Download 6/30/14 1 The only goal a virus has is to… General characteristics of

6/30/14
Viruses, Viroids and Prions
The only goal a virus has is to…
HOW??
All viruses use this general strategy: INVADE and HIJACK a host
cell, FORCE IT to make virus copies, which then ESCAPE
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encyclopedia2.thefreedictionary.com
www.infoniac.com
General characteristics of viruses
Video - a virus hijacks a bacterial cell
General characteristics of viruses (cont.)
§  Are inert when not within a host cell
§  Require a living host to multiply
§  obligate intracellular parasites
§  Multiply within living cells by using the host cell’s
enzymes, nucleic acids, amino acids etc
§  Contain DNA or RNA (not both)
§  Have a protein coat surrounding the nucleic acid
§  Host Range = the range of host cells a virus can infect
§  Determined by specific host attachment sites (e.g. cell walls,
flagella) and cellular factors necessary for viral multiplication
§  Most viruses only infect the cells of one host species
§  bacterial viruses = bacteriophages (phages)
§  Some are enclosed by an outer lipid envelope
§  No ATP generating mechanism
§  Display little or no metabolic activity
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Viruses - relative size
Viral size = 20 - 1000nm long (0.02 - 1um)
Viral structure
§  Virion: a single mature, complete, infectious virus particle
§  Nucleic acid:
§  DNA or RNA
§  Double-stranded (ds) or single-stranded (ss):
§  dsDNA
§  ssDNA
§  dsRNA
§  ssRNA
§  Linear or circular
§  some viruses (e.g. influenza virus) - in several segments
Viral structure (cont.)
§  Capsid: the protein coat surrounding the nucleic acid.
Viral structure (cont.)
VIRAL STRUCTURE
§  Composed of subunits - capsomeres
§  Capsomere proteins can be the same or several different
proteins
§  Envelope: Lipid layer (also has proteins and carbohydrates)
§  External to the capsid
§  May be covered with glycoprotein “spikes” (= antigens)
projecting from the envelope’s surface
§  can serve as attachment points for binding to host cells
----> Enveloped vs non-enveloped viruses
A non-enveloped virus
An enveloped virus
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Viral structure (cont.)
Viral structure (cont.)
§  General morphology (shape): viruses may be classified on
§  General morphology (cont.)
the basis of their capsid structure:
Helical viruses
Enveloped viruses
Complex viruses
Polyhedral (many-sided) viruses
Ebola
by SEM
§  e.g. bacteriophages
http://www.gettyimages.co.uk
A example of a complex virus
T-even bacteriophages
§  A T-even
(e.g. T2, T4, T6)
bacteriophage…
.. One of the weirder
looking viruses!
www.mansfield.ohio-state.edu
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Viral infection and host immune response
Viral Taxonomy
§  Infection with a virus:
§  Viruses - grouped into families, based on:
§  stimulates host to produce antibodies
§  Type of nucleic acid
§  Antibodies recognize & bind to viral surface proteins (antigens)
§  Replication strategy
§  Some viral proteins can mutate frequently
§  different viral strains can therefore infect the same individual
more than once
§  Morphology
§  Family names end in -viridae.
§  Genus names end in -virus.
§  Viral species: A group of viruses sharing the same genetic
§  e.g. influenza virus
information and ecological niche (host).
§  Descriptive common names are use; no specific epithet.
§  Subspecies are designated by a number.
Viral Taxonomy
Growing Bacteriophages in the lab
§  An example of viral taxonomy:
§  Family: Herpesviridae
§  Viruses must be
grown in living host
§  Genus: Simplexvirus
§  Common name: Human herpes virus (HHV)
§  Subspecies: HHV-1, HHV-2, HHV-3
cells.
§  Bacteriophages
form plaques on a
lawn of bacteria.
http://www.sciencedaily.com/releases/2005/07/050725065240.htm
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Viral multiplication
Viral multiplication
§  The virion genome only contains a few of the genes (e.g.
capsid protein genes) needed for new virus synthesis.
§  HIJACK!!…
§  Following infection, viruses take over the host cell’s metabolic
machinery:
HOW are new viruses made?
§  Uses the hosts' ribosomes, tRNA, amino acids, ATP and
(NOT by binary fission, budding or mitosis)
some enzymes.
§  The basic mechanism of viral multiplication is similar in all
viruses
www.mansfield.ohio-state.edu
Viral multiplication in bacteriophages
Viral multiplication in bacteriophages
§  Bacteriophages (phages) - viruses that infect bacteria
§  To make a new bacteriophage, what do we need (i.e. what
§  Use two mechanisms:
components need to be made) ?
§  Lytic cycle (host lysed and killed)
§  Lysogenic cycle (host remains alive)
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Viral multiplication:
the lytic cycle in bacteriophages
BACTERIOPHAGES (Bacterial Viruses):
dsDNA genome
§  5 stages for T-even phages:
§  Attachment:
BACTERIOPHAGES GENOME §  Phage attaches by tail fibers to host cell.
dsDNA
dsDNA
§  Phage lysozyme opens cell wall, tail sheath contracts to force tail
Progeny
virions
an
Tr
core and DNA into cell.
TranslaFon sc
rip
Fo
n CAPSID §  Penetration:
ReplicaFon mRNA
§  Biosynthesis:
Capsid
proteins
§  Production of phage DNA and proteins.
§  Maturation:
§  Assembly of phage particles.
§  Release:
§  Phage lysozyme breaks cell wall; host cell dies.
Figure modified from Koonin and Dolja (2014) Microbiol. Mol. Biol. Rev. 78:278-­‐303 Viral multiplication:
the lytic cycle in bacteriophages
Viral multiplication:
the lytic cycle in bacteriophages
§  Burst time: the time from
phage attachment to
1
release; averages 20 - 40
4
min.
2
§  Burst Size: The number of
newly synthesized virions
released from a single cell
3
(usually 50 - 200).
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Viral multiplication:
the lysogenic cycle in bacteriophages
The Lysogenic Cycle
§  Upon attachment of phage and penetration of DNA - two
things can happen
§  lytic cycle OR lysogenic cycle
§  Lysogenic phages can proceed through a lytic cycle, but…
…can also insert their DNA into the hosts’ DNA to
begin a lysogenic cycle
§  Phage remains inactive (latent)
§  Bacterial host cell termed a lysogenic cell
§  remains alive during lysogeny.
Consequences of lysogeny
The Lysogenic Cycle
1.  Lysogenic cells immune to infection by the same phage
2.  Phage conversion can occur - i.e. host cells may exhibit
new properties:
§ 
e.g. production of phage-encoded toxin by the host cell
3.  Specialized transduction can occur:
§ 
when prophage is excised from the host chromosome flanking genes may remain attached to the phage DNA…
Prophage DNA
incorporated into
host DNA.
Fig. 13
Repressor proteins
Keep phage "dormant"
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Multiplication of animal viruses
§  Follows the basic pattern of phage multiplication, but with
Enough about bacteriophages…
several differences.
§  Attachment: Viruses attach to cell membrane.
What about the multiplication of
ANIMAL viruses?
§  Penetration (entry): by endocytosis or fusion.
§  Uncoating: by viral or host enzymes.
§  Biosynthesis: Production of nucleic acid and proteins.
§  Maturation: Nucleic acid and capsid proteins assemble.
§  Release: by budding (enveloped viruses) or rupture.
Multiplication of animal viruses
Multiplication of animal viruses
§  Follows the basic pattern of phage multiplication, but with
§  Follows the basic pattern of phage multiplication, but has
several differences.
several differences.
§  Attachment: Viruses attach to cell membrane.
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Multiplication of animal viruses
Multiplication of animal viruses
§  Follows the basic pattern of phage multiplication, but has
§  Follows the basic pattern of phage multiplication, but has
several differences.
several differences.
§  Attachment: Viruses attach to cell membrane.
§  Attachment: Viruses attach to cell membrane.
§  Penetration (entry): by endocytosis *** or fusion.
§  Penetration (entry): by endocytosis or fusion.
*** NOTE - some editions of the textbook say "pinocytosis" - this is incorrect
Multiplication of animal viruses
Multiplication of animal viruses
§  Follows the basic pattern of phage multiplication, but has
§  Follows the basic pattern of phage multiplication, but has
several differences.
several differences.
§  Attachment: Viruses attach to cell membrane.
§  Attachment: Viruses attach to cell membrane.
§  Penetration (entry): by endocytosis or fusion.
§  Penetration (entry): by endocytosis or fusion.
§  Uncoating: by viral or host enzymes.
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Multiplication of animal viruses
§ 
Follows the basic pattern of phage multiplication, but has
several differences.
Multiplication of animal viruses
§ 
Follows the basic pattern of phage multiplication, but has
several differences.
§ 
Attachment: Viruses attach to cell membrane.
§ 
Attachment: Viruses attach to cell membrane.
§ 
Penetration (entry): by endocytosis or fusion.
§ 
Penetration (entry): by endocytosis or fusion.
§ 
Uncoating: by viral or host enzymes.
§ 
Uncoating: by viral or host enzymes.
§ 
Biosynthesis: Production of viral:
§ 
Biosynthesis: Production of viral:
1) Nucleic acid
1) Nucleic acid
2) Proteins
2) Proteins by viral or host enzymes.
§ 
Multiplication of animal viruses
§  Follows the basic pattern of phage multiplication, but has
several differences.
Maturation: virions assembled
§  Viral Nucleic acid:
§  dsDNA
§  ssDNA
§  Attachment: Viruses attach to cell membrane.
§  dsRNA
§  Penetration (entry): by endocytosis or fusion.
§  ssRNA
DNA viruses
RNA viruses
§  Uncoating: by viral or host enzymes.
§  Biosynthesis: Production of viral:
1) Nucleic acid
2) Proteins
§  Therefore… each virus type needs to have a slightly
different mechanism for multiplication and biosynthesis.
§  Maturation: virions assembled
§  Release: by budding (enveloped viruses)
or rupture (non-enveloped viruses).
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DNA-containing animal viruses
Adenovirus
Herpes Simplex virus
DNA
Transcription
+ strand RNA (sense strand)
mRNA
RNA-dependant
RNA polymerase
Translation
§  For DNA-containing viruses:
Protein
- strand RNA
(antisense strand)
§  DNA generally copied in the nucleus (using viral enzymes)
§  Capsids/other proteins synthesized in cytoplasm (using host
cell enzymes)
Multiplication of a DNA virus - in more detail
Multiplication of a DNA virus - in more detail
Once viral DNA enters the host nucleus, either:
Once viral DNA enters the host nucleus, either:
1. Recombination of viral and
host DNA - provirus formation
§ 
Provirus remain in host DNA
§ 
permanently (not excised)
Replicated with host DNA
§ 
§ 
Mutagens can induce
expression of a previously latent
provirus
Provirus can convert host cell to
a tumour cell
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Multiplication of a DNA virus - in more detail
ANIMAL DNA VIRUSES:
dsDNA genome
Once viral DNA enters the host nucleus, either:
ANIMAL dsDNA VIRUSES 1. Recombination of viral and
host DNA - provirus formation
§ 
Provirus remain in host DNA
2. Viral DNA expressed,
producing new virions.
§ 
GENOME dsDNA
Next slide…
TranslaFon sc
rip
n CAPSID Fo
Replicated with host DNA
Mutagens can induce
Progeny
virions
an
Tr
permanently (not excised)
§ 
§ 
ReplicaFon dsDNA
mRNA
Capsid
proteins
expression of a previously latent
provirus
§ 
Provirus can convert host cell to
a tumour cell
Multiplication of a DNA virus - in more detail
Figure modified from Koonin and Dolja (2014) Microbiol. Mol. Biol. Rev. 78:278-­‐303 RNA-containing animal viruses
Rubella virus
Vesicular stomatitis virus
Mouse mammary
tumor virus
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Multiplication of RNA viruses - in more detail
§ 
Essentially the same as for DNA viruses, with some
DNA
differences due to having an RNA instead of a DNA genome
§ 
§ 
Occurs in the cytoplasm
Transcription
Examples:
+ strand RNA (sense strand)
§ 
+ strand (sense strand) RNA animal viruses.
§ 
- strand (antisense strand) RNA animal viruses.
mRNA
RNA-dependant
RNA polymerase
Translation
Protein
- strand RNA
(antisense strand)
ANIMAL RNA VIRUSES:
ssRNA genome
+ sense strand RNA virus - multiplication
Attachment
ANIMAL -­‐ RNA VIRUS RdRp GENOME RdRp - RNA
+ RNA
Tra
CAPSID Capsid
- RNA
nsla
Fon
Cytoplasm
Host cell
Capsid
proteins
Entry
and uncoating
Maturation
and release
ANIMAL + RNA VIRUS RdRp = RNA dependent RNA polymerase
Translation and synthesis
of viral proteins
RNA replication by viral RNAdependent RNA polymerase
GENOME + RNA
RdRp - RNA
Figure modified from Koonin and Dolja (2014) Microbiol. Mol. Biol. Rev. 78:278-­‐303 Viral
genome
(RNA)
Viral
protein
+ RNA
TranslaFon CAPSID Uncoating releases
viral RNA and proteins.
– strand is transcribed
from + viral genome.
Capsid
protein
RdRp Nucleus
RNA
Progeny
virions
Progeny
virions
+ strand
mRNA is transcribed
from the – strand.
(a) ssRNA; +
or sense strand;
Picornaviridae
Capsid
proteins
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- antisense strand RNA virus - multiplication
Attachment
Multiplication of a Retrovirus (Retroviridae)
§ 
Capsid
Nucleus
RNA
§ 
Many retroviruses infect vertebrates
Includes lentivirus
Cytoplasm
§ 
Host cell
HIV-1, HIV-2 - cause AIDS
Entry
and uncoating
Maturation
and release
Translation and synthesis
of viral proteins
RNA replication by viral RNAdependent RNA polymerase
The + strand (mRNA) must first
be transcribed from the – viral
genome before proteins can
be synthesized.
Viral
genome
(RNA)
Uncoating releases
viral RNA and proteins.
Viral
protein
Capsid
protein
– strands are
incorporated
into capsid
Additional – strands are
transcribed from mRNA.
(b) ssRNA; – or
antisense strand;
Rhabdoviridae
Multiplication of a Retrovirus (Retroviridae)
§ 
§ 
Many retroviruses infect vertebrates
§ 
Includes lentivirus
§ 
§ 
Multiplication of a Retrovirus (Retroviridae)
Attachment, penetration and uncoating of retroviruses
basically like other animal viruses…
HIV-1, HIV-2 - cause AIDS
BUT… biosynthesis stage different
Retroviruses contain the gene for the enzyme Reverse
Transcriptase:
§ 
§ 
Next slide…
An RNA-dependent DNA polymerase
§  Makes DNA from an RNA template
§ 
"Retrovirus" … from reverse transcriptase
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Multiplication of a Retrovirus
(Retroviridae)
ANIMAL RNA VIRUSES:
ssRNA genome (continued)
Retrovirus: has 2
+ (sense) strands
of RNA
ANIMAL RETROVIRUSES TranscripFon Reverse GENOME + RNA
transcriptase dsDNA
+ RNA
Tra
nsla
Progeny
virions
Fon
CAPSID Capsid
proteins
Figure modified from Koonin and Dolja (2014) Microbiol. Mol. Biol. Rev. 78:278-­‐303 Viruses and Cancer
§  Some human cancers (approx. 10 - 15%) caused by viruses.
§  Examples:
§  Human papilloma virus - cervical cancer
§  Hepatitis B virus and hepatitis C virus - liver cancer
§  Recent study (Tang et al., 2013)
- breast cancer and glioblastoma
NOT associated with viruses
Viruses and Cancer
§  Cancer = uncontrolled growth of cells
§  Tumor cells undergo transformation, and acquire new
properties:
§  Dedifferentiated
§  Increased growth rate
§  Loss of contact inhibition
§  Transplantable
§  Invasive
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Viruses and Cancer
Two main ways viruses can cause cancer:
1) Formation of a provirus = a permanent genetic change in a cell…
… therefore, DNA viruses and retroviruses can cause cancer
2) Oncogenic viruses:
Oncogene = the mutated form of a normal, cellular gene involved
in cell growth
§  Mutated oncogenes transform normal cells into cancerous cells.
§  Can be transferred by DNA oncogenic viruses
§  The genetic material of oncogenic viruses becomes integrated
into the host cell's DNA.
Latent and Persistent viral infections
Latent and Persistent viral infections
§  Latent viral infections
§  Virus remains in asymptomatic host cell for long periods.
§  Suddenly becomes active
§  Cold sores (herpes simplex virus)
§  Shingles (chickenpox virus)
§  Trigger for disease manifestation is usually a stressor
§  Persistent viral infections
§  Disease processes occurs over a long period
§  Detectable infectious virus gradually builds up over a long period
§  Generally fatal
§  Subacute sclerosing panencephalitis (measles virus)
§  AIDS (HIV virus)
Latent and Persistent viral infections
§  Latent viral infections
§  Virus remains in asymptomatic host cell for long periods.
§  Suddenly becomes active
§  Cold sores (herpes simplex virus)
§  Shingles (chickenpox virus)
§  Trigger for disease manifestation usually a stressor
§  Persistent viral infections
§  Disease processes occurs over a long period
§  Detectable infectious virus gradually builds up over a long period
§  Generally fatal
§  Subacute sclerosing panencephalitis (measles virus)
§  AIDS (HIV virus)
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Prions (= proteinaceous infectious particle)
Prions (= proteinaceous infectious particle)
§  Infectious proteins
§  Infectious proteins
§  Inherited and transmissible by ingestion, transplant, and
surgical instruments
Prions (= proteinaceous infectious particle)
§  Infectious proteins
§  Inherited and transmissible by ingestion, transplant, and
surgical instruments
Prions (= proteinaceous infectious particle)
§  Kuru:
§  Disease seen in the Fore tribe in Papua New Guinea
§  Unable to walk, loss of the ability to swallow or chew. Drastic
weight loss, leading to death.
§  Cause spongiform encephalopathies (vacuoles in the brain)
in humans and other animals
§  How transmitted? the Fore
funeral rituals involved
§  Animals - bovine spongiform encephalopathy (mad cow disease),
women and children
sheep scrapie, cervid chronic wasting disease
cooking and eating their
§  Humans - Creutzfeldt-Jakob disease (CJD), vCJD, Gerstmann-
dead relatives.
Sträussler-Scheinker syndrome, fatal familial insomnia, kuru.
www.mastersinhealthcare.net
http://learn.genetics.utah.edu
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Spongiform encephalopathy brains
Prions (= proteinaceous infectious particle)
§  Mechanism of disease:
§  PrPC (normal) is a cell surface protein…
A cheetah with Spongiform Encephalopathy
http://learn.genetics.utah.edu
Prions (= proteinaceous infectious particle)
Prions (= proteinaceous infectious particle)
§  Mechanism of disease:
§  Mechanism of disease:
§ 
PrPC
(normal) is a cell surface protein…
§  PrPC (normal) is a cell surface protein…
Conversion of a normal host glycoprotein
Conversion of a normal host glycoprotein
(PrPC; normal cellular prion protein)
(PrPC; normal cellular prion protein)
Infectious form
Infectious form
(PrPSc; scrapie protein)
(PrPSc; scrapie protein)
§  Abnormal PrPSc accumulates in brain cells - forms plaques.
§  Cell death occurs.
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Plant viruses and Viroids
Plant viruses and Viroids
§  Plant viruses:
§  Viroids: Naked RNA
§  Resemble animal viruses in many ways
molecules (300 - 400
§  Cause many diseases of economically important crops -
nucleotides); no protein coat
§  Fold back on themselves
beanscorn, sugarcane, potatoes
§  Because of tough cell wall, enter through wounds or via
§  Plant pathogens only
§  Infectious RNA; e.g., potato
parasites (nematode fungus, insect)
spindle tuber viroid.
§  Infected plants can spread virus in pollen/seeds.
§  May have evolved from introns
New HUGE viruses!
www.stanford.edu
§  New, very large viruses that infect amoeba
have been discovered in the last few years:
§  Mimivirus
§  Pandoravirus
§  Pandoravirus-like (1.5 uM long!!)
www.livescience.com
www.nytimes.com
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