Characteristics of Viruses
• Extracellular state
• Called virion
• Protein coat (capsid) surrounding nucleic acid
• Nucleic acid and capsid, also called nucleocapsid
• Some have phospholipid envelope
• Outermost layer provides protection and recognition
sites for host cells
• Intracellular state
• Capsid removed
• Virus exists as nucleic acid
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Figure 13.1 Virions, complete virus particles, include a nucleic acid, a capsid, and in some cases an envelope.
Capsid (sectioned
to show interior)
Nucleic acid
(viral genome)
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Figure 13.6 The complex shape of bacteriophage T4.
Tail fibers
Base plate
Tail
Head
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Viral Replication
• Dependent on hosts' organelles and enzymes to
produce new virions
• Lytic replication
• Viral replication usually results in death and lysis of host
cell
• Five stages of lytic replication cycle
• Attachment
• Entry
• Synthesis
• Assembly
• Release 10/21/2014 MDufilho 4
Viral Replication: Overview
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Figure 13.8 The lytic replication cycle in bacteriophages.
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Figure 13.9 Pattern of virion abundance in lytic cycle.
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Viral Replication: Virulent Bacteriophages
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Viral Replication
• Lysogeny
• Modified replication cycle
• Infected host cells grow and reproduce normally for
generations before they lyse
• Temperate phages
• Prophages – inactive phages
• Lysogenic conversion
• Results when phages carry genes that alter phenotype of
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Figure 13.10 Bacteriophage lambda.
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Figure 13.11 The lysogenic replication cycle in bacteriophages.
1
2 3
Attachment
Lambda phage
Entry
Lytic
cycle
Release
Assembly
Synthesis
Induction
Prophage in chromosome
Lysogeny
Replication of chromosome and virus; cell division
Further replications and
cell divisions
1
2 3
4
5
6
7
8
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Viral Replication: Temperate Bacteriophages
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Culturing Viruses in the Laboratory
• Culturing Viruses in Mature Organisms
• Culturing viruses in bacteria
• Phages are grown in bacteria in liquid cultures or on agar
plates
• Lysis of bacteria produces plaques
• Allows estimation of phage numbers by plaque assay
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Bacterial lawn
Viral plaques
Figure 13.17 Viral plaques in a lawn of bacterial growth on the surface of an agar plate.
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