Virus
General Properties of Viruses
Virus have both an extracellular and an intracellular state,A virus is a
submicroscopic particle containing nucleic acid surrounded by protein
and occasionally containing other macromolecular components,In this
extracellular state,the virus particle,also called the virion,is
metabolically inert and does not carry out respiratory or biosynthetic
functions,The virion is the structure by which the virus genome is
carried from the cell in which it has been produced to another cell where
the viral nucleic acid can be introduced,In the intracellular state,virus
replication occurs,the virus genome is produced,and the components
that make up the virus coat are synthesized,When a virus genome is
introduced into a host cell and reproduces,the process is called infection.
(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:
General Properties of Viruses
Types of Viral Nucleic Acids
A
Diagrammatic
Description of
Families and
Genera of
Virus That
Infect Plants,
RT stands for
reverse
transcriptase
A
Diagrammatic
Description of
the Families
and Genera of
Viruses That
Infect
Vertebrates,RT
stand for
reverse
transcriptase
Major
Bacteriaophage
Families and
Genera.
The Myoviridae are
the only family with
contractile tails,
Plasmaviridae are
pleomorphic,
Tectiviridae have
distinctive double
capsids,whereas the
corticoviridae have
complex capsids
containing lipid.
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).
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.
?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
Tobacco Mosaic Virus
Structure,
(a) An electron
micrograph of the
negatively stained
helical capsid
(× 400,000),
(b) Illustration of TMV
structure,The
nucleocapsid is
composed of a helical
array of protomers with
the RNA spiraling on
the inside,
(c) A model of TMV.
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 !!
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 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
The One-Step Growth Curve,In the initial part of the latent period,the
eclipse period,the host cells do not contain any complete,infective virions,
During the remainder of the latent period,an increasing number of
infective virions are present,but none are released,The latent period ends
with host cell lysis and rapid release of virions during the rise period or
burst,In this figure the blue line represents the total number of complete
virions,The red line is the number of free viruses (the unabsorbed virions
plus those released from host cells),When E.coli is infected with T2 phage
at 37℃,the growth plateau is reached in about 30 minutes and the burst
size is approximately 100 or move virions per cell,The eclipse period is
11-12 minutes,and the latent period is around 21-22 minutes in length.
Electron Micrograph of E.coli Infected with Phage T4.
T4 Phage Adsorption and DNA Injection.
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.
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
The Assembly of T4
Bacteriophage.
The subassembly
lines for the baseplate,
tail tube and sheath,
tail fibers,and head.
Aniaml Virus Entry,Mechanisms of animal virus attachment
and entry into host cells
The Reproduction of φX174,a + Strand DNA Phages.
The
Reproduction of
Single-Stranded
RNA
Bacteriophages
RNA Animal Virus Reproductive Strategies
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
The Life Cycle of
Bacteriophage T4.
(a) A schematic
diagram depicting the
life cycle with the
minutes after DNA
injection given
beneath each stage,
mRNA is drawn in
only at the stage
during which its
synthesis begins,
(b) Electron
micrographs show the
development of T2
bacteriophages in
E.coli
(b1) Several phages
are near the bacterium,
and some are attached
and probably injecting
their DNA,
(b2) By about 30
minutes after infection,
the bacterium contains
numerous completed
phages.
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,
General Properties of Viruses
Virus have both an extracellular and an intracellular state,A virus is a
submicroscopic particle containing nucleic acid surrounded by protein
and occasionally containing other macromolecular components,In this
extracellular state,the virus particle,also called the virion,is
metabolically inert and does not carry out respiratory or biosynthetic
functions,The virion is the structure by which the virus genome is
carried from the cell in which it has been produced to another cell where
the viral nucleic acid can be introduced,In the intracellular state,virus
replication occurs,the virus genome is produced,and the components
that make up the virus coat are synthesized,When a virus genome is
introduced into a host cell and reproduces,the process is called infection.
(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:
General Properties of Viruses
Types of Viral Nucleic Acids
A
Diagrammatic
Description of
Families and
Genera of
Virus That
Infect Plants,
RT stands for
reverse
transcriptase
A
Diagrammatic
Description of
the Families
and Genera of
Viruses That
Infect
Vertebrates,RT
stand for
reverse
transcriptase
Major
Bacteriaophage
Families and
Genera.
The Myoviridae are
the only family with
contractile tails,
Plasmaviridae are
pleomorphic,
Tectiviridae have
distinctive double
capsids,whereas the
corticoviridae have
complex capsids
containing lipid.
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).
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.
?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
Tobacco Mosaic Virus
Structure,
(a) An electron
micrograph of the
negatively stained
helical capsid
(× 400,000),
(b) Illustration of TMV
structure,The
nucleocapsid is
composed of a helical
array of protomers with
the RNA spiraling on
the inside,
(c) A model of TMV.
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 !!
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 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
The One-Step Growth Curve,In the initial part of the latent period,the
eclipse period,the host cells do not contain any complete,infective virions,
During the remainder of the latent period,an increasing number of
infective virions are present,but none are released,The latent period ends
with host cell lysis and rapid release of virions during the rise period or
burst,In this figure the blue line represents the total number of complete
virions,The red line is the number of free viruses (the unabsorbed virions
plus those released from host cells),When E.coli is infected with T2 phage
at 37℃,the growth plateau is reached in about 30 minutes and the burst
size is approximately 100 or move virions per cell,The eclipse period is
11-12 minutes,and the latent period is around 21-22 minutes in length.
Electron Micrograph of E.coli Infected with Phage T4.
T4 Phage Adsorption and DNA Injection.
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.
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
The Assembly of T4
Bacteriophage.
The subassembly
lines for the baseplate,
tail tube and sheath,
tail fibers,and head.
Aniaml Virus Entry,Mechanisms of animal virus attachment
and entry into host cells
The Reproduction of φX174,a + Strand DNA Phages.
The
Reproduction of
Single-Stranded
RNA
Bacteriophages
RNA Animal Virus Reproductive Strategies
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
The Life Cycle of
Bacteriophage T4.
(a) A schematic
diagram depicting the
life cycle with the
minutes after DNA
injection given
beneath each stage,
mRNA is drawn in
only at the stage
during which its
synthesis begins,
(b) Electron
micrographs show the
development of T2
bacteriophages in
E.coli
(b1) Several phages
are near the bacterium,
and some are attached
and probably injecting
their DNA,
(b2) By about 30
minutes after infection,
the bacterium contains
numerous completed
phages.
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,
