Chapter 5 Viruses
5.1 General Properties of Viruses
5.2 General Features of Virus Reproduction
5.3 Overview of Bacterial Viruses
5.4 Temperate Bacteriophages,Lysogeny and Lambda
5.5 Overview of Animal Viruses
5.6 Pox Viruses
5.7 Adcnoviruses
5.8 Retroviruses
5.9 Viroids and Prions
Chapter outline
(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:
5.1 General Properties of Viruses
Virion,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.
Virus particles (virions) vary widely in size and shape,Viruses
are smaller than cells,ranging in size from 0.02 to 0.3 μm,A
common unit of measure for viruses is the nanometer,which is
1000 times smaller than 1 um and 1 million times smaller than
1 mm,Smallpox virus,one of the largest viruses,is about 200
nm in diameter (a bit smaller than the size of the smallest
bacteria); poliovirus,one of the smallest,is only 28 nm in
diameter (about the size of a ribosome).
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
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.
The comparative sizes of several viruses and bacteria:
? 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.
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.
Structure of viruses
? 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.
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 !!
A virus can have either DNA or RNA but never both !!
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
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.
MULTIPLICATION OF BACTERIOPHAGES
The multiplication cycle of bacteriophages,like
that of all viruses,can be divided into several
distinct stages-
attachment
penetration
biosynthesis of viral components
Maturation
Release
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,Assembly f st uctural
s bunits (and m mbran
omponents in enveloped
viruses) and packagi g of
nucleic acid nto new v rus
particles.
7 l as of ma ur ion
from the ell
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,
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
one-step growth curve
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,
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,What are the two pathways
available to temperate virus?
2,Describe how a single protein like
the lambda represor can act both as
an activator and a represor.
Think and answer following two questions !!
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
Some important characteristics for viral classification
(1)nucleic acid type
(2) nucleic acid strandedness
(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,
Schematic representations of the
main types of bacterial viruses
Tailed bacteriophage,
Genome,DNA,double-stranded,
Virion,complex shape,binary
symmetry,variable number of
capsomers,The tails of the
phage are long and contractile in
group A,long and noncontractile
in group B,and very short in
group C,Example,T-even
coliphages,
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,
Filamentous bacteriophage:
Genome,DNA,single-stranded,Virion,rod-shaped,helical
symmetry,Example,fd.
Plant Viruses,
Tobacco mosaic virus (TMV) as an example
(1) penetration by the virus of a susceptible plant cell-
generally through abrasions or insect bites,
(2) tincoating of the viral nucleic acid within the plant cell,
(3) assumption by the viral genome of control of the
synthetic activities of the host cell,
(4) expression of the viral genome so that viral nucleic acid
and capsid components are synthesized,
(5) assembly of the viral particles within the host cell,and,
(6) release of the complete viral particles from the host
plant cell,
Animal Viruses:
Viroids and Prions:
Viroids are small,circular,single-stranded RNA
molecules that are the smallest known pathogens,The
extracellular form of the viroid is naked RNA-there is
no capsid of any kind.
Prions 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
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
5.1 General Properties of Viruses
5.2 General Features of Virus Reproduction
5.3 Overview of Bacterial Viruses
5.4 Temperate Bacteriophages,Lysogeny and Lambda
5.5 Overview of Animal Viruses
5.6 Pox Viruses
5.7 Adcnoviruses
5.8 Retroviruses
5.9 Viroids and Prions
Chapter outline
(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:
5.1 General Properties of Viruses
Virion,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.
Virus particles (virions) vary widely in size and shape,Viruses
are smaller than cells,ranging in size from 0.02 to 0.3 μm,A
common unit of measure for viruses is the nanometer,which is
1000 times smaller than 1 um and 1 million times smaller than
1 mm,Smallpox virus,one of the largest viruses,is about 200
nm in diameter (a bit smaller than the size of the smallest
bacteria); poliovirus,one of the smallest,is only 28 nm in
diameter (about the size of a ribosome).
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
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.
The comparative sizes of several viruses and bacteria:
? 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.
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.
Structure of viruses
? 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.
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 !!
A virus can have either DNA or RNA but never both !!
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
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.
MULTIPLICATION OF BACTERIOPHAGES
The multiplication cycle of bacteriophages,like
that of all viruses,can be divided into several
distinct stages-
attachment
penetration
biosynthesis of viral components
Maturation
Release
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,Assembly f st uctural
s bunits (and m mbran
omponents in enveloped
viruses) and packagi g of
nucleic acid nto new v rus
particles.
7 l as of ma ur ion
from the ell
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,
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
one-step growth curve
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,
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,What are the two pathways
available to temperate virus?
2,Describe how a single protein like
the lambda represor can act both as
an activator and a represor.
Think and answer following two questions !!
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
Some important characteristics for viral classification
(1)nucleic acid type
(2) nucleic acid strandedness
(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,
Schematic representations of the
main types of bacterial viruses
Tailed bacteriophage,
Genome,DNA,double-stranded,
Virion,complex shape,binary
symmetry,variable number of
capsomers,The tails of the
phage are long and contractile in
group A,long and noncontractile
in group B,and very short in
group C,Example,T-even
coliphages,
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,
Filamentous bacteriophage:
Genome,DNA,single-stranded,Virion,rod-shaped,helical
symmetry,Example,fd.
Plant Viruses,
Tobacco mosaic virus (TMV) as an example
(1) penetration by the virus of a susceptible plant cell-
generally through abrasions or insect bites,
(2) tincoating of the viral nucleic acid within the plant cell,
(3) assumption by the viral genome of control of the
synthetic activities of the host cell,
(4) expression of the viral genome so that viral nucleic acid
and capsid components are synthesized,
(5) assembly of the viral particles within the host cell,and,
(6) release of the complete viral particles from the host
plant cell,
Animal Viruses:
Viroids and Prions:
Viroids are small,circular,single-stranded RNA
molecules that are the smallest known pathogens,The
extracellular form of the viroid is naked RNA-there is
no capsid of any kind.
Prions 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
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
