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VIRUSES, V IR O IDS A ND

PRIONS

Chapter 13

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L iving Cells or Nonliving Entities?

A . Inert outside the Host

B . Nucleic acids become active onceinside the host

C . Obligate Intracellular Parasites

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Vir ion Structure

1. Non-enveloped or Naked viruses

Nucleocapsid

2.

Enveloped virusesIn addition to Nucleocapsid,some virusescontain lipid

envelope,spikes

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Vir ion Structure

1.

Nucleocapsid

A . Nucleic acid:

DN A/RN A , ds/ss,

ci rcular/linear/fragmented

B . Capsid: protein coat

Capsomere arrangement

and shape

2. Envelope: lipid

3. Spikes

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Bacteriophages: Complex Viruses

Tail fibers,

involved in

attachment.

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Figure 13.1

Virus Sizes

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General Characteristics of V iruses

Obligatory intracellular parasites Contain DNA or RNA

Contain a protein coat

Some are enclosed by an envelope

Some viruses have spikes

Most viruses infect only specific types ofcells in one host

Host range is determined by specific hostattachment sites and cellular factors

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General Characteristics of V iruses

MultiplicationInsidelivingcells using host cell

machinery

Regulation of vi ral synthesis

Very few viral enzymes

Viruses are in charge

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How do Viruses differ from bacteria?

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Virus Interactions with Host Cells

1. Productive Infection

Lysis of Host cel l : V irulent phage T4

2. Latent Infection

Te

mpe

rate

phage

Lambda

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The Lytic Cycle

Attachment: Phage attaches by tail fibers tohost cell

Penetration: Phage lysozyme opens cell

wall; tail sheath contracts to force tail coreand DNA into cell

Biosynthesis: Production of phage DNA and

proteins Maturation: Assembly of phage particles

Release: Phage lysozyme breaks cell wall

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Lytic Cycle of a T-Even Bacteriophage1

2

3

Figure 13.11

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4

Figure 13.11

Lytic Cycle of a T-Even Bacteriophage

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Figure 13.10

A Viral One-Step Growth Curve

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The Lysogenic Cycle

1.

Temperate Phage Lambda causesLatent Infection/Lysogeny

2. Host = Lysogen

3. Replication of Lambda: Lytic or Lysogenic

1. Attachment

2. Penetration

3. Prophage

4. Gett ing out of lysogeny

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Figure 13.12

The Lysogenic Cycle

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Advantages of Lysogeny to the

Host

1. No direct lysis of the host

2.

Immunity3. Lysogenic Conversion: Lysogenic host

acquires new properties

4.

Transduction