19lecturepresentation 110329072641-phpapp02

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Chapter 19
Viruses

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Overview: A Borrowed Life
• Viruses called bacteriophages can infect and
set in motion a genetic takeover of bacteria,
such as Escherichia coli
• Viruses lead “a kind of borrowed life” between
life-forms and chemicals
• The origins of molecular biology lie in early
studies of viruses that infect bacteria

Concept 19.1: A virus consists of a nucleic acid
surrounded by a protein coat
• Viruses were detected indirectly long before
they were actually seen

The Discovery of Viruses: Scientific Inquiry
• Tobacco mosaic disease stunts growth of
tobacco plants and gives their leaves a mosaic
coloration
• In the late 1800s, researchers hypothesized
that a particle smaller than bacteria caused the
disease
• In 1935, Wendell Stanley confirmed this
hypothesis by crystallizing the infectious
particle, now known as tobacco mosaic virus
(TMV)

Fig. 19-2
RESULTS
1 2 3Extracted sap
from tobacco
plant with
tobacco
mosaic
disease
Passed sap
through a
porcelain
filter known
to trap
bacteria
Rubbed filtered
sap on healthy
tobacco plants
4 Healthy plants
became infected

Structure of Viruses
• Viruses are not cells
• Viruses are very small infectious particles
consisting of nucleic acid enclosed in a protein
coat and, in some cases, a membranous
envelope

Viral Genomes
• Viral genomes may consist of either
– Double- or single-stranded DNA, or
– Double- or single-stranded RNA
• Depending on its type of nucleic acid, a virus is
called a DNA virus or an RNA virus

Capsids and Envelopes
• A capsid is the protein shell that encloses the
viral genome
• Capsids are built from protein subunits called
capsomeres
• A capsid can have various structures

Fig. 19-3
RNA
Capsomere
Capsomere
of capsid
DNA
Glycoprotein
18 × 250 nm 70–90 nm (diameter)
Glycoproteins
80–200 nm (diameter) 80 × 225 nm
Membranous
envelope RNA
Capsid
Head
DNA
Tail
sheath
Tail
fiber
50 nm50 nm50 nm20 nm
(a) Tobacco mosaic
virus
(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

Fig. 19-3a
(a) Tobacco mosaic
virus
20 nm
18 × 250 nm
Capsomere
of capsid
RNA

Fig. 19-3b
DNA
Capsomere
Glycoprotein
70–90 nm (diameter)
50 nm
(b) Adenoviruses

Fig. 19-3c
Membranous
envelope RNA
Capsid
Glycoproteins
80–200 nm (diameter)
50 nm
(c) Influenza viruses

Fig. 19-3d
Head
DNA
Tail
sheath
Tail
fiber
80 × 225 nm
50 nm
(d) Bacteriophage T4

• Some viruses have membranous envelopes
that help them infect hosts
• These viral envelopes surround the capsids of
influenza viruses and many other viruses found
in animals
• Viral envelopes, which are derived from the
host cell’s membrane, contain a combination of
viral and host cell molecules

• Bacteriophages, also called phages, are
viruses that infect bacteria
• They have the most complex capsids found
among viruses
• Phages have an elongated capsid head that
encloses their DNA
• A protein tail piece attaches the phage to the
host and injects the phage DNA inside

Concept 19.2: Viruses reproduce only in host cells
• Viruses are obligate intracellular parasites,
which means they can reproduce only within a
host cell
• Each virus has a host range, a limited number
of host cells that it can infect

General Features of Viral Reproductive Cycles
• Once a viral genome has entered a cell, the
cell begins to manufacture viral proteins
• The virus makes use of host enzymes,
ribosomes, tRNAs, amino acids, ATP, and
other molecules
• Viral nucleic acid molecules and capsomeres
spontaneously self-assemble into new viruses
Animation: Simplified Viral Reproductive CycleAnimation: Simplified Viral Reproductive Cycle

Transcription
and manufacture
of capsid proteins
Self-assembly of
new virus particles
and their exit from
the cell
Entry and
uncoating
Fig. 19-4
VIRUS1
2
3
DNA
Capsid
4
Replication
HOST CELL
Viral DNA
mRNA
Capsid
proteins
Viral DNA

Reproductive Cycles of Phages
• Phages are the best understood of all viruses
• Phages have two reproductive mechanisms:
the lytic cycle and the lysogenic cycle

The Lytic Cycle
• The lytic cycle is a phage reproductive cycle
that culminates in the death of the host cell
• The lytic cycle produces new phages and
digests the host’s cell wall, releasing the
progeny viruses
• A phage that reproduces only by the lytic cycle
is called a virulent phage
• Bacteria have defenses against phages,
including restriction enzymes that recognize
and cut up certain phage DNA
Animation: Phage T4 Lytic CycleAnimation: Phage T4 Lytic Cycle

Fig. 19-5-2
Entry of phage
DNA and
degradation of
host DNA
Attachment1
2

Fig. 19-5-3
Synthesis of viral
genomes and
proteins
Entry of phage
DNA and
degradation of
host DNA
Attachment1
2
3

Fig. 19-5-4
Phage assembly
Assembly
Synthesis of viral
genomes and
proteins
Entry of phage
DNA and
degradation of
host DNA
Attachment1
2
4
Head Tail Tail fibers
3

Fig. 19-5-5
Phage assembly
Head Tail Tail fibers
Assembly
Release
Synthesis of viral
genomes and
proteins
Entry of phage
DNA and
degradation of
host DNA
Attachment1
2
4
5
3

The Lysogenic Cycle
• The lysogenic cycle replicates the phage
genome without destroying the host
• The viral DNA molecule is incorporated into the
host cell’s chromosome
• This integrated viral DNA is known as a
prophage
• Every time the host divides, it copies the phage
DNA and passes the copies to daughter cells
Animation: Phage Lambda Lysogenic and Lytic CyclesAnimation: Phage Lambda Lysogenic and Lytic Cycles

• An environmental signal can trigger the virus
genome to exit the bacterial chromosome and
switch to the lytic mode
• Phages that use both the lytic and lysogenic
cycles are called temperate phages

Fig. 19-6
Phage
DNA
Phage
The phage injects its DNA.
Bacterial
chromosome
Phage DNA
circularizes.
Daughter cell
with prophage
Occasionally, a prophage
exits the bacterial
chromosome,
initiating a lytic cycle.
Cell divisions
produce
population of
bacteria infected
with the prophage.
The cell lyses, releasing phages.
Lytic cycle
Lytic cycle
is induced
or
Lysogenic cycle
is entered
Lysogenic cycle
Prophage
The bacterium reproduces,
copying the prophage and
transmitting it to daughter cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
New phage DNA and proteins
are synthesized and
assembled into phages.

Reproductive Cycles of Animal Viruses
• There are two key variables used to classify
viruses that infect animals:
– DNA or RNA?
– Single-stranded or double-stranded?

Viral Envelopes
• Many viruses that infect animals have a
membranous envelope
• Viral glycoproteins on the envelope bind to
specific receptor molecules on the surface of a
host cell
• Some viral envelopes are formed from the host
cell’s plasma membrane as the viral capsids
exit

• Other viral membranes form from the host’s
nuclear envelope and are then replaced by an
envelope made from Golgi apparatus
membrane

Fig. 19-7
Capsid
RNA
Envelope (with
glycoproteins)
Capsid and viral genome
enter the cell
HOST CELL
Viral genome (RNA)
Template
mRNA
ER
Glyco-
proteins
Capsid
proteins Copy of
genome (RNA)
New virus

RNA as Viral Genetic Material
• The broadest variety of RNA genomes is found
in viruses that infect animals
• Retroviruses use reverse transcriptase to
copy their RNA genome into DNA
• HIV (human immunodeficiency virus) is the
retrovirus that causes AIDS (acquired
immunodeficiency syndrome)

Fig. 19-8
Glycoprotein Viral envelope
Capsid
RNA (two
identical
strands)Reverse
transcriptase HIV
HIV
Membrane of
white blood cell
HIV entering a cell
0.25 µm
Viral RNA
RNA-DNA
hybrid
HOST CELL
Reverse
transcriptase
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
mRNA
New virus
New HIV leaving a cell

Fig. 19-8a
Glycoprotein
Reverse
transcriptase HIV
RNA (two
identical
strands)
Capsid
Viral envelope
HOST CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
mRNA
New virus

Fig. 19-8b
HIV
Membrane of
white blood cell
HIV entering a cell
0.25 µm
New HIV leaving a cell

• The viral DNA that is integrated into the host
genome is called a provirus
• Unlike a prophage, a provirus remains a
permanent resident of the host cell
• The host’s RNA polymerase transcribes the
proviral DNA into RNA molecules
• The RNA molecules function both as mRNA for
synthesis of viral proteins and as genomes for
new virus particles released from the cell
Animation: HIV Reproductive CycleAnimation: HIV Reproductive Cycle

Evolution of Viruses
• Viruses do not fit our definition of living
organisms
• Since viruses can reproduce only within cells,
they probably evolved as bits of cellular nucleic
acid
• Candidates for the source of viral genomes are
plasmids, circular DNA in bacteria and yeasts,
and transposons, small mobile DNA segments
• Plasmids, transposons, and viruses are all
mobile genetic elements

• Mimivirus, a double-stranded DNA virus, is the
largest virus yet discovered
• There is controversy about whether this virus
evolved before or after cells

Concept 19.3: Viruses, viroids, and prions are
formidable pathogens in animals and plants
• Diseases caused by viral infections affect
humans, agricultural crops, and livestock
worldwide
• Smaller, less complex entities called viroids
and prions also cause disease in plants and
animals, respectively

Viral Diseases in Animals
• Viruses may damage or kill cells by causing the
release of hydrolytic enzymes from lysosomes
• Some viruses cause infected cells to produce
toxins that lead to disease symptoms
• Others have envelope proteins that are toxic

• Vaccines are harmless derivatives of
pathogenic microbes that stimulate the immune
system to mount defenses against the actual
pathogen
• Vaccines can prevent certain viral illnesses
• Viral infections cannot be treated by antibiotics
• Antiviral drugs can help to treat, though not
cure, viral infections

Emerging Viruses
• Emerging viruses are those that appear
suddenly or suddenly come to the attention of
scientists
• Severe acute respiratory syndrome (SARS)
recently appeared in China
• Outbreaks of “new” viral diseases in humans
are usually caused by existing viruses that
expand their host territory

• Flu epidemics are caused by new strains of
influenza virus to which people have little
immunity
• Viral diseases in a small isolated population
can emerge and become global
• New viral diseases can emerge when viruses
spread from animals to humans
• Viral strains that jump species can exchange
genetic information with other viruses to which
humans have no immunity

• These strains can cause pandemics, global
epidemics
• The “avian flu” is a virus that recently appeared
in humans and originated in wild birds

Fig. 19-9
(a) The 1918 flu pandemic
(b) Influenza A
H5N1 virus
(c) Vaccinating ducks
0.5 µm

Fig. 19-9a
(a) The 1918 flu pandemic

Fig. 19-9b
(b) Influenza A H5N1
virus
0.5 µm

Fig. 19-9c
(c) Vaccinating ducks

Viral Diseases in Plants
• More than 2,000 types of viral diseases of
plants are known and cause spots on leaves
and fruits, stunted growth, and damaged
flowers or roots
• Most plant viruses have an RNA genome

• Plant viruses spread disease in two major
modes:
– Horizontal transmission, entering through
damaged cell walls
– Vertical transmission, inheriting the virus from
a parent

Viroids and Prions: The Simplest Infectious
Agents
• Viroids are circular RNA molecules that infect
plants and disrupt their growth
• Prions are slow-acting, virtually indestructible
infectious proteins that cause brain diseases in
mammals
• Prions propagate by converting normal proteins
into the prion version
• Scrapie in sheep, mad cow disease, and
Creutzfeldt-Jakob disease in humans are all
caused by prions

Fig. 19-11
Prion
Normal
protein
Original
prion
New
prion
Aggregates
of prions

Fig. 19-UN1
Phage
DNA
Bacterial
chromosome
The phage attaches to a
host cell and injects its DNA
Prophage
Lysogenic cycle
• Temperate phage only
• Genome integrates into bacterial
chromosome as prophage, which
(1) is replicated and passed on to
daughter cells and
(2) can be induced to leave the
chromosome and initiate a lytic cycle
Lytic cycle
• Virulent or temperate phage
• Destruction of host DNA
• Production of new phages
• Lysis of host cell causes release
of progeny phages

Fig. 19-UN2
Time Time
A B
Numberofbacteria
Numberofviruses

You should now be able to:
1. Explain how capsids and envelopes are formed
2. Distinguish between the lytic and lysogenic
reproductive cycles
3. Explain why viruses are obligate intracellular
parasites
4. Describe the reproductive cycle of an HIV
retrovirus
5. Describe three processes that lead to the
emergence of new diseases
6. Describe viroids and prions

19lecturepresentation 110329072641-phpapp02

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