Transcript File - Microbiology

An Introduction
to the Viruses
Chapter 6
Part 2
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Nucleic Acids: At the Core of a
Virus
• Genome- the sum total of the genetic information
carried by an organism
• Contain DNA or RNA, not both
• Number of viral genes compared with a callquite small
• They only have the genes necessary to invade
host cells and redirect their activity
• Some viruses are exceptions to typical DNA and
RNA formations
• Parvoviruses contain single-stranded DNA
• Reoviruses contain double-stranded RNA
DNA Viruses of Eukaryotes
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Table 6.2 Medically Relevant DNA Virus Groups
DNA Viruses
Nonenveloped
Enveloped
Double-stranded
genome
Double-stranded
genome
Poxviruses
Adenoviruses
(linear dsDNA)
Herpesviruses
Papovaviruses
(circular dsDNA)
Single-stranded
genome
Parvoviruses
Source: Adapted from: Poxviridae from Buller et al., National Institute of Allergy & Infectious Disease,
Department of Health & Human Services.
RNA Viruses
• Mostly single-stranded
• Positive-sense RNA: genomes that are ready
for immediate translation into proteins
• Negative-sense RNA: genomes have to be
converted into the proper form to be made into
proteins
• Segmented- individual genes exist on separate
pieces of RNA
RNA Viruses of Eukaryotes
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Table 6.3
Medically Relevant RNA Viruses
RNA Viruses
Enveloped
Single-stranded
genome
Segmented
genome
Orthomyxoviruses
Single-stranded genome
encodes reverse
transcriptase
Nonsegmented genome
Retroviruses
Paramyxoviruses
Bunyaviruses
Arenaviruses
Rhabdoviruses
Filoviruses
Coronaviruses
Nonenveloped
Single-stranded
genome
Picornaviruses
Caliciviruses
Double-stranded
genome
Reoviruses
Viral Genomes
• One kind of nucleic acid (either RNA or DNA)
• DS DNA
Adenovirus, Herpes virus, Poxvirus
• SS DNA
Parvovirus
• DS RNA
Reovirus
• SS RNA (+) Poliovirus, Rhinovirus
• SS RNA (-) Influenza, Rabies virus
• SS RNA (+) HIV (becomes DNA in cell)
• SS RNA (+) Hepatitis B (becomes DNA in virus)
Viral Enzymes
• Found in
nucleocapsid
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• Polymerases
• Reverse
transcriptase
• Found in envelope
• Hemagglutinin
Envelope
• Neuraminidase
Nucleocapsid
Concept Check
What type of genetic material does the influenza
virus contain?
A. DS DNA
B. DS RNA
C. SS (+) RNA
D. SS (–) RNA
6.4 How Viruses are Classified and
Named
• Main criteria
• Structure
• Chemical composition
• Similarities in genetic makeup
• International Committee on the Taxonomy of Viruses,
2005
• 3 orders
• 73 families “-viridae”
• 287 genera “-virus”
• Some virologists use a species naming system, but it
is not an official designation
Viral Classification
6.5 Modes of Viral Multiplication
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• Host cell is
required for viral
multiplication.
Host Cell Cytoplasm
Spikes
Cell membrane
1
1 Adsorption. The virus
attaches to its host cell
by specific binding of its
spikes to cell receptors.
Receptors
2
Penetration. The virus is
engulfed into a vesicle and
its envelope is
3
Uncoated, thereby freeing
the viral RNA into the cell
cytoplasm.
2
3
• Specific
interaction
between host
cell and virus.
(+)
4
RNA
4
(–)
(+)
New
spikes
(+)
(+)
(+)
New
RNA
5 Assembly. Viral spike
proteins are inserted
into the cell membrane
for the viral envelope;
nucleocapsid is formed
from RNA and
capsomers.
New
capsomers
(+)
6 Release. Enveloped viruses bud
off of the membrane, carrying
away an envelope with the spikes.
This complete virus or virion is
ready to infect another cell.
(a)
Synthesis: Replication and Protein
Production. Under the control of viral
genes, the cell synthesizes the basic
components of new viruses: RNA
molecules, capsomers, spikes.
5
6
Multiplication Cycles in Animal
Viruses
• Adsorption – specific attachment
• Penetration – entry of viral genome
• Uncoating – release of viral genome
• Synthesis – new viral products made
• Assembly – new viruses are made in the cell
• Release – often causes the host cell to lyse
Adsorption
• Virus encounters
susceptible host cells
• Adsorbs specifically to
receptor sites on the cell
membrane
• Because of the exact fit
required, viruses have
a limited host range
Penetration
• Flexible cell membrane of the host is penetrated by the
whole virus or its nucleic acid
• Endocytosis: entire virus engulfed by the cell and
enclosed in a vacuole or vesicle
• The viral envelope can also directly fuse with the host cell
membrane
Uncoating
• Enzymes in the vacuole dissolve the envelope
and capsid
• The virus is now uncoated
Synthesis
• Free viral nucleic acid exerts control over the host’s
synthetic and metabolic machinery
• DNA viruses- enter host cell’s nucleus where they
are replicated and assembled
• RNA viruses- replicated and assembled in the
cytoplasm
Synthesis
RNA virus
DNA virus
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Host Cell Cytoplasm
Spikes
Cell membrane
1
1 Adsorption. The virus
attaches to its host cell
by specific binding of its
spikes to cell receptors.
Receptors
2
Penetration. The virus is
engulfed into a vesicle and
its envelope is
3
Uncoated, thereby freeing
the viral RNA into the cell
cytoplasm.
2
3
(+)
4
RNA
4
(–)
(+)
New
spikes
(+)
(+)
(+)
New
RNA
5 Assembly. Viral spike
proteins are inserted
into the cell membrane
for the viral envelope;
nucleocapsid is formed
from RNA and
capsomers.
New
capsomers
(+)
6 Release. Enveloped viruses bud
off of the membrane, carrying
away an envelope with the spikes.
This complete virus or virion is
ready to infect another cell.
(a)
Synthesis: Replication and Protein
Production. Under the control of viral
genes, the cell synthesizes the basic
components of new viruses: RNA
molecules, capsomers, spikes.
5
6
Assembly and Release
• Mature virus particles are constructed from the
growing pool of parts
• Nonenveloped and complex viruses are released
when the cell lyses or ruptures
• Enveloped viruses are liberated by budding or
exocytosis
• Anywhere from 3,000 to 100,000 virions may be
released, depending on the virus
• Entire length of cycle- anywhere from 8 to 36
hours
Release of Enveloped Virus