Download HOST CELL

CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
17
Viruses
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
© 2014 Pearson Education, Inc.
Overview: A Borrowed Life
 A virus is an infectious particle consisting of little
more than genes packaged into a protein coat
 Viruses lead “a kind of borrowed life,” existing in a
shady area between life-forms and chemicals
© 2014 Pearson Education, Inc.
Figure 17.1
0.25 m
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.2
RNA
Capsomere
DNA
Membranous
envelope
RNA
Capsid
Head
Capsomere
of capsid
DNA
Tail
sheath
Tail
fiber
Glycoprotein
18  250 nm
70–90 nm (diameter)
20 nm
50 nm
(a) Tobacco mosaic
(b) Adenoviruses
virus
© 2014 Pearson Education, Inc.
Glycoproteins
80–200 nm (diameter)
50 nm
(c) Influenza viruses
80  225 nm
50 nm
(d) Bacteriophage T4
Figure 17.2a
RNA
Capsomere
DNA
Capsomere
of capsid
18  250 nm
© 2014 Pearson Education, Inc.
Glycoprotein
70–90 nm (diameter)
20 nm
50 nm
(a) Tobacco mosaic
(b) Adenoviruses
virus
 Some viruses have membranous envelopes that
help them infect hosts
 These viral envelopes are derived from the host
cell’s membrane and contain a combination of viral
and host cell molecules
© 2014 Pearson Education, Inc.
 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
© 2014 Pearson Education, Inc.
Concept 17.2: Viruses replicate only in host cells
 Viruses are obligate intracellular parasites, which
means they can replicate only within a host cell
 Each virus has a host range, a limited number of
host cells that it can infect
© 2014 Pearson Education, Inc.
General Features of Viral Replicative 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
 These exit from the host cell, usually damaging or
destroying it
Animation: Simplified Viral Replicative Cycle
© 2014 Pearson Education, Inc.
Figure 17.3
DNA
1 Entry and
uncoating
VIRUS
3 Transcription and
Capsid
2 Replication
manufacture of
capsid proteins
HOST
CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
4 Self-assembly of
new virus particles
and their exit from
the cell
© 2014 Pearson Education, Inc.
Replicative Cycles of Phages
 Phages are the best understood of all viruses
 Phages have two reproductive mechanisms: the
lytic cycle and the lysogenic cycle
© 2014 Pearson Education, Inc.
The Lytic Cycle
 The lytic cycle is a phage replicative cycle that
culminates in the death of the host cell
 The lytic cycle produces new phages and lyses
(breaks open) 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 Cycle
© 2014 Pearson Education, Inc.
Figure 17.4-1
1 Attachment
© 2014 Pearson Education, Inc.
Figure 17.4-2
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
© 2014 Pearson Education, Inc.
Figure 17.4-3
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
3 Synthesis of
viral genomes
and proteins
© 2014 Pearson Education, Inc.
Figure 17.4-4
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
Phage assembly
4 Assembly
Head
Tail
Tail
fibers
© 2014 Pearson Education, Inc.
3 Synthesis of
viral genomes
and proteins
Figure 17.4-5
1 Attachment
2 Entry of phage
5 Release
DNA and
degradation
of host DNA
Phage assembly
4 Assembly
Head
Tail
Tail
fibers
© 2014 Pearson Education, Inc.
3 Synthesis of
viral genomes
and proteins
The Lysogenic Cycle
 The lysogenic cycle replicates the phage genome
without destroying the host
 Phages that use both the lytic and lysogenic cycles
are called temperate phages
 The viral DNA molecule is incorporated into the
host cell’s chromosome
 This integrated viral DNA is known as a prophage
© 2014 Pearson Education, Inc.
 Every time the host divides, it copies the phage DNA
and passes the copies to daughter cells
 A single infected cell can give rise to a large
population of bacteria carrying the virus in prophage
form
 An environmental signal can trigger the virus genome
to exit the bacterial chromosome and switch to the
lytic mode
Animation: Phage  Lysogenic and Lytic Cycles
© 2014 Pearson Education, Inc.
Figure 17.5a
Phage
DNA
The phage injects its DNA.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Lytic cycle
The cell lyses, releasing phages.
Phage DNA and proteins are
synthesized and assembled.
© 2014 Pearson Education, Inc.
Figure 17.5b
Daughter cell
with prophage
Many cell
divisions
create many
infected
bacteria.
Prophage exits
chromosome.
Lysogenic cycle
Prophage
Prophage is copied
with bacterial
chromosome.
Phage DNA integrates into
bacterial chromosome.
© 2014 Pearson Education, Inc.
Replicative Cycles of Animal Viruses
 There are two key variables used to classify viruses
that infect animals
 The nature of the viral genome (single- or doublestranded DNA or RNA)
 The presence or absence of an envelope
© 2014 Pearson Education, Inc.
Viral Envelopes
 An animal virus with an envelope uses it to enter the
host cell
 The envelope is derived from the plasma membrane
of a host cell, although some of the molecules on the
envelope are specified by the genome of the virus
© 2014 Pearson Education, Inc.
Figure 17.6
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Template
Viral genome
(RNA)
mRNA
ER
Capsid
proteins
Copy of genome
(RNA)
Glycoproteins
© 2014 Pearson Education, Inc.
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)
© 2014 Pearson Education, Inc.
 Viral DNA that is integrated into the host genome is
called a provirus
 Unlike a prophage, a provirus is 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
viruses released from the cell
Animation: HIV Replicative Cycle
© 2014 Pearson Education, Inc.
Figure 17.7
Glycoprotein
Viral envelope
HIV
Membrane of
white blood cell
Capsid
HIV
Reverse
transcriptase
RNA (two
identical
strands)
HOST
CELL
Viral RNA
Reverse
transcriptase
RNA-DNA
hybrid
0.25 m
HIV entering a cell
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the next
mRNA
viral generation
New virus
New HIV leaving a cell
© 2014 Pearson Education, Inc.
Figure 17.7a
Glycoprotein
Viral envelope
Capsid
HIV
Reverse
transcriptase
RNA (two
identical
strands)
HOST
CELL
Viral RNA
Reverse
transcriptase
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the next
mRNA
viral generation
New virus
© 2014 Pearson Education, Inc.
Figure 17.7aa
Viral envelope
Glycoprotein
Capsid
HIV
Reverse
transcriptase
RNA (two
identical
strands)
HOST
CELL
Viral RNA
RNA-DNA
hybrid
DNA
© 2014 Pearson Education, Inc.
Reverse
transcriptase
Figure 17.7ab
NUCLEUS
Chromosomal
DNA
RNA genome
for the next
mRNA
viral generation
New virus
© 2014 Pearson Education, Inc.
Provirus
Figure 17.7b
HIV
Membrane of
white blood cell
0.25 m
HIV entering a cell
© 2014 Pearson Education, Inc.
New HIV leaving a cell
Evolution of Viruses
 Viruses do not fit our definition of living organisms
 Since viruses can replicate only within cells, they
probably evolved after the first cells appeared
 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
© 2014 Pearson Education, Inc.
Concept 17.3: Viruses are formidable pathogens
in animals and plants
 Diseases caused by viral infections afflict humans,
agricultural crops, and livestock worldwide
© 2014 Pearson Education, Inc.
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 molecular components such as
envelope proteins that are toxic
© 2014 Pearson Education, Inc.
 A vaccine is a harmless derivative of a pathogen
that stimulates the immune system to mount
defenses against the harmful 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
© 2014 Pearson Education, Inc.
Emerging Viruses
 Viruses that suddenly become apparent are called
emerging viruses.
 HIV is a classic example
 The West Nile virus appeared in North America
first in 1999 and has now spread to all 48
contiguous states
© 2014 Pearson Education, Inc.
 In 2009 a general outbreak, or epidemic, of a flulike illness occurred in Mexico and the United
States; the virus responsible was named H1N1
 H1N1 spread rapidly, causing a pandemic, or global
epidemic
© 2014 Pearson Education, Inc.
Figure 17.8
1 m
(a) 2009 pandemic H1N1
influenza A virus
© 2014 Pearson Education, Inc.
(b) 2009 pandemic screening
Figure 17.8a
1 m
(a) 2009 pandemic H1N1
influenza A virus
© 2014 Pearson Education, Inc.
 Three processes contribute to the emergence of viral
diseases
 The mutation of existing viruses, which is especially
high in RNA viruses
 Dissemination of a viral disease from a small, isolated
human population, allowing the disease to go
unnoticed before it begins to spread
 Spread of existing viruses from animal populations;
about three-quarters of new human diseases
originate this way
© 2014 Pearson Education, Inc.
 Strains of influenza A are given standardized names
 The name H1N1 identifies forms of two viral surface
proteins, hemagglutinin (H) and neuraminidase (N)
 There are numerous types of hemagglutinin and
neuraminidase, identified by numbers
© 2014 Pearson Education, Inc.
Viral Diseases in Plants
 More than 2,000 types of viral diseases of plants
are known; these have enormous impacts on the
agricultural and horticultural industries
 Plant viruses have the same basic structure and
mode of replication as animal viruses
 Most plant viruses known thus far have an RNA
genome and many have a helical capsid
© 2014 Pearson Education, Inc.
 Plant viral diseases spread by two major routes
 Infection from an external source of virus is called
horizontal transmission
 Herbivores, especially insects, pose a double threat
because they can both carry a virus and help it get
past the plant’s outer layer of cells
 Inheritance of the virus from a parent is called
vertical transmission
© 2014 Pearson Education, Inc.
Figure 17.UN01
© 2014 Pearson Education, Inc.
Figure 17.UN02a
A/California/07/2009 Group 1
A/Taiwan/1164/2010
A/Taiwan/T1773/2009 Group 6
A/Taiwan/T1338/2009
A/Taiwan/T0724/2009
A/Taiwan/T1821/2009
A/Taiwan/937/2009
A/Taiwan/T1339/2009
A/Taiwan/940/2009
A/Taiwan/7418/2009
A/Taiwan/8575/2009
A/Taiwan/4909/2009
A/Taiwan/8542/2009
Group 3
Group 7
A/Taiwan/1018/2011
A/Taiwan/552/2011
Group 9
A/Taiwan/2826/2009
A/Taiwan/T0826/2009
A/Taiwan/1017/2009
A/Taiwan/7873/2009
A/Taiwan/11706/2009
Group 8
A/Taiwan/6078/2009
A/Taiwan/6341/2009
A/Taiwan/6200/2009
A/Taiwan/5270/2010
Group 8-1
A/Taiwan/3994/2010
A/Taiwan/2649/2011
Group 10
A/Taiwan/1102/2011
A/Taiwan/4501/2011
A/Taiwan/67/2011
A/Taiwan/1749/2011
A/Taiwan/4611/2011
A/Taiwan/5506/2011
Group 11
A/Taiwan/1150/2011
A/Taiwan/2883/2011
A/Taiwan/842/2010
A/Taiwan/3697/2011
© 2014 Pearson Education, Inc.
Figure 17.UN03
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Bacterial
chromosome
Lytic cycle
• Virulent or temperate phage
• Destruction of host DNA
• Production of new phages
• Lysis of host cell causes release
of progeny phages
© 2014 Pearson Education, Inc.
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