in the Laboratory
• In Whole Organisms
– Bacteria
– Plants and Animals
• Embryonated Chicken Eggs
• In Cell (Tissue Culture)
Culturing Viruses in Bacteria
Plaque assay
PFU – plaque
forming unit
Lawn of bacteria
Figure 13.16
Culturing Viruses in Embryonated
Chicken Eggs
Figure 13.17
Culturing Viruses in Cell (Tissue)
Culture
Figure 13.18
Gene therapy
• In vitro
• In vivo
• Single-gene defects
Characteristics of Viroids
• Extremely small, circular pieces of RNA that are
infectious and pathogenic in plants
• Similar to RNA viruses, but lack capsid
• May appear linear due to H bonding
How viroids differ from viruses
• Single circular RNA molecule with low molecular
weight
• Exist inside cells, usually in nucleoli, and
particles of RNA w/o capsids or envelopes
• Does not produce proteins from RNA
• Viroid RNA always processed in host nucleus
• Not apparent in infected tissues without the use
of special techniques to identify nucleotide
sequences in the RNA
Characteristics of Prions
• Proteinaceous infectious agents
• Composed of single protein PrP
• All mammals contain gene that codes for
primary sequence of amino acids in PrP
• Two stable tertiary structures of PrP
– Normal functional structure with α-helices called
cellular PrP
– Disease-causing form with β-sheets called prion PrP
• Prion PrP converts cellular PrP into prion PrP by
inducing conformational change
Characteristics of Prions
• Normally, nearby proteins and polysaccharides
force PrP into cellular shape
• Excess PrP production or mutations in PrP gene
result in initial formation of prion PrP
• When prions present, they cause newly
synthesized cellular PrP to refold into prion PrP
Prion Diseases
• All involve fatal neurological degeneration,
deposition of fibrils in brain, and loss of brain
matter
• Large vacuoles form in brain; characteristic spongy
appearance
• Spongiform encephalopathies – BSE, CJD, kuru
• Only destroyed by incineration; not cooking or
sterilization
Scrapie
Prion-associated diseases
• Scrapie
• Common in several countries around the world, sheep,
goats
• Kuru
• Once common in Papua New Guinea, now rare, humans
• Cruetzfeldt-Jakob disease (CJD)
• Worldwide, humans
• Bovine spongiform enchephalopathy (BSE) or “mad
cow disease”
• Mainly Europe, UK, cattle
• Feline spongiform encephalopathy (FSE)
• UK, domestic cats