18.2 Viral Structure and Reproduction
KEY CONCEPT
Viruses exist in a variety of shapes and sizes.
18.2 Viral Structure and Reproduction
Viruses differ in shape and in ways of entering host
cells.
• Viruses have a simple structure.
– genetic material
– capsid, a protein shell
– maybe a lipid envelope, a protective outer coat
enveloped
(influenza)
capsid
nucleic acid
lipid
envelope
helical
(rabies)
Surface proteins
capsid
nucleic acid
surface
proteins
lipid envelope
polyhedral
(foot-and-mouth
disease)
surface
proteins
capsid
nucleic acid
18.2 Viral Structure and Reproduction
• Bacteriophages infect bacteria.
capsid
DNA
tail sheath
tail fiber
18.2 Viral Structure and Reproduction
• Viruses enter cells in various ways.
– bacteriophages pierce host cells
colored SEM; magnifications:
large photo 25,000; inset 38,000x
18.2 Viral Structure and Reproduction
• Viruses enter cells in various ways.
– viruses of eukaryotes enter by endocytosis
18.2 Viral Structure and Reproduction
• Viruses enter cells in various ways.
– viruses of eukaryotes also fuse with membrane
18.2 Viral Structure and Reproduction
Viruses cause two types of infections.
• A lytic infection causes the host cell to burst.
host bacterium
Event 1
Event 4
The host bacterium breaks apart,
or lyses. Bacteriophages are able
to infect new host cells.
The bacterophage attaches
and injects it DNA into a host
bacterium.
Event 2
The viral DNA
forms a circle.
Event 3
The viral DNA directs the host
cell to produce new viral parts.
The parts assemble into new
bacteriophages.
The virus may enter the
lysogenic cycle, in which the
host cell is not destroyed.
18.2 Viral Structure and Reproduction
• A lysogenic infection does no immediate harm.
– Event 1 was injection - shown on previous slide.
The prophage may leave the
host’s DNA and enter the
lytic cycle.
Event 2
The viral DNA is called a prophage
when it combines with
the host cell’s DNA.
Triggered by
environmental
factors such as
stress.
Event 3
Event 4
Many cell divisions produce a
colony of bacteria infected
with prophage.
Although the prophage is not
active, it replicates along with
the host cell’s DNA.
18.2 Viral Structure and Reproduction
Viruses - The Bad House Guests
• Using the analogy of viral infections resembling houseguests
described on page 515, explain which describes a lytic infection and
which describes a lysogenic infection.
– Lytic infection is like the bad house guest that eats all the food
and blows up the house on the way out because a lytic virus
destroys the host cell in the end.
– A lysogenic infection is like the houseguest that never leaves.
– Neither is good for the host because in the end they can both be
harmful.
18.2 Viral Structure and Reproduction
Interpreting Models – Lytic and Lysogenic cycle
D
A
C
F
H
B
E
G
18.2 Viral Structure and Reproduction
Two possible paths for a lysogenic viral infection.
•
Viruses in the lysogenic phase are sometimes called dormant or hidden
because while they are part of the host cell’s DNA, they are not active or
infectious. What are the two possible paths this type of virus can take after
the viral DNA becomes incorporated?
– They can stay “hidden” or dormant and remain incorporated in the host
cell’s DNA. It will not affect the host but will be copied every time the cell
divides.
– A “trigger” such as stress can cause the virus to become activated and
enter the lytic cycle. When this happens it can now destroy the host cell.
– Shingles is an example of this type of virus.