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Cecie Starr
Christine Evers
Lisa Starr
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Chapter 19
Viruses, Bacteria, and Archaeans
(Sections 19.6 - 19.8)
Albia Dugger • Miami Dade College
19.6 Bacterial Reproduction
and Gene Exchange
• Bacteria reproduce by binary fission: replication of a single,
circular chromosome and division of a parent cell into two
genetically equivalent descendants
• binary fission
• Method of asexual reproduction that divides one bacterial
or archaean cell into two identical descendant cells
4 Steps in Binary Fission
1. Circular chromosome
attaches to inside of
plasma membrane
2. Cell duplicates
chromosome, attaches
copy beside the original,
and adds membrane
and wall material
between them
4 Steps in Binary Fission
3. When cell has almost
doubled in size, new
membrane and wall are
deposited
4. Two genetically
identical cells result
4 Steps in Binary Fission
A bacterium has one circular
chromosome that attaches to the
inside of the plasma membrane.
1
The cell duplicates its
chromo-some, attaches the
copy beside the original, and
adds membrane and wall
material between them.
2
When the cell has almost
doubled in size, new membrane
and wall are deposited across its
midsection.
3
Two genetically identical
cells result.
4
Fig. 19.7, p. 303
4 Steps in Binary Fission
A bacterium has one circular
chromosome that attaches to the
inside of the plasma membrane.
1
The cell duplicates its
chromo-some, attaches the
copy beside the original, and
adds membrane and wall
material between them.
2
When the cell has almost
doubled in size, new membrane
and wall are deposited across its
midsection.
3
Two genetically identical
cells result.
4
Stepped Art
Fig. 19.7, p. 303
Animation: Prokaryotic Fission
Horizontal Gene Transfers
• Horizontal gene transfers move genes between existing
individuals (cells)
• In conjugation, one cell gives a plasmid to the other
• One cell extends a sex pilus out to another cell
• The cell that made the sex pilus passes a copy of its
plasmid to its partner
Key Terms
• horizontal gene transfer
• Transfer of genetic material
• conjugation
• Mechanism of gene exchange in which one bacterial or
archaean cell passes a plasmid to another
• plasmid
• Of many bacteria and archaeans, a small ring of
nonchromosomal DNA replicated independently of the
chromosome
Prokaryotic Conjugation
• One cell extends a sex pilus to another, draws it close, and
gives it a copy of a plasmid
Horizontal Gene Transfers (cont.)
• Two other processes can also introduce new genes:
1. A cell can take up DNA from its environment,
(transformation)
2. Viruses that infect bacteria sometimes move genes
between their hosts
• Gene transfers help genes spread through a population,
accelerating response to selective pressure
Key Concepts
• Replication and Gene Exchange
• Bacteria have a single chromosome and some also have
one or more plasmids
• They reproduce by fission, a type of asexual reproduction
• Cells can exchange genes by swapping plasmids, and by
other processes
ANIMATION: Prokaryotic Conjugation
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19.7 Bacterial Diversity
• Bacteria are widespread, abundant, and diverse
• Most bacteria are either harmless or beneficial
• Many bacteria have essential ecological roles :
• Decomposers
• Cycle nutrients
• Form partnerships with other species
Heat-Loving Bacteria
• Modern heat-loving bacteria may resemble early cells
• One species, Thermus aquaticus, was discovered in a
volcanic spring in Yellowstone National Park
• A heat-stable enzyme from T. aquaticus (DNA polymerase)
was used in the first PCR reactions
Oxygen-Producing Cyanobacteria
• Cyanobacteria produce oxygen during photosynthesis
• Some carry out nitrogen fixation, producing ammonia
that algae and plants need as a nutrient
• Some partner with fungi and form lichens
• Spirulina is grown commercially
• cyanobacteria
• Photosynthetic, oxygen-producing bacteria
Key Terms
• nitrogen fixation
• Incorporation of gaseous nitrogen (N=N) into ammonia
(NH3)
• Plants and algae need nitrogen but cannot use nitrogen
gas because they cannot break its triple bond – they can
take up ammonia produced by nitrogen fixers
Important Cyanobacteria
• Chain of cyanobacteria,
with many
photosynthetic, oxygenproducing cells and one
nitrogen-fixing cell
Important Cyanobacteria
nitrogenfixing
cell
A
Fig. 19.9a, p. 304
Highly Diverse Proteobacteria
• Proteobacteria are the largest bacterial lineage:
• Some are nitrogen-fixers (Rhizobium)
• Myxobacteria show remarkable cooperative behavior
(multicelled fruiting bodies with spores)
• Escherichia coli are part of our normal flora
• Some important pathogens (Salmonella, Campylobacter,
Helicobactor pylori, Vibrio cholerae)
• Rickettsias (intracellular parasites) are the closest
relatives of mitochondria
Key Terms
• proteobacteria
• Largest bacterial lineage
• normal flora
• Normally harmless or beneficial microorganisms that
typically live in or on a body
Important Proteobacteria
• Thiomargarita
namibiensis, the largest
known bacterium
• It has an enormous
vacuole that holds sulfur
and nitrate
The Thick-Walled Gram Positives
• Gram-positive bacteria have thick walls
• Some Gram-positive soil bacteria (Clostridium, Bacillus)
produce endospores that allow them to survive boiling,
freezing, radiation, and disinfectants
• endospore
• Resistant resting stage of some soil bacteria
The Thick-Walled Gram Positives
• Gram-positive bacteria have thick walls
• Some Gram-positive soil bacteria (Clostridium , Bacillus)
produce endospores that allow them to survive boiling,
freezing, radiation, and disinfectants
• endospore
• Resistant resting stage of some soil bacteria
Gram-Staining
• Gram staining is a method used to prepare bacteria for
examination under a microscope
• Gram staining
• Process used to prepare bacterial cells for microscopy,
and to distinguish groups based on cell wall structure
• Thick walled Gram-positive bacteria are tinted purple
• Thinner-walled Gram-negative bacteria such as
cyanobacteria and proteobacteria are stained pink
Important Gram Positive
• Lactate-fermenting
bacteria (Lactobacillus)
in yogurt
• Related cells are
decomposers or part of
the human normal flora
Some Dangerous Gram-Positives
• Some endospore-forming bacteria make deadly toxins:
• Bacillus anthracis: anthrax
• Clostridium tetani: tetanus
• C. botulinum: botulism
• Dangerous infections:
• Mycobacterium tuberculosis: tuberculosis
• Streptococcus: strep throat, “flesh-eating bacteria”
• Staphylococcus: Antibiotic-resistant staph infections
Dangerous Gram-Positives
• Staphylococci, common
skin bacteria
• Abscess caused by an
antibiotic-resistant staph
infection
Spring-Shaped Spirochetes
• Spirochetes are spiral-shaped bacteria
• Include aquatic decomposers, nitrogen fixers, and bacteria
that live in cattle gut and break down cellulose
• Some are pathogens:
• Cause of sexually-transmitted disease syphilis
• Cause of Lyme disease, transmitted by ticks
• spirochetes
• Bacteria that resemble a stretched-out spring
Spirochete That Causes Lyme Disease
• Bull’s eye rash around a tick bite is a sign of infection
Parasitic Chlamydias
• Chlamydias are tiny cocci that live inside vertebrate cells
• Chlamydia infection is the most common sexually transmitted
bacterial disease in the United States
• chlamydias
• Tiny round bacteria that are intracellular parasites of
eukaryotic cells
Key Concepts
• Bacterial Diversity
• Bacteria are well studied and highly diverse
• They put oxygen into the air, supply nutrients to plants,
and break down wastes and remains
• Some live in or on our bodies and have beneficial effects
• Others are pathogens that cause human disease
ANIMATION: Gram Staining
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19.8 The Archaeans
• Archaeans superficially resemble bacteria, but comparisons
of structure, function, and genetic sequences position them in
a separate domain, closer to eukaryotes and bacteria
Discovery of the Third Domain
• Biologists historically divided all life into two groups,
prokaryotes and eukaryotes
• New evidence revealed differences in structure, organization,
and gene sequences between bacteria and archaeans
• The three domain model is now considered correct
The Third Domain
The Third Domain
prokaryotes
A
eukaryotes
bacteria
archaea
eukaryotes
B
Fig. 19.12, p. 306
Archaean Diversity
• Archaeans are more diverse and widely distributed than
previously thought
• In their physiology, many archaeans are methanogens
(methane makers), extreme halophiles (salt lovers), and
extreme thermophiles (heat lovers)
• Archaeans coexist with bacteria in many habitats and can
exchange genes with them
Methanogens
• methanogen
• Organism that produces
methane gas (CH4) as a
metabolic by-product
• Adapted to anaerobic
conditions (deep-sea
hydrothermal vents,
soils, cow guts)
Extreme Halophiles
• extreme halophile
• Organism adapted to life
in a highly salty
environment
Extreme Thermophiles
• extreme thermophile
• Organism adapted to life
in a very hightemperature
environment; such as a
hot spring
Key Concepts
• Archaean Diversity
• Archaeans were discovered relatively recently
• Many are adapted to life in very hot or very salty places
• Others live in low-oxygen environments and make
methane
• Still others live beside bacteria in soils and seas
• None cause human disease
Evolution of a Disease (revisited)
• Pathogens coevolve with their hosts
• Example: HIV seems to be adapting to human immune
defenses; escape mutations help HIV evade detection by
recognition proteins of white blood cells
• Selection also favors hosts that can fight off a pathogen
• Example: About 10 percent of people of European
ancestry have a mutation that lessens the likelihood of
infection by most HIV strains