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Chapter 19
Viruses
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Structure of Viruses
• Viruses:
– not cells
– very small infectious particles:
• nucleic acid (DNA or RNA)
• protein coat (caspid)
• (in some cases) membranous envelope
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Capsids and Envelopes
• Capsid- protein shell that encloses the viral
genome
– can have various structures
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 19-3
RNA
DNA
Capsomere
Membranous
envelope
RNA
Head
DNA
Capsid
Tail
sheath
Capsomere
of capsid
Glycoproteins
Glycoprotein
18  250 nm
70–90 nm (diameter) 80–200 nm (diameter)
20 nm
50 nm
50 nm
Tail
fiber
80  225 nm
50 nm
(a) Tobacco mosaic (b) Adenoviruses
virus
(c) Influenza viruses (d) Bacteriophage T4
-Helical capsid
-8 RNA molecules
Wrapped in Helical
Caspid
-Icoshedral caspid
-Icosahedral head and
Tail apparatus
Fig. 19-3d
Head
DNA
Tail
sheath
Tail
fiber
80  225 nm
50 nm
(d) Bacteriophage T4
Fig. 19-1
0.5 µm
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Bacteriophages (phages)-
• Viruses that infect bacteria
• Most complex capsids found among viruses
Elongated capsid
head that encloses
their DNA
Protein tail piece attaches the
phage to the host and injects the
phage DNA inside
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Reproductive Cycles of Phages
• Phages- best understood of all viruses
– Ex: Phage 
• Phages have two reproductive mechanisms:
– Lytic cycle
– Lysogenic cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Lytic Cycle
• Lytic cycle• phage reproductive cycle that culminates in the
death of the host cell
• produces new phages and digests the host’s
cell wall, releasing the progeny viruses
Animation: Phage T4 Lytic Cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Lytic cycle
• 1. The phage injects its DNA
• 2. Phage DNA circularizes
1.
2.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Lytic Cycle
• Environmental cues =
LYTIC cycle is induced
• 3. New phage DNA
and proteins are
synthesized and
assembled into
phages.
1.
4.
• 4. The cell lyses,
releasing phages
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
2.
3.
LYTIC
The Lysogenic Cycle
• Lysogenic cycle replicates the phage genome
without destroying the host
• Viral DNA molecule incorporated into the host
cell’s chromosome
– Integrated viral DNA = prophage
• Every time the host divides, it copies the phage
DNA and passes the copies to daughter cells
Animation: Phage Lambda Lysogenic and Lytic Cycles
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 1. The phage injects its DNA
• 2. Phage DNA circularizes
1.
2.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Environmental cue = Lysogenic cycle induced
• 3. Phage DNA integrates into the bacterial
chromosome, becoming a Prophage
1.
2.
Lysogenic
3.
Prophage
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Environmental cue = Lysogenic cycle induced
• 4. The bacterium reproduces, copying the
prophage and transmitting it to daughter cells
Daughter cell
with prophage
1.
2.
4.
Lysogenic
3.
Prophage
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Environmental cue = Lysogenic cycle induced
• 5. Occasionally, a prophage exits the bacterial
chromosome, initiating a lytic cycle.
Daughter cell
with prophage
1.
5.
2.
Lysogenic
4.
3.
Prophage
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 19-6
Phage
DNA
Daughter cell
with prophage
The phage injects its DNA.
Cell divisions
produce
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial
chromosome,
initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
The bacterium reproduces,
copying the prophage and
transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle
is induced
or
Lysogenic cycle
is entered
New phage DNA and proteins
are synthesized and
assembled into phages.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Reproductive Cycles of Animal Viruses
• Many viruses that infect animals have:
• RNA genome
• Membranous envelope
• Glycoproteins
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Reproductive Cycles of Animal Viruses
•
1. Glycoproteins on viral envelope bind to specific receptors on the
host cellentry
•
2. Caspid and viral genome enter the cell, caspid digested by host cell
enzymes
•
3. Viral genome serves as template for complementary RNA strands by
a viral enzyme
•
4. New copies of viral RNA genome are made using the
complementary strands of RNA by host enzyme
•
5. Complementary RNA strands also serve as mRNA which is
translated into caspid proteins and glycoproteins for the new viral
envelopes.
•
6. Vesicles transport glycoproteins to the plasma membrane of host
cell
•
7. A caspid assembles around viral genome molecule
•
8. Each new virus buds from the host cell with viral glycoproteins
embedded in membrane derived from the host cell.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 19-7
Capsid and viral genome
enter the cell
1.
Capsid
RNA
2.
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
5.
3.
mRNA
Capsid
proteins
ER
Glycoproteins
6.
4.
Copy of
genome (RNA)
7.
8.
New virus
Fig. 19-8b
HIV
Membrane of
white blood cell
0.25 µm
HIV entering a cell
New HIV leaving a cell
You should now be able to:
1. Describe virus structure
2. Describe and Distinguish between the lytic and
lysogenic reproductive cycles
3. Explain why viruses are obligate intracellular
parasites
4. Describe the generalized reproductive cycle of
an animal virus with a membranous envelope
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Phage replication: Lytic vs. Lysogenic
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings