Microbiology – Chapter 8 – Viruses and their replication
8.1 What is a virus?
Virus; genetic element that cannot replicate independently of a living (host) cell
A virus possesses (bezit) their own genome and in this sense are independent of the
host’s genome.
Virology; the study of viruses
Virus particle (virion); extracellular form of a virus, it allows the virus to
travel form one host cell to another. It contains a nucleic acid genome
surrounded by a protein coat, in some cases, and other layers of
material.
Viral component and activities
Capsid; protein shell around the virus genome.
Nucleocapsid; the inner structure of nucleic acid + capsid protein
See picture on page 263; for head, collar, tail, tail pins, baseplate and tail fibers.
The virion protects the viral genome when the virus is outside the host cell
The proteins on the virion can attach to the host cell.
Once inside the host cell 2 events;
• Lytic infection; virus may replicate and destroy the host cell
• Lysogenic infection; the host cell is not destroyed but is genetically altered
because the viral genome becomes part of the host genome.
Viral genomes
Most viral genomes are linear, some circular
Viruses and the viral genomes are smaller than prokaryotic
genomes.
Viral genomes contain DNA or RNA genomes, and are
further subdivided in single-stranded or double-stranded.
Single stranded; have a plus sense or a minus sense
• Plus sense; exact same base sequence as the viral mRNA
• Minus sense; complementary in base sequence to viral mRNA
Some viruses use both DNA and RNA as genetic material, but at different stages of
their life cycle.
Retroviruses can go from RNA to DNA.
Viruses can be classified on the bases of the hosts they infect;
• Bacterial viruses (bacteriophages)
• Archaeal viruses
• Animal viruses
• Plant viruses
• Other viruses
, 8.2 Structure of the virion
Virion structure
Capsid; protein shell that surrounds the genome of a virus
Capsomere; subunit of the capsid
Nucleocapsid; complete complex of nucleic acid and protein packaged
(=naked virus)
Enveloped virus; virus that contains additional layer around the nucleocapsid
Virus symmetry
Virions are high symmetric structures.
• Helical symmetry; rod-shaped viruses; TMV virus (first discovered virus)
o Length of virus determined by length of nucleic acid
o Width of virus determined by size and packing of the capsomeres
• Icosahedral symmetry; spherical viruses
o Most efficient arrangement of subunits in a closed shell
o Icosahedron = 20 triangular faces
Enveloped viruses
They have a lipoprotein membrane surrounding the nucleocapsid,
with embedded proteins in it.
Envelope makes initial contact with the host cell for attachment of the
virion to the host cell during infection and for release of the virion
form the host cell after replication.
Enzymes inside virions
• Lysozyme; makes hole in cell wall (link between G and M sugars of
peptidoglycan is broken) Active during the infection process of the host cell.
• Nucleic acid polymerases; replicated the viral genome and produce viral
mRNA.
o Retroviruses replicate via DNA intermediates. They make DNA from an
RNA template; reverse transcriptase.
• Neuraminidases; enzymes that cleave glyosidic bonds in glycoproteins and
glycolipids. Allows liberation (vrijlating) of viruses from the host cell
8.3 Overview of the virus life cycle
Phases of bacterial viral replication
• Attachment (adsorption); of the virus to a host cell
• Entry (penetration/injection); of the virion or tis
nucleic acid
• Synthesis; of virus nucleic acid and protein by cell
metabolism
• Assembly; of capsids and packaging of viral
genomes into new virions
• Release; of mature virions from host cell
Watch out in Bacteria and Archaea, only the viral nucleic acid enters the host cell. In
plant and animal cells the entire virion is taken up.
8.1 What is a virus?
Virus; genetic element that cannot replicate independently of a living (host) cell
A virus possesses (bezit) their own genome and in this sense are independent of the
host’s genome.
Virology; the study of viruses
Virus particle (virion); extracellular form of a virus, it allows the virus to
travel form one host cell to another. It contains a nucleic acid genome
surrounded by a protein coat, in some cases, and other layers of
material.
Viral component and activities
Capsid; protein shell around the virus genome.
Nucleocapsid; the inner structure of nucleic acid + capsid protein
See picture on page 263; for head, collar, tail, tail pins, baseplate and tail fibers.
The virion protects the viral genome when the virus is outside the host cell
The proteins on the virion can attach to the host cell.
Once inside the host cell 2 events;
• Lytic infection; virus may replicate and destroy the host cell
• Lysogenic infection; the host cell is not destroyed but is genetically altered
because the viral genome becomes part of the host genome.
Viral genomes
Most viral genomes are linear, some circular
Viruses and the viral genomes are smaller than prokaryotic
genomes.
Viral genomes contain DNA or RNA genomes, and are
further subdivided in single-stranded or double-stranded.
Single stranded; have a plus sense or a minus sense
• Plus sense; exact same base sequence as the viral mRNA
• Minus sense; complementary in base sequence to viral mRNA
Some viruses use both DNA and RNA as genetic material, but at different stages of
their life cycle.
Retroviruses can go from RNA to DNA.
Viruses can be classified on the bases of the hosts they infect;
• Bacterial viruses (bacteriophages)
• Archaeal viruses
• Animal viruses
• Plant viruses
• Other viruses
, 8.2 Structure of the virion
Virion structure
Capsid; protein shell that surrounds the genome of a virus
Capsomere; subunit of the capsid
Nucleocapsid; complete complex of nucleic acid and protein packaged
(=naked virus)
Enveloped virus; virus that contains additional layer around the nucleocapsid
Virus symmetry
Virions are high symmetric structures.
• Helical symmetry; rod-shaped viruses; TMV virus (first discovered virus)
o Length of virus determined by length of nucleic acid
o Width of virus determined by size and packing of the capsomeres
• Icosahedral symmetry; spherical viruses
o Most efficient arrangement of subunits in a closed shell
o Icosahedron = 20 triangular faces
Enveloped viruses
They have a lipoprotein membrane surrounding the nucleocapsid,
with embedded proteins in it.
Envelope makes initial contact with the host cell for attachment of the
virion to the host cell during infection and for release of the virion
form the host cell after replication.
Enzymes inside virions
• Lysozyme; makes hole in cell wall (link between G and M sugars of
peptidoglycan is broken) Active during the infection process of the host cell.
• Nucleic acid polymerases; replicated the viral genome and produce viral
mRNA.
o Retroviruses replicate via DNA intermediates. They make DNA from an
RNA template; reverse transcriptase.
• Neuraminidases; enzymes that cleave glyosidic bonds in glycoproteins and
glycolipids. Allows liberation (vrijlating) of viruses from the host cell
8.3 Overview of the virus life cycle
Phases of bacterial viral replication
• Attachment (adsorption); of the virus to a host cell
• Entry (penetration/injection); of the virion or tis
nucleic acid
• Synthesis; of virus nucleic acid and protein by cell
metabolism
• Assembly; of capsids and packaging of viral
genomes into new virions
• Release; of mature virions from host cell
Watch out in Bacteria and Archaea, only the viral nucleic acid enters the host cell. In
plant and animal cells the entire virion is taken up.
