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Viruses and Prokaryotes
Chapter 24
Learning Objective 1
•
What is the structure of a virus?
•
Contrast a virus with a living cell
Virus (Virion)
•
Subcellular particle
•
Consists of
•
•
DNA or RNA genome
surrounded by protein coat (capsid)
Virus Structure
RNA inside
capsid
Capsid
0.1 µm
Fig. 24-1a, p. 502
Capsid with
antenna-like
fibers
DNA inside
capsid
0.05 µm
Fig. 24-1b, p. 502
DNA inside
capsid
Capsid
Tail
Tail fibers
Emerging
DNA
0.1 µm
Fig. 24-1c, p. 502
Viruses
•
Cannot metabolize on their own
•
Contain nucleic acids necessary to make
copies of themselves
•
but must invade and use metabolic machinery
of living cells in order to reproduce
KEY CONCEPTS
•
A virus is a small particle consisting of a
DNA or RNA genome surrounded by a
protein coat
Learn more about virus structure
by clicking on the figure in
ThomsonNOW.
Learning Objective 2
•
According to current hypotheses, what is
the evolutionary origin of viruses?
Origin of Viruses
•
Viruses may be bits of nucleic acid that
originally “escaped” from animal, plant, or
bacterial cells
Hypothesis
•
Viruses must have evolved before the
three domains diverged
•
It is unlikely that similar viruses that infect
archaea and bacteria evolved twice
Learning Objective 3
•
Characterize bacteriophages (phages)
•
•
viruses that infect bacteria
What is the difference between a lytic
cycle and a lysogenic cycle?
Viral Reproductive Cycles
•
Lytic cycle
•
virus destroys host cell
• Temperate viruses
•
do not always destroy their hosts
• Lysogenic cycle
•
viral genome replicated along with host DNA
Lytic Cycle
•
5 steps:
•
•
•
•
•
attachment to host cell
penetration of viral nucleic acid into host cell
replication of viral nucleic acid
assembly of components into new viruses
release from host cell
Lytic Cycle
Phages
Bacterium
1 Attachment.
Phage attaches to
cell surface of
bacterium.
Bacterial DNA
2 Penetration.
Phage DNA enters
bacterial cell.
Phage protein
Phage DNA
3 Replication and
synthesis. Phage
DNA is replicated.
Phage proteins are
synthesized.
Fig. 24-2a (1), p. 504
4
Assembly. Phage
components are
assembled into new
viruses.
5 Release.
Bacterial cell lyses and
releases many phages
that can then infect
other cells.
Fig. 24-2a (2), p. 504
0.25 µm
Fig. 24-2b, p. 504
Lysogenic Cycle
•
Prophage
•
•
Lysogenic cells
•
•
nucleic acid of phage becomes integrated into
bacterial DNA
bacterial cells that carry prophages
Lysogenic conversion
•
bacterial cells containing certain temperate
viruses exhibit new properties
Lysogenic Cycle
1 Attachment.
Phage attaches
to cell surface
of bacterium.
2 Penetration.
Phage DNA
enters bacterial
cell.
Prophage
3 Integration.
Phage DNA
integrates into
bacterial DNA.
These cells may exhibit
new properties.
4 Replication.
Integrated
prophage
replicates when
bacterial DNA
replicates.
Fig. 24-3, p. 504
KEY CONCEPTS
•
Evolution occurs rapidly in prokaryotes;
natural selection acts on the genetic
variation provided by mutations and
genetic recombination and is facilitated by
rapid reproduction
Insert “The two different
ways that viruses replicate
(lytic and lysogenic cycles)”
Tbd
*suggested by Mary Durant, who will
review existing animations currently
slated for pickup (cd)
Watch the lytic and lysogenic
cycles by clicking on the figure
in ThomsonNOW.
Learning Objective 4
•
Compare viral infection of animals and
plants
•
Identify specific diseases caused by
animal viruses
Animal Viruses
•
Viruses enter animal cells by membrane
fusion or endocytosis
•
Viral nucleic acid replicated in host cell
•
•
proteins synthesized
new viruses assembled and released from cell
Envelope proteins
1 Virus attaches to specific
receptors on plasma membrane
of host cell.
Envelope
Capsid
Nucleic acid
Membrane
Fusion
2 Membrane fusion.
Viral envelope fuses
with plasma
membrane.
Receptors
Host-cell plasma
membrane
Cytoplasm
Capsid
Nucleus
Nucleic
acid
Ribosomes
ER
mRNA
10
Viruses are
released from
host cell.
3 Virus is released into
host-cell cytoplasm.
4 Viral nucleic acid
separates from its
capsid.
5 Viral nucleic acid
enters host-cell
nucleus and
replicates.
6 Viral nucleic acid is
transcribed into mRNA.
7 Host ribosomes are
directed by mRNA to
synthesize viral proteins.
8 Vesicles transport
glycoproteins to hostcell plasma membrane.
New viruses are assembled and enveloped
9 by host-cell plasma membrane.
Fig. 24-4b, p. 508
Endocytosis
3 Endosomal vesicle forms and
moves into cytoplasm.
2 Host-cell plasma
membrane
surrounds virus.
1 Virus makes contact with
plasma membrane of
host cell.
Host-cell cytoplasm
Host-cell plasma
membrane
4 Virus is released into
host-cell cytoplasm.
5 Viral envelope fuses
with host-cell plasma
membrane (not shown).
Fig. 24-4c, p. 508
Viral Diseases
•
DNA viruses cause
•
•
smallpox, herpes, respiratory infections,
gastrointestinal disorders
RNA viruses cause
•
influenza, upper respiratory infections, AIDS,
some types of cancer
Rubella
•
An RNA virus
Plant Viruses
•
Mostly RNA viruses
•
Spread among plants by insect vectors
•
Spread through plant via plasmodesmata
Plant Viruses
Learning Objective 5
•
Describe the reproductive cycle of a
retrovirus, such as human
immunodeficiency virus (HIV)
Retroviruses
•
Use reverse transcriptase
•
Transcribe RNA genome into DNA
intermediate
•
•
becomes integrated into host DNA
Synthesize copies of viral RNA
HIV
Envelope protein
Envelope
Capsid
Enzymes
(reverse
transcriptase,
ribonuclease,
integrase,
protease)
Host-cell
plasma
membrane
HIV
Nucleic
acid
(RNA)
1 HIV attaches to host-cell
plasma membrane.
2 HIV enters host-cell cytoplasm.
CD4 Receptors
Viral nucleic
Reverse acid (RNA)
transcriptase
Cytoplasm
ssDNA
Nucleus
Host
chromosome
Viral
RNA
dsDNA
3 Capsid is removed by enzymes.
Reverse transcriptase catalyzes
synthesis of single-stranded (ss)
DNA that is complementary to viral
RNA.
4 The DNA strand then serves as
template for synthesis of complementary DNA strand, resulting in
double-stranded (ds) DNA.
5 dsDNA is transferred to host nucleus
and enzyme integrase integrates
DNA into host chromosome.
6 When activated, viral DNA uses
host enzymes to transcribe viral
RNA.
7 Viral RNA leaves nucleus, viral proteins
are synthesized on host ribosomes, and
virus is assembled.
8 Virus buds from host cell, using host-cell plasma
membrane to make viral envelope.
Fig. 24-5, p. 509
Watch the HIV life cycle by
clicking on the figure in
ThomsonNOW
Learning Objective 6
•
What are viroids and prions?
Viroids and Prions
•
Viroids
•
•
short strands of RNA with no protein coat
Prions
•
•
consists only of protein
cause transmissible spongiform
encephalopathies (TSEs)
Prions
Contacts
Prion
Normal protein (PrP)
1 Prion induces normal
PrP to misfold, forming
another prion.
Contacts
Contacts
2 Each prion can induce
additional PrP proteins
to misfold.
3 Proteins aggregate.
Fig. 24-7, p. 511
Learning Objective 7
•
Describe the structure and common
shapes of prokaryotic cells
Prokaryotic Cells
•
Do not have membrane-enclosed
organelles
•
such as nuclei and mitochondria
Prokaryotic Cell
Outer membrane
Pili
(structures
used for
attachment)
Peptidoglycan
layer
Cell wall
Nuclear
area
(nucleoid)
Storage granule
Plasmid
(DNA)
Flagellum
Ribosomes
Bacterial chromosome
(DNA)
Capsule
Plasma
membrane
Fig. 24-9, p. 513
Bacterial Shapes
•
Spherical (cocci)
Bacterial Shapes
•
Rod shaped (bacilli)
Bacterial Shapes
•
Spiral
•
•
rigid helix (spirillum)
flexible helix (spirochete)
Bacteria Structure
•
Cell walls composed of peptidoglycan
•
Some have capsule surrounding cell wall
Bacterial Cell Walls
•
Gram-positive bacteria
•
•
•
walls very thick
consist mainly of peptidoglycan
Gram-negative bacteria
•
•
walls have thin peptidoglycan layer
outer membrane (like plasma membrane)
Gram-Positive
Cell Wall
Gram-Negative
Cell Wall
Thick peptidoglycan
layer
Cell
wall
Plasma membrane
(inner membrane)
Transport protein
(a) Gram-positive cell wall.
Fig. 24-10a, p. 514
Polysaccharides
Lipoprotein
Outer membrane
Cell
wall
Thin peptidoglycan
layer
Plasma membrane
Transport protein
(b) Gram-negative cell wall.
Fig. 24-10b, p. 514
Bacterial Pili
•
Protein structures extending from cell
•
help bacteria adhere to one another or to
other surfaces
Bacterial Flagella
•
Different from eukaryotic flagella
•
Consist of
•
•
•
•
basal body
hook
filament
Produce rotary motion
Bacterial Flagella
Plasma
membrane
Cytoplasm
Basal
body
Peptidoglycan
layer
Outer
membrane
Protein
rings
Hook
Filament
Fig. 24-11b, p. 515
Learn more about the structure
of prokaryotes and their cell
walls by clicking on the figures
in ThomsonNOW
Learning Objective 8
•
Describe asexual reproduction in
prokaryotes
•
Summarize three mechanisms
(transformation, transduction, and
conjugation) that may lead to genetic
recombination
Prokaryote Genes
•
Genetic material consists of
•
•
1 circular DNA molecule
1 or more plasmids (circular DNA fragments)
Asexual Reproduction
•
Binary fission
•
•
Budding
•
•
cell divides, forming two cells
bud forms, separates from mother cell
Fragmentation
•
•
walls form inside cell
separates into several cells
Genetic Material Exchange
•
Transformation
•
•
Transduction
•
•
bacterial cell takes in DNA fragments
released by another cell
phage carries bacterial DNA from one
bacterial cell into another
Conjugation
•
two cells of different mating types exchange
genetic material
Transformation
1 Bacterium dies and releases DNA.
2 Fragments of foreign DNA bind to
proteins on surface of living
bacterium.
3 DNA enters cell, and some DNA is
incorporated into host cell by
reciprocal recombination.
DNA exchanged
Fig. 24-12, p. 515
Transduction
1 DNA of a phage
penetrates bacterial
cell.
2 Phage DNA may become
integrated with host-cell
DNA as a prophage.
Phage DNA with
bacterial genes
Fragmented
bacterial DNA
3 When the prophage
becomes lytic, bacterial
DNA is degraded and new
phages are produced. New
phages may contain some
bacterial DNA.
Fig. 24-13a, p. 516
4 Bacterial cell lyses and
releases many phages, which
can then infect other cells.
5 Phage infects new host cell.
6 Bacterial genes introduced into
new host cell are integrated into
host's DNA. They become a part
of bacterial DNA and are
replicated along with it.
Fig. 24-13b, p. 516
1
DNA of a phage penetrates bacterial cell.
2
Phage DNA may become integrated with hostcell DNA as a prophage.
3
When the prophage becomes lytic, bacterial
DNA is degraded and new phages are
produced. New phages may contain some
bacterial DNA.
4
Bacterial cell lyses and
releases many phages, which
can then infect other cells.
5
Phage infects new host cell.
6
Bacterial genes introduced into new host cell
are integrated into host's DNA. They become
a part of bacterial DNA and are replicated
along with it.
Stepped Art
Phage DNA with
bacterial genes
Fragmented
bacterial DNA
Fig. 24-13b, p. 516
Conjugation
F+ (donor) cell
F– (recipient) cell
1 F+ (donor) cell produces
sex pilus.
Bacterial
chromosome
F plasmid
2 Sex pilus develops into
conjugation bridge.
3
DNA replicates, and single
strand of F plasmid DNA is
transferred from F+ cell to
F– cell.
Both bacterial cells now
4 contain double-stranded F
plasmid. The F– cell has been
converted to an F+ cell.
Fig. 24-14b, p. 517
.
Learning Objective 9
•
What are the modes of nutrition and
metabolic adaptations of prokaryotes?
Prokaryote Nutrition
•
Most are heterotrophs
•
•
obtain energy and carbon from other
organisms
Some are autotrophs
•
make their own organic molecules from
simple raw materials
Heterotrophs
•
Chemoheterotrophs
•
•
•
free-living decomposers
obtain carbon and energy from dead organic
matter
Photoheterotrophs
•
•
obtain carbon from other organisms
photosynthetic pigments trap light energy
Autotrophs
•
Photoautotrophs
•
•
obtain energy from sunlight
Chemoautotrophs
•
obtain energy by oxidizing inorganic
chemicals such as ammonia
Aerobes and Anaerobes
•
Aerobic bacteria
•
•
Facultative anaerobes
•
•
require oxygen for cellular respiration
metabolize anaerobically when necessary
Obligate anaerobes
•
only metabolize anaerobically
Learning Objective 10
•
Compare the three domains: Archaea,
Bacteria, and Eukarya
3 Domains
•
Domain Archaea (prokaryotes)
•
•
Domain Bacteria (prokaryotes)
•
•
cell walls have peptidoglycan
cell walls do not have peptidoglycan
Domain Eukarya
•
four kingdoms of eukaryotes
3 Domains
Proteobacteria
Domain Archaea
Eukaryotes
Nanoarchaeota
Crenarchaeota
Euryarchaeota
Korarchaeota
Spirochetes
Chlamydias
Gram-positives
Cyanobacteria
Gram-positives
Epsilon
Delta
Gamma
Beta
Alpha
Domain Bacteria
Domain Eukarya
Common ancestor of
all living organisms
Fig. 24-16, p. 519
Learning Objective 11
•
Distinguish among the main groups of
archaea based on their ecology
•
Identify the archaean phyla (Table 24-3)
•
Describe the main groups of bacteria
(Table 24-4)
Archaea
•
Methanogens
•
•
•
Extreme halophiles
•
•
produce methane from carbon compounds
inhabit anaerobic environments
inhabit saturated salt solutions
Extreme thermophiles
•
live at temperatures greater than 100° C
Extreme Halophiles
Archaeans
•
•
•
•
Crenarchaeota
Euryarchaeota
Nanoarchaeota
Korarchaeota
Archaeans
Bacteria
•
Gram-negative
•
•
•
•
•
Proteobacteria
Cyanobacteria
Chlamydias
Spirochetes
Gram-positive bacteria
Cyanobacterium
Heterocysts
50 µm
Fig. 24-18, p. 523
KEY CONCEPTS
•
•
•
•
Viroids and prions are smaller than viruses
A prion consists only of proteins
Unlike eukaryotic cells, prokaryotic cells
do not have membrane-enclosed
organelles such as nuclei and
mitochondria
Prokaryotes make up two of the three
domains: Bacteria and Archaea
Learning Objective 12
•
What are the ecological roles of
prokaryotes, their importance as
pathogens, and their commercial
importance?
Ecological Roles of Prokaryotes
•
Essential decomposers
•
recycle nutrients
•
Some carry out photosynthesis
•
Many are symbiotic with other organisms
Symbiosis
•
Mutualism
•
•
Commensalism
•
•
•
both partners benefit
1 partner benefits
other not harmed or helped
Parasitism
•
•
parasite benefits
host is harmed
Bacteria and Disease
•
Pioneers in microbiology
•
•
•
•
Anton van Leeuwenhoek
Louis Pasteur
Robert Koch
Koch’s postulates
•
guidelines to demonstrate specific pathogen
causes specific disease symptoms
Heliobacter
pylori
Fig. 24-19a, p. 525
Fig. 24-19b, p. 525
Pathogenic Bacteria
•
Exotoxins
•
•
strong poisons released by pathogenic
bacteria
Endotoxins
•
•
poisonous components of cell walls
released when bacteria die
Pathogenic Bacteria
Antibiotic Resistance
•
Many bacteria have become resistant to
antibiotics
•
R factors
•
plasmids with genes for antibiotic resistance
Commercial Importance
•
Some bacteria produce antibiotics
•
Some bacteria used to produce cheese
•
Lactic acid bacteria used in yogurt,
pickles, olives, sauerkraut
Lactic Acid Bacteria
5 µm
Fig. 24-20, p. 526
KEY CONCEPTS
•
Great diversity has evolved in the mode of
nutrition, the metabolism, and the
ecological roles of prokaryotes