Chapter

Viruses

Ch 19

Viruses

Characteristics of Viruses

1. Tiny, nonliving particles a. do not respire, move, grow

2. Must reproduce inside of a host

3. Pathogens: disease-causinga. Virulence: degree of

response caused in a host

4. Not given latin names a. named for disease caused,

organ infected, or region first detected in

Viral Structure

1. Inner core of nucleic acid surrounded by protein coat called capsid a. head contains either DNA or RNA

body

T4 BacteriophageTobacco Mosaic

Virus Influenza Virus

Head

Tail sheath

DNA

Tail fiber

RNACapsid

Surfaceproteins Membrane

envelope

RNA

Capsidproteins

Section 19-3

Figure 19-13 Virus Structures

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Recognition & Attachment of Viruses

1. Virus must recognize specific receptor site on host. A. part of the virus matches site

on host B. lands on host and “locks in”

2. Attachment is specific a. virus can enter & reproduce in only a few kinds of cells

Example: tobacco mosaic virus cannot affect human cells even though we are exposed to it.

There are 2 ways viruses can reproduce inside a host.

1. Lytic cycle-fast

2. Lysogenic cycle-slow

Lysogenic Cycle( virus can enter a period of inactivity before becoming lytic)

STEPS

1. Attachment- virus attaches to the host cell

2. Entry- whole virus or just nucleic acid enters the cell

3. Replication- viral nucleic acid joins w/DNA of host cell. It may stay there inactive for years. Host cell continues to divide

If the cell reproduces, the viral nucleic acid is also copied so each new cell is infected.

4. the virus becomes active and enters the lytic cycle.

5. Assembly,lysis, & release- new viral nucleic acids & protein coats are assembled. The virus exits the host cell.

Example- AIDS, Herpes

Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles

Bacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles that can attack other cells

Bacteriophage DNA inserts itself into bacterial chromosome

Bacteriophage DNA (prophage) may replicate with bacterium for many generations

Bacteriophage DNA (prophage) can exit the bacterial chromosome

Bacteriophage DNA forms a circle

Bacteriophage DNA

Bacterialchromosome

Bacteriophage injects DNA into bacterium

Prophage

Lytic Cycle

Lysogenic Cycle

Section 19-3

Figure 19-15 A Lysogenic Infection

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Section 19-3

Common Diseases Caused by Viruses

Oncogenic viruses

Retroviruses

Adenoviruses

Herpesviruses

Poxviruses

DNA

RNA

DNA

DNA

DNA

cancer

cancer, AIDS

respiratory infections

chickenpox

smallpox

Type of Virus Nucleic Acid Disease

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