1. Chapter
Viruses
Ch 19
Viruses

2. Characteristics of Viruses
1. Tiny, nonliving particles
a. do not respire, move, grow
2. Must reproduce inside of a host
3. Pathogens: disease-causing
a. 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

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

4. Figure 19-13 Virus Structures
Section 19-3
Tobacco Mosaic
T4 Bacteriophage Virus
Influenza
DNA RNA Virus
Head
RNA
Capsid Capsid
proteins
Tail
sheath
Tail
fiber
Surface
proteins Membrane
envelope
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5. 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”

6. 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.

7. There are 2 ways viruses can reproduce
inside a host.
1. Lytic cycle-fast
2. Lysogenic cycle-slow

8. 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

9. 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

10. Figure 19-15 A Lysogenic Infection
Section 19-3
Bacteriophage Bacterial
DNA chromosome
Bacteriophage injects Bacteriophage DNA
DNA into bacterium (prophage) can exit the
bacterial chromosome
Lytic Lysogenic
Cycle Cycle
Bacteriophage enzyme lyses the Bacteriophage DNA Bacteriophage DNA
forms a circle (prophage) may replicate
bacterium’s cell wall, releasing
with bacterium for many
new bacteriophage particles that
generations
can attack other cells Prophage
Bacteriophage proteins and Bacteriophage DNA
nucleic acids assemble into inserts itself into
complete bacteriophage particles bacterial chromosome
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11. Common Diseases Caused by Viruses
Section 19-3
Type of Virus Nucleic Acid Disease
Oncogenic viruses DNA cancer
Retroviruses RNA cancer, AIDS
Adenoviruses DNA respiratory infections
Herpesviruses DNA chickenpox
Poxviruses DNA smallpox
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