Chapter 6 Viruses
?Virus, a noncellular genetic element that enlists a
cell for its own replication,it has an extracellular
state.
?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.
Classification of Viruses
Viruses can have either DNA or RNA as genetic material
?According to hosts viruses infect:
– Bacterial viruses (Bacteriophage or 噬菌体）
– Animal viruses (well studied)
– Plant viruses (less studied)
?According to nucleic acid structures:
– DNA viruses (ss DNA,ds DNA)
– RNA viruses (ss RNA,ds RNA)
– RNA DNA viruses (ss RNA,ds DNA)
Nature of the virion
Virus Size,20 nm to 300 nm
Largest virus,Smallpox (天花病毒 ) is 200 nm
Smallest virus,Polio virus (脊髄灰质炎病毒 ) is 28 nm
Virus genome,Smaller than those of cells,the
largest known viral genome,vaccinia (牛痘 )
is only 190 KB.
Bacterial genomes,1000-9000 KB
Virus Structure
Viruses consist of nucleic acids,capsid,and/or envelope.
Virus Symmetry
Helical symmetry,Tobacco mosaic virus (TMV).
Icosahedron,The most efficient arrangement for
subunits in a closed shell.
Enveloped viruses,envelope consists of lipid
bilayer with glycoproteins embedded in it,
Its symmetry refers to its nucleocapside.
Complex viruses,T4 bacteriophage
Virus Morphology
Enzymes in viruses
?Some viruses contain their own enzymes for
their own purposes,such as
– Reverse transcriptase in retroviruses
– Neuraminadase,break down glycoprotein,
aiding liberation of the virus
– Lysozyme,bacteriophage
Cultivating Viruses
?Bacteriophage,bacterial Cultures
?Animal viruses,Tissue or cell cultures
Quantification
of viruses
? Plaque assay technique
for quantification of
bacterial viruses.
Quantification of animal viruses
Tumor viruses
may bot 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.
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.
Virus Life Cycle
1,Attachment (adsorption)
2,Penetration (injection)
3,Early steps in replication
4,Replication
5,Synthesis of protein subunits
6,Assembly and packaging
7,Release
Virus Life
Cycle
The one-step
growth curve of
virus replication
How a bacteriophage
injects its nucleic acid
into a host cell?
How host cell protect themselves?
?Restriction enzyme destroy invasion of
foreign DNA.
?Cells protect themselves from destruction
by restriction enzymes via methylation of
purine and pyrimidine bases,
?Some T-even bacteriophages have their
DNA glycosylated to varying degrees.
Virus messenger RNA
mRNA is
equal to
RNA(+)
DNA (+) is
the
template
DNA for
making
mRNA
Bacterial
Viruses
Main types of
bacterial
viruses.
Sizes are
approximat
e scale
RNA bacteriophages
?Characteristics,small (26 nm),icosahedral,
with 180 copies of coat protein per virus
particle
– RNA phage MS2,3569 nucleotides long,
(+)RNA
RNA
Bacteriophage
Flow of events
during
multiplication
of RNA virus
MS2
Single-Stranded Icosahedral DNA
Bacteriophages
?Phage fC174 (infect E,coli) has
overlapping genes,consisting of circular
single-stranded molecule of 5386 Bases
Overlapping
Genes
The DNA of
fC174 was
the first
DNA to be
completely
sequenced,
Phage fC174
Rolling Circle
Replication of
Single-Stranded
Icosahedral
DNA
Bacteriophage
Questions
? What are viruses? How to classify viruses?
? How are viruses different from bacteria?
? Describe viral structures.
? What are two common symmetry structures?
? What are the most common enzymes brought with by viruses
themselves? How to grow viruses?
? How to quantify viruses?
? What is the virus life circle?
? How host cell protect themselves from virus attacks?
? How do minus,plus or ss-,ds DNA viruses replicate?
? What are the major bacteriophages? Describe Phage fC174 and its
genome.
? Describe RNA Bacteriophage MS2 and its genome.
? What is rolling circle replication?
?Virus, a noncellular genetic element that enlists a
cell for its own replication,it has an extracellular
state.
?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.
Classification of Viruses
Viruses can have either DNA or RNA as genetic material
?According to hosts viruses infect:
– Bacterial viruses (Bacteriophage or 噬菌体）
– Animal viruses (well studied)
– Plant viruses (less studied)
?According to nucleic acid structures:
– DNA viruses (ss DNA,ds DNA)
– RNA viruses (ss RNA,ds RNA)
– RNA DNA viruses (ss RNA,ds DNA)
Nature of the virion
Virus Size,20 nm to 300 nm
Largest virus,Smallpox (天花病毒 ) is 200 nm
Smallest virus,Polio virus (脊髄灰质炎病毒 ) is 28 nm
Virus genome,Smaller than those of cells,the
largest known viral genome,vaccinia (牛痘 )
is only 190 KB.
Bacterial genomes,1000-9000 KB
Virus Structure
Viruses consist of nucleic acids,capsid,and/or envelope.
Virus Symmetry
Helical symmetry,Tobacco mosaic virus (TMV).
Icosahedron,The most efficient arrangement for
subunits in a closed shell.
Enveloped viruses,envelope consists of lipid
bilayer with glycoproteins embedded in it,
Its symmetry refers to its nucleocapside.
Complex viruses,T4 bacteriophage
Virus Morphology
Enzymes in viruses
?Some viruses contain their own enzymes for
their own purposes,such as
– Reverse transcriptase in retroviruses
– Neuraminadase,break down glycoprotein,
aiding liberation of the virus
– Lysozyme,bacteriophage
Cultivating Viruses
?Bacteriophage,bacterial Cultures
?Animal viruses,Tissue or cell cultures
Quantification
of viruses
? Plaque assay technique
for quantification of
bacterial viruses.
Quantification of animal viruses
Tumor viruses
may bot 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.
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.
Virus Life Cycle
1,Attachment (adsorption)
2,Penetration (injection)
3,Early steps in replication
4,Replication
5,Synthesis of protein subunits
6,Assembly and packaging
7,Release
Virus Life
Cycle
The one-step
growth curve of
virus replication
How a bacteriophage
injects its nucleic acid
into a host cell?
How host cell protect themselves?
?Restriction enzyme destroy invasion of
foreign DNA.
?Cells protect themselves from destruction
by restriction enzymes via methylation of
purine and pyrimidine bases,
?Some T-even bacteriophages have their
DNA glycosylated to varying degrees.
Virus messenger RNA
mRNA is
equal to
RNA(+)
DNA (+) is
the
template
DNA for
making
mRNA
Bacterial
Viruses
Main types of
bacterial
viruses.
Sizes are
approximat
e scale
RNA bacteriophages
?Characteristics,small (26 nm),icosahedral,
with 180 copies of coat protein per virus
particle
– RNA phage MS2,3569 nucleotides long,
(+)RNA
RNA
Bacteriophage
Flow of events
during
multiplication
of RNA virus
MS2
Single-Stranded Icosahedral DNA
Bacteriophages
?Phage fC174 (infect E,coli) has
overlapping genes,consisting of circular
single-stranded molecule of 5386 Bases
Overlapping
Genes
The DNA of
fC174 was
the first
DNA to be
completely
sequenced,
Phage fC174
Rolling Circle
Replication of
Single-Stranded
Icosahedral
DNA
Bacteriophage
Questions
? What are viruses? How to classify viruses?
? How are viruses different from bacteria?
? Describe viral structures.
? What are two common symmetry structures?
? What are the most common enzymes brought with by viruses
themselves? How to grow viruses?
? How to quantify viruses?
? What is the virus life circle?
? How host cell protect themselves from virus attacks?
? How do minus,plus or ss-,ds DNA viruses replicate?
? What are the major bacteriophages? Describe Phage fC174 and its
genome.
? Describe RNA Bacteriophage MS2 and its genome.
? What is rolling circle replication?
