1. An Introduction to the Viruses
Microbiology: A Systems Approach
An Introduction to the Viruses
Chapter 6
Part 2
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Chapter 6, pages 139 to 167

2. 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 call- quite 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

3. DNA Viruses of Eukaryotes
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Table 6.2
Medically Relevant DNA Virus Groups
DNA Viruses
Enveloped
Nonenveloped
Double-stranded
genome
Double-stranded
genome
Single-stranded
genome
Poxviruses
Adenoviruses
(linear dsDNA)
Herpesviruses
Papovaviruses
(circular dsDNA)
Parvoviruses
Source: Adapted from: Poxviridae from Buller et al., National Institute of Allergy & Infectious Disease,
Department of Health & Human Services.

4. 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

5. RNA Viruses of Eukaryotes
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Table 6.3
Medically Relevant RNA Viruses
RNA Viruses
Enveloped
Nonenveloped
Single-stranded
genome
Single-stranded genome
encodes reverse
transcriptase
Single-stranded
genome
Double-stranded
genome
Segmented
genome
Nonsegmented genome
Retroviruses
Picornaviruses
Reoviruses
Orthomyxoviruses
Paramyxoviruses
Caliciviruses
Bunyaviruses
Arenaviruses
Rhabdoviruses
Filoviruses
Coronaviruses

6. 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)

7. Viral Enzymes Found in nucleocapsid Polymerases Reverse transcriptase
Found in envelope
Hemagglutinin
Neuraminidase
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Envelope
Nucleocapsid

8. An Introduction to the Viruses
Concept Check
Microbiology: A Systems Approach
What type of genetic material does the influenza virus contain? A. DS DNA B. DS RNA C. SS (+) RNA D. SS (–) RNA
Correct answer is D; The influenza virus contains eight pieces of ss (-) RNA
KNOWLEDGE (Bloom’s taxonomy): Simple recall of material from the previous slides.
Chapter 6, pages 139 to 167

9. 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

10. Viral Classification

11. 6.5 Modes of Viral Multiplication
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Host cell is required for viral multiplication.
Specific interaction between host cell and virus.
Host Cell Cytoplasm
Cell membrane
Spikes 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
Uncoated, thereby freeing
the viral RNA into the cell
cytoplasm. 3 2 3 N u c l e u s R N A 4
(+)
Synthesis: Replication and Protein
Production. Under the control of viral
genes, the cell synthesizes the basic
components of new viruses: RNA
molecules, capsomers, spikes. 4
(–)
(+)
New
spikes
(+) 5
(+)
Assembly. Viral spike
proteins are inserted
into the cell membrane
for the viral envelope;
nucleocapsid is formed
from RNA and
capsomers.
(+)
New
capsomers
(+) 5
New
RNA 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. 6
(a)

12. 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

13. 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

14. 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

15. Uncoating Enzymes in the vacuole dissolve the envelope and capsid
The virus is now uncoated

16. 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

17. Synthesis RNA virus DNA virus
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Host Cell Cytoplasm
Cell membrane
Spikes 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
Uncoated, thereby freeing
the viral RNA into the cell
cytoplasm. 2 3 3 N u c l e u s R N A 4
(+)
Synthesis: Replication and Protein
Production. Under the control of viral
genes, the cell synthesizes the basic
components of new viruses: RNA
molecules, capsomers, spikes. 4
(–)
(+)
New
spikes
(+)
(+) 5
Assembly. Viral spike
proteins are inserted
into the cell membrane
for the viral envelope;
nucleocapsid is formed
from RNA and
capsomers.
(+)
New
capsomers 5
New
RNA
(+) 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. 6
(a)

18. 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

19. Release of Enveloped Virus