6.1 General Properties of Viruses
6.2 Structure of viruses
6.3 The cultivation of viruses
6.4 General Features of Virus Reproduction
6.5 one-step growth curve
6.6 Temperate Bacteriophages,Lysogeny and Lambda
6.7 viral classification
6.8 Overview of Animal Viruses
6.9 plant viruses
6.10 Viroids and Prions
Chapter 6 Viruses
The Viruses
Introduction and general characteristics
? Virus, simple,acellular entities consisting of one or more molecules
of either DNA or RNA enclosed in a coat of protein,They are
reproduced only within living cells and are obligately intracellular
parasites
? The nucleic acid strands can be linear,closed cycle,or able to
assume either shape
? A virus particle containing nucleic acid surrounded by protein and
other macromolecular components is called virion.
? Viruses have a heavy dependence on host-cell structural and
metabolic components.
? Viruses can confer important new properties on their host cell.
(1) their simple,acellular organization
(2) the absence of both DNA and RNA in the same
virion,
(3) their inability to reproduce independently of
cells and carry out cell division as prokaryotes
and eukaryotes do.
Viruses differ from living cells in at least three ways:
6.1 General Properties of Viruses
The extracellular phase,posses few if any enzymes
and can not reproduce independently of living cells.
In the intracellular phase,viruses exist primarily as
replicating nucleic acids that induce host metabolism to
synthesize virion components; eventually complete virus
particles or virions are released,
extracellular and intracellular
Viruses can exist in two phases
Hosts and size
Three main classes - animal viruses,bacterial viruses
(bacteriophages),and plant viruses.
The particular host range of a virus is determined by the
virus's requirements for its specific attachment to the
host cell and the availability within the potential host of
cellular factors required for viral multiplication.
Viruses vary considerably in size,Although most are
quite a bit smaller than bacteria,some of the larger
viruses (such as the smallpox virus) are about the
same size as some very small bacteria (such as the
mycoplasmas,rickettsias,and chlamydias).
Size
Viruses range from 20 to 300 nm in diameter
The comparative sizes of several viruses and bacteria:
Genome in virion
Viral genomes,The genomes of viruses can be composed
of either DNA or RNA,and some use both as their
genomic material at different stages in their life cycle,
However,only one type of nucleic acid is found in the
virion of any particular type of virus,This can be single-
stranded (ss),double-stranded (ds),or in the case of the
hepadnaviruses,partially double-stranded.
? Most viruses are too small to be seen under light microscope.
?All viruses consists of an RNA or DNA core genome
surrounded by a protein coat capsid.
? The combined viral genome and capsid is called the
nucleocapsid.
6.2 Structure of viruses
The nucleic acid of a virus is surrounded by a protein
coat called the capsid
Each capsid is composed of protein subunits called
capsomeres.
In some viruses,the capsid is covered by an envelope,
which usually consists of some combination of lipids,
proteins,and carbohydrates.
Depending on the virus,envelopes may or may not be
covered by spikes,which are carbohydrateprotein
complexes that project from the surface of the envelope.
Viruses may be classified into several
morphological types on the basis of their
capsid architecture as revealed by electron
microscopy and a technique called x-ray
crystallography.
GENERAL MORPHOLOGY
A,Some viruses,such as tobacco mosaic
virus,have a helical symmetry with the
capsid surrounding an RNA genome.
B,Many viruses that infect bacteria,such as
the T-even bacteriophage,have a complex
capsid with DNA contained within the head
structure.
C,Some animal viruses,such as adenovirus,
have isometric symmetry and a DNA
genome.
D,others,such as coronavirus(冠状病毒 ）,
have complex capsids and an envelope with
protruding proteins surrounding an RNA
genome.
Helical viruses resemble
long rods that may be
rigid or flexible,
Surrounding the nucleic
acid,their capsid is a
hollow cylinder with a
helical structure,An
example of a helical virus
that is a rigid rod is the
tobacco mosaic virus
Helical viruses
The capsid of most polyhedral
viruses is in the shape of a
regular polyhedron with 20
triangular faces and 12 corners,
The capsomeres of each face
form an equilateral triangle,
An example of a polyhedral
virus in the shape of an
icosahedron is the adenovirus,
Another icosahedral virus is
the poliovirus,
Polyhedral viruses
The capsid of some viruses is covered by an envelope,
Enveloped viruses are roughly spherical but highly variable
in shape because the envelope is not rigid,When helical or
polyhedral viruses are enclosed by envelopes,they are
called enveloped helical and enveloped polyhedral viruses,
An example of an enveloped helical virus is the influenza
virus.
Enveloped viruses
Some viruses,particularly bacterial
viruses,have very complicated
structures and are called complex
viruses,Examples of complex
viruses are poxviruses,which do not
contain clearly identifiable capsids
but have several coats around the
nucleic acid,Certain bacteriophages
have capsids to which additional
structures are attached.
Complex viruses
A virus can have either DNA or RNA but never both !!
6.3 Cultivating Viruses
? Bacteriophage,bacterial Cultures
? Animal viruses,Tissue or cell cultures
? Plant viruses,tissue or separated cell
cultures,or cultures of protoplasts
Quantification
of viruses
? Plaque assay technique
for quantification of
bacterial viruses.
1,A dilution of a suspension containing the virus material
is mixed in a small amount of melted agar with the
sensitive host bacteria.
3,The host bacteria begin
to grow,and after overnight
incubation form a lawn of
confluent growth.
Quantification of bacterial virus by plaque assay
2,The mixture poured on
the surface of a nutrient
agar plate.
Photograph of a plate showing plaques formed by
bacteriophage on a lawn of sensitive bacteria,
The plaques shown are about 1-2 mm in diameter.
The size of the plaque formed depends on the virus,
the host,and conditions of culture.
Phage
plaques
Attentions
? Young,actively growing bacterial cells
? Plaque appearance often is characteristic of
the phage being cultivated.
? Plaque-forming units(PFU),each plaque
in a layer of bacterial or animal cells is
assumed to have arisen from the
reproduction of a single virus particle,
Efficiency of Plating
? Plaque assay has low efficiency:
– With bacteriophages,>50%
– With animal virues,0.1-1%
? Therefore,to quantify virus,it is accurate to
express the concentration (called titer) of the virus
suspension not as the absolute number of virion
units,but as the number of plaque-forming units,
The number of PFU does not equal the number of
virus particles,but their ratios are proportional.
The cultivation of animal viruses
? For many years researchers have cultivated animal viruses
by inoculating suitable host animals or embryonated eggs-
fertilized chicken eggs incubated about 6 to 8 days after
laying.
? More recently animal viruses have been grown in
tissue(cell) culture on monolayers of animal cells,This
technique is made possible by the development of growth
media for animal cells and by the advent of antibiotics that
can prevent bacterial and fungal contamination.
? Animal viruses can cause microscopic or macroscopic
degenerative changes or abnormalities in host cells and in
tissues called cytopathic effects.
Quantification of animal viruses
Tumor viruses
may not destroy
cells but cause
cells to grow
faster than
uninfected cells,
a phenomenon
called
transformation,
Transformed cells can be recognized as a small
focus of growth.
6.4 Viral Multiplication
For a virus to multiply,it must invade a host cell
and take over the host's metabolic machinery,A
single virus can give rise to several or even
thousands of similar viruses in a single host cell.
This process drastically changes the host cell and
often causes its death.
1,Attachment (adsorption) of the virion to a
susceptible host cell.
2,Penetration (injection) of the virion or its nucleic
acid into the cell.
3,Early steps in replication during which the host
cell biosynthetic machinery is altered as a prelude
to virus nucleic acid synthesis,Virus-specific
enzymes are typically made.
Multiplication of bacteriophages
4,Replication of the virus nucleic acid.
5,Synthesis of proteins used as structural
subunits of the virus coat.
6,Assembly of structural subunits (and
membrane components in enveloped viruses)
and packaging of nucleic acid into new virus
particles.
7,Release of mature virions from the cell.
Attachment of phage
to host cell,
After a chance
collision between
phage particles and
bacteria,an
attachment site on
the virus attaches to
a complementary
receptor site on the
bacterial cell,
Penetration:
During the process of penetration,the bacteriophage's
tail releases an enzyme,phage lysozyme,which breaks
down a portion of the bacterial cell wall,then the
bacteriophage injects its DNA (nucleic acid) into the
bacterium,
Biosynthesis of viral components:
Any RNA transcribed in the cell is mRNA
transcribed from phage DNA for biosynthesis of
phage enzymes and capsid protein,The host cell's
ribosomes,enzymes,and amino acids are used for
translation
Formation of mRNA after infection of cells by viruses of different
types,The chemical sense of the mRNA is considered as plus (+),
The sense of the various virus nucleic acids are indicated as + if the
same as mRNA,as – if opposite,or as +- if double-stranded,
Examples are indicated next to the virus nucleic acid,Although
examples of viruses containing SS DNA of the - sense are known,
none are discussed in the text.
Maturation
and release:
The phage heads and tails are separately
assembled from protein subunits,the
head is packaged with phage DNA,and
the tail is attached
Release:
Lysozyme,whose code is provided by a phage gene,
is synthesized within the cell,This enzyme causes a
breakdown of the bacterial cell wall,and the newly
produced bactedophages are released from the host
cell.
The number of newly synthesized phage particles
released from a single cell usually ranges from about
50 to 200,
1,Attachment (adsorption) of
the virion to a susceptible host
cell.
2,Penetration (injection) of
irion or its nucl ic acid
into the cell.
3,Early steps in r pl cati n
during which the host cel
biosynthetic machinery is altered
as a prelude to virus nucleic acid
synthesis,Virus-specific
enzymes are typically made.
4 Replica io of the virus
n cle c acid.
5,Sy thes s of proteins used
as structural ubunits of the
v rus coat.
6,ssembly st uctural
s bun ts (and membran
ompon nts in enveloped
viruses) and packaging of
nucleic acid nto new virus
particl,
7 l as of matur ion
from the ell
6.5 one-step growth curve
1,Latent period,no
release of virions
2,Rise period or
burst,the host
cells rapidly lyse
and release
infective phages
The one-step growth curve of virus replication
There are genetic controls that regulate when different
regions of phage DNA are transcribed into mRNA during
the multiplication cycle.
? There are early messages that are translated into early
phage proteins,the enzymes used in the synthesis of phage
DNA.
? There are late messages that are translated into late phage
proteins for the synthesis of capsid proteins,
This control mechanism is mediated by RNA polymerase.
eclipse period
Time course of events in phage T4 infection
Latent period
6.6 Lysogeny
Some phages can incorporate their DNA into
the host cell's DNA,The phage remains
latent and does not cause lysis of the host cell,
Such a state is called lysogeny.
Such phages are called lysogenic phages or
temperate phages,The participating
bacterial host cells are known as lysogenic
cells.
Lysogenic cycle of bacteriophage lambda in E.coli
The consequences of
infection by a temperate
bacteriophage,The
alternatives on infection
are integration of the
virus DNA into the host
DNA (lysogenization) or
replication and release
of mature virus (lysis),
The lysogenic cell can
also be induced to
produce mature virus
and lyse,
1,Nature of the host-animal,plant,bacterial,insect,fungal
2,Nucleic acid characteristics-DNA or RNA,single or double
stranded,molecular weight
3,Capsid symmetry-icosahedral,helical
4,Presence of an envelope and ether sensitivity
5,Diameter of the virion or nucleocapsid
6,Number of capsomers in icosahedral viruses
7,Immunologic properties
8,Intracellular location of viral replication
6.7 Some important characteristics for viral
classification
(1) nucleic acid type
(2) nucleic acid strand
(3) presence or absence of an envelope
Recently,the International Committee for Taxonomy
of Viruses has developed a uniform classification
system and divided viruses into 50 families,The
committee places greatest weight on three properties,
Cubic bacteriophage:
Group 1 Genome:DNA,single-stranded,Virion,icosahedral,
cubic symmetry,12 capsomers,Example,X174
Group 2 Genome,DNA,double-stranded,Virion,cubic
symmetry,enveloped,Example,PM-2.
Group 3 Genome,RNA,single-stranded,Virion,icosahedral,
cubic symmetry,32 capsomers.
Group 4 Genome,RNA,double-stranded,Virion,cubic
symmetry,enveloped,Example,06,
6.8 Animal Viruses
Uptake of an enveloped
virion by an animal cell.
(a)The process by which
the viral nucleocapsid is
separated from its
envelope,
(b)Electron micrograph of
adenovirus virions
entering a cell,Each
particle is about 70 nm in
diameter.
Animal
virus
entry
Viruses can have varied effects on cells,Lytic
infection results in the destruction of the host
cell,However,there are several other possible
effects,
In the case of enveloped viruses,release of
virions,which occurs by a kind of budding
process and the host cell may not be lysed,The
cell may remain alive and continue to produce
virus over a long period of time,Such infections
are referred to as persistent infections
Possible effects that animal viruses may have
on cells they effect
Viruses and cancer
A number of animal viruses participate in the
events that change a cell from a normal one
to a cancer or tumor cell, Cancer is a cellular
phenomenon of uncontrolled growth,Most
cells in a mature animal,although alive,do
not divide extensively.
Because cancerous cells in the animal body
have fewer growth requirements,they grow
profusely,leading to the formation of large
masses of cells,called tumors,
The development of cancer is clearly a multistep
process,There seem to be many different causes
of cancer,including mutations arising either
spontaneously or as the result of exposure to
certain chemicals,called carcinogens,or by
physical stimuli,such as ultraviolet radiation or
X-rays,Certain viruses also bring about the
genetic change that results in initiation of tumor
formation.
How does a normal cell become cancerous?
Some human cancers where viruses play a role
Cancer Virus Family Genome in virion
Adult T-cell
leukemia
Human T-cell leukemia
virus(typeⅠ )
Retrovirus RNA
Burkitt’s
lymphoma
Epstein-Barr virus Herpes DNA
Nasopharyngeal
carcinoma
Epstein-Barr virus Herpes DNA
Hepatocellular
carcinoma(liver
cancer)
Hepatitis B virus Hepadna DNA
Skin and
cervical cancers
Papilloma virus Papova DNA
6.8.1 Pox Viruses
The most complex and largest animal viruses
known and have some characteristics that
approach those of primitive cells,
The pox viruses are not able to metabolize
and thus depend on the host for the complete
machinery of protein synthesis.
These viruses are also unique in that they are
DNA viruses that replicate in the cytoplasm,
Smallpox was the first virus to be studied in any
detail and was the first virus for which a vaccine
was developed,
The pox viruses are very large,so large that they
can actually be seen under the light microscope.
Vaccinia virions are taken up into cells via a
phagocytic process from which the cores are
liberated into the cytoplasm.
General Properties of Pox Viruses
6.8.2 Adenoviruses
The genomes of the adenoviruses consist of linear
double-stranded DNA of about 36 kilobase pairs,
Attached in covalent linkage to the 5'-terminus of
the DNA is a protein component essential for
infectivity of the DNA,
The DNA has inverted terminal repeats of 100-1800
base pairs (this varies with the virus strain),
The DNA of the adenoviruses is six to seven times
the size of the DNA of the papovavirus SV40.
Replication of the viral DNA occurs in the nucleus.
After the virus particle has been transported to the
nucleus,the core is released and converted to a
viral DNA-histone complex,
Early transcription is carried out by an RNA
polymerase of the host,and a number of primary
transcripts are made,
The transcripts are spliced,capped,and
polyadenylated,giving sever al different mRNAs.
Replication of adenovirus DNA
(See text for details)
6.8.3 Retroviruses
One important group of animal viruses,those
called the retroviruses,have both an RNA and a
DNA phase of replication,
The retroviruses are RNA viruses,but they
replicate by means of a DNA intermediate using
the enzyme reverse transcriptase,
3,The enzyme reverse transcriptase has become
a major tool in genetic engineering.
2,One retrovirus,the one causing acquired
immunodeficiency syndrome (AIDS),
1,They were the first viruses shown to cause
cancer and have been studied most extensively
for their carcinogenic characteristics,
Importance
Retrovirus structure and
function
(a)Structure of a retrovirus,
(b) Genetic map of a typical
retrovirus genome,
(c)Genetic map of Rous
sarcoma virus,a retrovirus
that causes malignant
tumors in birds,Each end
of the genomic RNA
contains direct repeats(R),
and this RNA also has a 5’-
cap and a 3’-poly-A tail,
See text for more details.
6.8.4 Insect viruses
? Many insect virus infections are accomanied by the
formation of inclusion bodies within the infected cells.
1,cytoplasmic polyhedrosis
2,nuclear polyhedrosis
3,cytoplasmic and nuclear polyhedrosis
? The inclusion bodies are protein in nature and enclose one
or more virions,Polyhedral bodies protect the virions
against heat,low pH,and many chemicals.
? Insect larvae are infected when they feed on leaves
contaminated with inclusion bodies,When in insect gut
contents,inclusion bodies dissolve to liberate the virions.
Insect viruses as biological control agents
? Baculoviruses have received the most attention for at
least three reasons:
1,They attack only invertebrates and have considerable
host specificity.
2,They are encased in protective inclusion bodies.
3,They are well suited for commercial production since
they often reach extremely hegh concentrations in larval
tissue.
? Cotton bollworm,Douglas fir tussock moth,gypsy moth,
alfalfa looper
? Inclusion bodies are sprayed on foliage consumed by the
target insects
6.9 Plant Viruses,
Virion morphology,
1,Helical capsids
2,Icosahedral
?Almost all plant viruses are RNA viruses,either
single stranded or double stranded.
?Taxomomy standard,nucleic acid type,strandedness,
capsid symmetry and size,envelope presence
?Transmission of plant viruses,leaves are
mechanically damaged.
6.10 Viroids and Prions
Viroids are small,circular,single-stranded RNA
molecules that are the smallest known pathogens,
? about 250 to 370 nucleotides long,
? Viroids are found principally in the nucleolus of
infected cells; 200-2000 copies
? They do not act as mRNAs to direct protein
synthesis,How to cause disease symptoms
? Could be replicated by an RNA-dependent
RNA polymerase,a rolling-circle type
mechanism
Prions represent the other extreme from
viroids,They have a distinct extracellular form,
but the extracellular form seems to be entirely
protein,It apparently does not contain any nucleic
acid,or if it does,the molecule is not long enough
to encode the single kind of protein of which the
prion is composed.
Prions
? Proteinaceous infectious particle
? Cause a degenerative disorder of the central
nervous system.
? No nucleic acid has yet been detected in the agent,
it seems to be a 33 to 35 kDa hydrophobic
membrane protein,often called PrP.
? The PrP is bound to the surface of neurons,
presumably an altered PrP is at least partly
responsible for the disease.
? PrPsc,?-helix,?-sheet
PrPc,?-helix,?-sheet
6.2 Structure of viruses
6.3 The cultivation of viruses
6.4 General Features of Virus Reproduction
6.5 one-step growth curve
6.6 Temperate Bacteriophages,Lysogeny and Lambda
6.7 viral classification
6.8 Overview of Animal Viruses
6.9 plant viruses
6.10 Viroids and Prions
Chapter 6 Viruses
The Viruses
Introduction and general characteristics
? Virus, simple,acellular entities consisting of one or more molecules
of either DNA or RNA enclosed in a coat of protein,They are
reproduced only within living cells and are obligately intracellular
parasites
? The nucleic acid strands can be linear,closed cycle,or able to
assume either shape
? A virus particle containing nucleic acid surrounded by protein and
other macromolecular components is called virion.
? Viruses have a heavy dependence on host-cell structural and
metabolic components.
? Viruses can confer important new properties on their host cell.
(1) their simple,acellular organization
(2) the absence of both DNA and RNA in the same
virion,
(3) their inability to reproduce independently of
cells and carry out cell division as prokaryotes
and eukaryotes do.
Viruses differ from living cells in at least three ways:
6.1 General Properties of Viruses
The extracellular phase,posses few if any enzymes
and can not reproduce independently of living cells.
In the intracellular phase,viruses exist primarily as
replicating nucleic acids that induce host metabolism to
synthesize virion components; eventually complete virus
particles or virions are released,
extracellular and intracellular
Viruses can exist in two phases
Hosts and size
Three main classes - animal viruses,bacterial viruses
(bacteriophages),and plant viruses.
The particular host range of a virus is determined by the
virus's requirements for its specific attachment to the
host cell and the availability within the potential host of
cellular factors required for viral multiplication.
Viruses vary considerably in size,Although most are
quite a bit smaller than bacteria,some of the larger
viruses (such as the smallpox virus) are about the
same size as some very small bacteria (such as the
mycoplasmas,rickettsias,and chlamydias).
Size
Viruses range from 20 to 300 nm in diameter
The comparative sizes of several viruses and bacteria:
Genome in virion
Viral genomes,The genomes of viruses can be composed
of either DNA or RNA,and some use both as their
genomic material at different stages in their life cycle,
However,only one type of nucleic acid is found in the
virion of any particular type of virus,This can be single-
stranded (ss),double-stranded (ds),or in the case of the
hepadnaviruses,partially double-stranded.
? Most viruses are too small to be seen under light microscope.
?All viruses consists of an RNA or DNA core genome
surrounded by a protein coat capsid.
? The combined viral genome and capsid is called the
nucleocapsid.
6.2 Structure of viruses
The nucleic acid of a virus is surrounded by a protein
coat called the capsid
Each capsid is composed of protein subunits called
capsomeres.
In some viruses,the capsid is covered by an envelope,
which usually consists of some combination of lipids,
proteins,and carbohydrates.
Depending on the virus,envelopes may or may not be
covered by spikes,which are carbohydrateprotein
complexes that project from the surface of the envelope.
Viruses may be classified into several
morphological types on the basis of their
capsid architecture as revealed by electron
microscopy and a technique called x-ray
crystallography.
GENERAL MORPHOLOGY
A,Some viruses,such as tobacco mosaic
virus,have a helical symmetry with the
capsid surrounding an RNA genome.
B,Many viruses that infect bacteria,such as
the T-even bacteriophage,have a complex
capsid with DNA contained within the head
structure.
C,Some animal viruses,such as adenovirus,
have isometric symmetry and a DNA
genome.
D,others,such as coronavirus(冠状病毒 ）,
have complex capsids and an envelope with
protruding proteins surrounding an RNA
genome.
Helical viruses resemble
long rods that may be
rigid or flexible,
Surrounding the nucleic
acid,their capsid is a
hollow cylinder with a
helical structure,An
example of a helical virus
that is a rigid rod is the
tobacco mosaic virus
Helical viruses
The capsid of most polyhedral
viruses is in the shape of a
regular polyhedron with 20
triangular faces and 12 corners,
The capsomeres of each face
form an equilateral triangle,
An example of a polyhedral
virus in the shape of an
icosahedron is the adenovirus,
Another icosahedral virus is
the poliovirus,
Polyhedral viruses
The capsid of some viruses is covered by an envelope,
Enveloped viruses are roughly spherical but highly variable
in shape because the envelope is not rigid,When helical or
polyhedral viruses are enclosed by envelopes,they are
called enveloped helical and enveloped polyhedral viruses,
An example of an enveloped helical virus is the influenza
virus.
Enveloped viruses
Some viruses,particularly bacterial
viruses,have very complicated
structures and are called complex
viruses,Examples of complex
viruses are poxviruses,which do not
contain clearly identifiable capsids
but have several coats around the
nucleic acid,Certain bacteriophages
have capsids to which additional
structures are attached.
Complex viruses
A virus can have either DNA or RNA but never both !!
6.3 Cultivating Viruses
? Bacteriophage,bacterial Cultures
? Animal viruses,Tissue or cell cultures
? Plant viruses,tissue or separated cell
cultures,or cultures of protoplasts
Quantification
of viruses
? Plaque assay technique
for quantification of
bacterial viruses.
1,A dilution of a suspension containing the virus material
is mixed in a small amount of melted agar with the
sensitive host bacteria.
3,The host bacteria begin
to grow,and after overnight
incubation form a lawn of
confluent growth.
Quantification of bacterial virus by plaque assay
2,The mixture poured on
the surface of a nutrient
agar plate.
Photograph of a plate showing plaques formed by
bacteriophage on a lawn of sensitive bacteria,
The plaques shown are about 1-2 mm in diameter.
The size of the plaque formed depends on the virus,
the host,and conditions of culture.
Phage
plaques
Attentions
? Young,actively growing bacterial cells
? Plaque appearance often is characteristic of
the phage being cultivated.
? Plaque-forming units(PFU),each plaque
in a layer of bacterial or animal cells is
assumed to have arisen from the
reproduction of a single virus particle,
Efficiency of Plating
? Plaque assay has low efficiency:
– With bacteriophages,>50%
– With animal virues,0.1-1%
? Therefore,to quantify virus,it is accurate to
express the concentration (called titer) of the virus
suspension not as the absolute number of virion
units,but as the number of plaque-forming units,
The number of PFU does not equal the number of
virus particles,but their ratios are proportional.
The cultivation of animal viruses
? For many years researchers have cultivated animal viruses
by inoculating suitable host animals or embryonated eggs-
fertilized chicken eggs incubated about 6 to 8 days after
laying.
? More recently animal viruses have been grown in
tissue(cell) culture on monolayers of animal cells,This
technique is made possible by the development of growth
media for animal cells and by the advent of antibiotics that
can prevent bacterial and fungal contamination.
? Animal viruses can cause microscopic or macroscopic
degenerative changes or abnormalities in host cells and in
tissues called cytopathic effects.
Quantification of animal viruses
Tumor viruses
may not destroy
cells but cause
cells to grow
faster than
uninfected cells,
a phenomenon
called
transformation,
Transformed cells can be recognized as a small
focus of growth.
6.4 Viral Multiplication
For a virus to multiply,it must invade a host cell
and take over the host's metabolic machinery,A
single virus can give rise to several or even
thousands of similar viruses in a single host cell.
This process drastically changes the host cell and
often causes its death.
1,Attachment (adsorption) of the virion to a
susceptible host cell.
2,Penetration (injection) of the virion or its nucleic
acid into the cell.
3,Early steps in replication during which the host
cell biosynthetic machinery is altered as a prelude
to virus nucleic acid synthesis,Virus-specific
enzymes are typically made.
Multiplication of bacteriophages
4,Replication of the virus nucleic acid.
5,Synthesis of proteins used as structural
subunits of the virus coat.
6,Assembly of structural subunits (and
membrane components in enveloped viruses)
and packaging of nucleic acid into new virus
particles.
7,Release of mature virions from the cell.
Attachment of phage
to host cell,
After a chance
collision between
phage particles and
bacteria,an
attachment site on
the virus attaches to
a complementary
receptor site on the
bacterial cell,
Penetration:
During the process of penetration,the bacteriophage's
tail releases an enzyme,phage lysozyme,which breaks
down a portion of the bacterial cell wall,then the
bacteriophage injects its DNA (nucleic acid) into the
bacterium,
Biosynthesis of viral components:
Any RNA transcribed in the cell is mRNA
transcribed from phage DNA for biosynthesis of
phage enzymes and capsid protein,The host cell's
ribosomes,enzymes,and amino acids are used for
translation
Formation of mRNA after infection of cells by viruses of different
types,The chemical sense of the mRNA is considered as plus (+),
The sense of the various virus nucleic acids are indicated as + if the
same as mRNA,as – if opposite,or as +- if double-stranded,
Examples are indicated next to the virus nucleic acid,Although
examples of viruses containing SS DNA of the - sense are known,
none are discussed in the text.
Maturation
and release:
The phage heads and tails are separately
assembled from protein subunits,the
head is packaged with phage DNA,and
the tail is attached
Release:
Lysozyme,whose code is provided by a phage gene,
is synthesized within the cell,This enzyme causes a
breakdown of the bacterial cell wall,and the newly
produced bactedophages are released from the host
cell.
The number of newly synthesized phage particles
released from a single cell usually ranges from about
50 to 200,
1,Attachment (adsorption) of
the virion to a susceptible host
cell.
2,Penetration (injection) of
irion or its nucl ic acid
into the cell.
3,Early steps in r pl cati n
during which the host cel
biosynthetic machinery is altered
as a prelude to virus nucleic acid
synthesis,Virus-specific
enzymes are typically made.
4 Replica io of the virus
n cle c acid.
5,Sy thes s of proteins used
as structural ubunits of the
v rus coat.
6,ssembly st uctural
s bun ts (and membran
ompon nts in enveloped
viruses) and packaging of
nucleic acid nto new virus
particl,
7 l as of matur ion
from the ell
6.5 one-step growth curve
1,Latent period,no
release of virions
2,Rise period or
burst,the host
cells rapidly lyse
and release
infective phages
The one-step growth curve of virus replication
There are genetic controls that regulate when different
regions of phage DNA are transcribed into mRNA during
the multiplication cycle.
? There are early messages that are translated into early
phage proteins,the enzymes used in the synthesis of phage
DNA.
? There are late messages that are translated into late phage
proteins for the synthesis of capsid proteins,
This control mechanism is mediated by RNA polymerase.
eclipse period
Time course of events in phage T4 infection
Latent period
6.6 Lysogeny
Some phages can incorporate their DNA into
the host cell's DNA,The phage remains
latent and does not cause lysis of the host cell,
Such a state is called lysogeny.
Such phages are called lysogenic phages or
temperate phages,The participating
bacterial host cells are known as lysogenic
cells.
Lysogenic cycle of bacteriophage lambda in E.coli
The consequences of
infection by a temperate
bacteriophage,The
alternatives on infection
are integration of the
virus DNA into the host
DNA (lysogenization) or
replication and release
of mature virus (lysis),
The lysogenic cell can
also be induced to
produce mature virus
and lyse,
1,Nature of the host-animal,plant,bacterial,insect,fungal
2,Nucleic acid characteristics-DNA or RNA,single or double
stranded,molecular weight
3,Capsid symmetry-icosahedral,helical
4,Presence of an envelope and ether sensitivity
5,Diameter of the virion or nucleocapsid
6,Number of capsomers in icosahedral viruses
7,Immunologic properties
8,Intracellular location of viral replication
6.7 Some important characteristics for viral
classification
(1) nucleic acid type
(2) nucleic acid strand
(3) presence or absence of an envelope
Recently,the International Committee for Taxonomy
of Viruses has developed a uniform classification
system and divided viruses into 50 families,The
committee places greatest weight on three properties,
Cubic bacteriophage:
Group 1 Genome:DNA,single-stranded,Virion,icosahedral,
cubic symmetry,12 capsomers,Example,X174
Group 2 Genome,DNA,double-stranded,Virion,cubic
symmetry,enveloped,Example,PM-2.
Group 3 Genome,RNA,single-stranded,Virion,icosahedral,
cubic symmetry,32 capsomers.
Group 4 Genome,RNA,double-stranded,Virion,cubic
symmetry,enveloped,Example,06,
6.8 Animal Viruses
Uptake of an enveloped
virion by an animal cell.
(a)The process by which
the viral nucleocapsid is
separated from its
envelope,
(b)Electron micrograph of
adenovirus virions
entering a cell,Each
particle is about 70 nm in
diameter.
Animal
virus
entry
Viruses can have varied effects on cells,Lytic
infection results in the destruction of the host
cell,However,there are several other possible
effects,
In the case of enveloped viruses,release of
virions,which occurs by a kind of budding
process and the host cell may not be lysed,The
cell may remain alive and continue to produce
virus over a long period of time,Such infections
are referred to as persistent infections
Possible effects that animal viruses may have
on cells they effect
Viruses and cancer
A number of animal viruses participate in the
events that change a cell from a normal one
to a cancer or tumor cell, Cancer is a cellular
phenomenon of uncontrolled growth,Most
cells in a mature animal,although alive,do
not divide extensively.
Because cancerous cells in the animal body
have fewer growth requirements,they grow
profusely,leading to the formation of large
masses of cells,called tumors,
The development of cancer is clearly a multistep
process,There seem to be many different causes
of cancer,including mutations arising either
spontaneously or as the result of exposure to
certain chemicals,called carcinogens,or by
physical stimuli,such as ultraviolet radiation or
X-rays,Certain viruses also bring about the
genetic change that results in initiation of tumor
formation.
How does a normal cell become cancerous?
Some human cancers where viruses play a role
Cancer Virus Family Genome in virion
Adult T-cell
leukemia
Human T-cell leukemia
virus(typeⅠ )
Retrovirus RNA
Burkitt’s
lymphoma
Epstein-Barr virus Herpes DNA
Nasopharyngeal
carcinoma
Epstein-Barr virus Herpes DNA
Hepatocellular
carcinoma(liver
cancer)
Hepatitis B virus Hepadna DNA
Skin and
cervical cancers
Papilloma virus Papova DNA
6.8.1 Pox Viruses
The most complex and largest animal viruses
known and have some characteristics that
approach those of primitive cells,
The pox viruses are not able to metabolize
and thus depend on the host for the complete
machinery of protein synthesis.
These viruses are also unique in that they are
DNA viruses that replicate in the cytoplasm,
Smallpox was the first virus to be studied in any
detail and was the first virus for which a vaccine
was developed,
The pox viruses are very large,so large that they
can actually be seen under the light microscope.
Vaccinia virions are taken up into cells via a
phagocytic process from which the cores are
liberated into the cytoplasm.
General Properties of Pox Viruses
6.8.2 Adenoviruses
The genomes of the adenoviruses consist of linear
double-stranded DNA of about 36 kilobase pairs,
Attached in covalent linkage to the 5'-terminus of
the DNA is a protein component essential for
infectivity of the DNA,
The DNA has inverted terminal repeats of 100-1800
base pairs (this varies with the virus strain),
The DNA of the adenoviruses is six to seven times
the size of the DNA of the papovavirus SV40.
Replication of the viral DNA occurs in the nucleus.
After the virus particle has been transported to the
nucleus,the core is released and converted to a
viral DNA-histone complex,
Early transcription is carried out by an RNA
polymerase of the host,and a number of primary
transcripts are made,
The transcripts are spliced,capped,and
polyadenylated,giving sever al different mRNAs.
Replication of adenovirus DNA
(See text for details)
6.8.3 Retroviruses
One important group of animal viruses,those
called the retroviruses,have both an RNA and a
DNA phase of replication,
The retroviruses are RNA viruses,but they
replicate by means of a DNA intermediate using
the enzyme reverse transcriptase,
3,The enzyme reverse transcriptase has become
a major tool in genetic engineering.
2,One retrovirus,the one causing acquired
immunodeficiency syndrome (AIDS),
1,They were the first viruses shown to cause
cancer and have been studied most extensively
for their carcinogenic characteristics,
Importance
Retrovirus structure and
function
(a)Structure of a retrovirus,
(b) Genetic map of a typical
retrovirus genome,
(c)Genetic map of Rous
sarcoma virus,a retrovirus
that causes malignant
tumors in birds,Each end
of the genomic RNA
contains direct repeats(R),
and this RNA also has a 5’-
cap and a 3’-poly-A tail,
See text for more details.
6.8.4 Insect viruses
? Many insect virus infections are accomanied by the
formation of inclusion bodies within the infected cells.
1,cytoplasmic polyhedrosis
2,nuclear polyhedrosis
3,cytoplasmic and nuclear polyhedrosis
? The inclusion bodies are protein in nature and enclose one
or more virions,Polyhedral bodies protect the virions
against heat,low pH,and many chemicals.
? Insect larvae are infected when they feed on leaves
contaminated with inclusion bodies,When in insect gut
contents,inclusion bodies dissolve to liberate the virions.
Insect viruses as biological control agents
? Baculoviruses have received the most attention for at
least three reasons:
1,They attack only invertebrates and have considerable
host specificity.
2,They are encased in protective inclusion bodies.
3,They are well suited for commercial production since
they often reach extremely hegh concentrations in larval
tissue.
? Cotton bollworm,Douglas fir tussock moth,gypsy moth,
alfalfa looper
? Inclusion bodies are sprayed on foliage consumed by the
target insects
6.9 Plant Viruses,
Virion morphology,
1,Helical capsids
2,Icosahedral
?Almost all plant viruses are RNA viruses,either
single stranded or double stranded.
?Taxomomy standard,nucleic acid type,strandedness,
capsid symmetry and size,envelope presence
?Transmission of plant viruses,leaves are
mechanically damaged.
6.10 Viroids and Prions
Viroids are small,circular,single-stranded RNA
molecules that are the smallest known pathogens,
? about 250 to 370 nucleotides long,
? Viroids are found principally in the nucleolus of
infected cells; 200-2000 copies
? They do not act as mRNAs to direct protein
synthesis,How to cause disease symptoms
? Could be replicated by an RNA-dependent
RNA polymerase,a rolling-circle type
mechanism
Prions represent the other extreme from
viroids,They have a distinct extracellular form,
but the extracellular form seems to be entirely
protein,It apparently does not contain any nucleic
acid,or if it does,the molecule is not long enough
to encode the single kind of protein of which the
prion is composed.
Prions
? Proteinaceous infectious particle
? Cause a degenerative disorder of the central
nervous system.
? No nucleic acid has yet been detected in the agent,
it seems to be a 33 to 35 kDa hydrophobic
membrane protein,often called PrP.
? The PrP is bound to the surface of neurons,
presumably an altered PrP is at least partly
responsible for the disease.
? PrPsc,?-helix,?-sheet
PrPc,?-helix,?-sheet
