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

a bacterium 10/21/2014 MDufilho 9

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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