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Chapter 17: Viruses
Following the Big Ideas
•
Evolution– Changes in DNA can lead to phenotypic variation which can be acted
upon by natural selection.
– Viral and bacterial genomes benefit from their ability to evolve rapidly.
• Information and Signaling– DNA, and in some cases RNA, is the primary source of inheritable
information.
– A variety of intercellular and intracellular signal transmissions mediate
gene expression.
– Gene regulation results in differential gene expression, leading to cell
specialization.
– Biological systems have multiple processes that increase genetic
variation.
– Viral replication results in genetic variation, and viral infection can
introduce genetic variation into the hosts.
• Interactions and Systems– Regulation of gene activity involves intricate interactions with internal
and external factors.
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
Viral Structures
Microbial Model Systems
• Viruses called bacteriophages can infect and set
in motion a genetic takeover of bacteria, such as
Escherichia coli
• E. coli and its viruses are called model systems
because of their frequent use by researchers in
studies that reveal broad biological principles
• Beyond their value as model systems, viruses
and bacteria have unique genetic mechanisms
that are interesting in their own right
A virus has a genome but can
reproduce only within a host cell
• Bacteria are prokaryotes with cells much smaller
and more simply organized than those of
eukaryotes
• Viruses are smaller and simpler than bacteria
• Scientists detected viruses indirectly long before
they could see them
• The story of how viruses were discovered begins
in the late 1800s
LE 18-2
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 µm
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)
Concept 17.1: A virus consists of a nucleic
acid surrounded by a protein coat
• Even the largest known virus is barely visible
under the light microscope
• Some viruses can be crystalized
• See Bozeman video on viruses:
– https://www.youtube.com/watch?v=L8oHs7G_syI
Structure of Viruses/ Viral Genome
• 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 may consist of
– Double- or single-stranded DNA
– Double- or single-stranded RNA
• Depending on its type of nucleic acid, a virus is
called a DNA virus or an RNA virus
LE 18-4a
Capsomere
of capsid
RNA
18  250 mm
20 nm
Tobacco mosaic virus
Capsids and Envelopes
♥ A capsid is the protein
shell that encloses the
viral genome
♥ A capsid can have
various structures
♥ Capsomeres make up
capsids!
LE 18-4b
Capsomere
DNA
Glycoprotein
70–90 nm (diameter)
50 nm
Adenoviruses
• Some viruses have structures 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
LE 18-4c
Membranous
envelope
Capsid
RNA
Glycoprotein
80–200 nm (diameter)
50 nm
Influenza viruses
• 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 tailpiece attaches the phage to the
host and injects the phage DNA inside
LE 18-4d
Head
Tail
sheath
Tail
fiber
DNA
80  225 nm
50 nm
Bacteriophage T4
Concept 17.2: Viruses replicate 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, recognized by
compatible receptor proteins on the virus and the
host
• Viruses use enzymes, ribosomes, and small host
molecules to synthesize progeny viruses
Animation: Simplified Viral Reproductive Cycle
LE 18-5
Entry into cell and
uncoating of DNA
VIRUS
DNA
Capsid
Transcription
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Self-assembly of
new virus particles
and their exit from cell
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
LE 18-8
Capsid
Capsid and viral genome
enter cell
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
ER
Glycoproteins
Capsid
proteins
Copy of
genome (RNA)
New virus
Reproductive Cycles of Phages
• Phages are the best understood of all viruses
• Phages have two reproductive mechanisms: the
lytic cycle and the lysogenic cycle
• See Bozeman video on viral replication:
– https://www.youtube.com/watch?v=EqK1CYYQIug&l
ist=PLFCE4D99C4124A27A&index=44
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
LE 18-6
Attachment
Phage assembly
Head
Tails
Release
Entry of phage DNA
and degradation of
host DNA
Tail fibers
Assembly
Synthesis of viral
genomes and proteins
The Lysogenic Cycle
• The lysogenic cycle replicates the phage genome
without destroying the host
• The viral DNA molecule is incorporated by genetic
recombination 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
• Phages that use both the lytic and lysogenic cycles
are called temperate phages
LE 18-7
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Daughter cell
with prophage
Many cell divisions
produce a large
population of
bacteria infected with
the prophage.
Phage DNA
circularizes
Phage
Bacterial
chromosome
Lytic cycle
The cell lyses, releasing phages.
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lysogenic cycle
Certain factors
determine whether
Lytic cycle or Lysogenic cycle
is induced
is entered
New phage DNA and proteins are
synthesized and assembled into phages.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Prophage
Phage DNA integrates into the
bacterial chromosomes, becoming a
prophage.
Reproductive Cycles of Animal Viruses
• Two key variables in classifying viruses that
infect animals:
– DNA or RNA?
– Single-stranded or double-stranded?
Class/Family Envelope Examples/Disease
I. Double-stranded DNA (dsDNA)
No
Respiratory diseases, animal tumors
Papovavirus No
Papillomavirus (warts, cervical
cancer): polyomavirus (animal
tumors)
Herpes simplex I and II (cold sores,
genital sores); varicella zoster
(shingles, chicken pox); Epstein-Barr
virus (mononucleosis, Burkitt’s
lymphoma)
Smallpox virus, cowpox virus
Adenovirus
Herpes virus Yes
Pox virus
Yes
Class/Family
Envelope Examples/Disease
II. Single-stranded DNA (ssDNA)
Parvovirus
No
B19 parvovirus (mild rash)
III. Double-stranded RNA (dsRNA)
Reovirus
No
Rotavirus (diarrhea), Colorado
tick fever virus
Class/Family
Envelope Examples/Disease
IV. Single-stranded RNA (ssRNA); serves as mRNA
Picornavirus
No
Coronavirus
Yes
Flavivirus
Yes
Togavirus
Yes
Rhinovirus (common cold);
poliovirus, hepatitis A virus, and
other enteric (intestinal) viruses
Severe acute respiratory
syndrome (SARS)
Yellow fever virus, West Nile
virus, hepatitis C virus
Rubella virus, equine
encephalitis viruses
Class/Family
Envelope Examples/Disease
V. ssRNA; template for mRNA synthesis
Filovirus
Yes
Ebola virus (hemorrhagic fever)
Orthomyxovirus Yes
Influenza virus
Paramyxovirus
Yes
Measles virus; mumps virus
Rhabdovirus
Yes
Rabies virus
VI. ssRNA; template for DNA synthesis
Retrovirus
Yes
HIV (AIDS); RNA tumor viruses
(leukemia)
RNA as Viral Genetic Material
• The broadest variety of RNA genomes is found in viruses that
infect animals
• RNA viruses lack replication error-checking mechanisms, thus
have higher rates of mutation.
• Related viruses can combine/recombine information if they
infect the same host cell. (flu)
• Retroviruses use reverse transcriptase to copy their RNA
genome into DNA
• Reverse transcriptase is a type of polymerase that transcribes
DNA from an RNA template.
• HIV is the retrovirus that causes AIDS- rapidly evolves!
• Retroviruses have the most complicated reproductive cycles
because of the reverse transcriptase stage.
LE 18-9
Glycoprotein
HIV
Reverse
transcriptase
Viral envelope
Capsid
RNA
(two identical
strands)
• 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
LE 18-10
Proviral
genes are
transcribed
into RNA, the
genomes for
the next HIV
generation
and mRNA
for the
translation
into viral
proteins.
HIV
Membrane of
white blood cell
HOST CELL
Reverse
transcription
Viral RNA
0.25 µm
HIV entering a cell
RNA-DNA
hybrid
DNA
NUCLEUS
The viral
proteins
include
capsid
proteins and
reverse
transcriptase
(made in the
cytosol) and
envelop
glycoproteins
(made in the
ER)
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
New HIV leaving a cell
mRNA
Reverse
transcriptase
catalyzes the
synthesis of a
DNA strand
complementary
to the viral RNA.
Reverse
Transcriptase
then catalyzes
the synthesis of
a second DNA
strand
complementary
to the first.
The double
stranded DNA
incorporates as
a provirus.
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
• Coevolution exists between virus and host.
– Defenses that bacteria have against invasion from phage
infection• Mutant receptor sits no longer recognized by phage
proteins
• Enzymes that digest foreign DNA evolved (restriction
endonucleases- cut up foreign DNA but not host DNA)
• As bacteria evolve to resist viral infection, the virus
evolves to become pathogenic again.
• Lysogenic cycle has evolved to allow bacteria and virus to
co-exist for a time before virus becomes pathogenic.
Concept 17.3: Viruses are formidable
pathogens in animals and plants
• Diseases caused by viral infections afflict
humans, agricultural crops, and livestock
worldwide
– Damage or kill cells by releasing hydrolytic enzymes
from lysosomes.
– Causes infected cells to produce toxins that lead to
disease symptoms
– Some viruses have toxic components, such as
envelope proteins
– Many symptoms are due to the body’s attempt at
defense.
Vaccines and Antiviral Drugs
• 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
• Antiviral drugs work by interfering with viral nucleic
acid synthesis
Emerging Viruses
• Emerging viruses are those that appear suddenly or
suddenly come to the attention of scientists
• Emergent diseases are scientific evidence that
evolution continues to occur!
• 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
• In 2009 a general outbreak, or epidemic, of a flu-like illness
occurred in Mexico and the United States; the virus
responsible was named H1N1
• H1N1 spread rapidly, causing a pandemic, or global epidemic
• Severe acute respiratory syndrome (SARS) recently
appeared in China
Figure 17.8
1 m
(a) 2009 pandemic H1N1
influenza A virus
(b) 2009 pandemic screening
LE 18-11
Young ballet students in Hong
Kong wear face masks to
protect themselves from the
virus causing SARS.
The SARS-causing agent is a
coronavirus like this one
(colorized TEM), so named for
the “corona” of glyco-protein
spikes protruding form the
envelope.
• 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
• 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
Viruses and Cancer
• Tumor viruses- All tumor viruses transform
cells into cancer cells through the integration
of viral nucleic acid into host cell DNA.
Through a variety of possible mechanisms, cell
cycle control is altered
Viral Group
Ex. /Diseases
Cancer Type
Retrovirus
HTLV-1
Adult leukemia
Herpesvirus
EpsteinBarr/infectious
mononucleosis
Burkitt’s
Lymphoma
Papovavirus
Papilloma/human
warts
Cervical cancer
Hepatitis B virus
Chronic hepatitis
Liver cancer
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
• Some symptoms are spots on leaves and fruits, stunted growth, and
damaged flowers or roots
• Plant viruses spread disease in two major
modes:
– Horizontal transmission, entering through damaged
cell walls (infection is from an outside source- wind)
– 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
– Vertical transmission, inheriting the virus from a
parent (virus spreads through plasmodesmata).
Connecting the Concepts
to the Big Ideas
• Evolution– All living things, and viruses, are subject to genetic
mutations or changes in the base sequence of
DNA (or RNA in the case of viruses); this is a
source of genetic variability that is acted upon by
natural selection, allowing for evolutionary change
over time.
– The phenomena of emergent diseases supports
present-day evolution.
Connecting the Concepts
to the Big Ideas
• Interactions and Systems– Environmental and internal cues may affect gene
regulation, which ultimately effects their survival.
– Unicellular populations are capable of interactions which
affect their efficiency and productivity.
• Information and Signaling
– Viruses evolve rapidly engaging in lytic and lysogenic
cycles as well as transduction.
– Retroviruses use reverse transcriptase.