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Fig. 27-13
Chapter 27
F plasmid
Bacteria and Archaea
Bacterial chromosome
F+ cell
F+ cell
Mating
bridge
F– cell
F+ cell
Bacterial
chromosome
(a) Conjugation and transfer of an F plasmid
Hfr cell
A+
A+
A+
F factor
F– cell
A+
A–
A–
{
(b) Conjugation and transfer of part of an Hfr bacterial chromosome
Recombinant
F– bacterium
A–
A+
A–
A+
Overview: Masters of Adaptation
• Prokaryotes thrive almost
everywhere, including places too
acidic, salty, cold, or hot for most
other organisms
• There are more in a handful of
fertile soil than the number of
people who have ever lived
• They have an astonishing
genetic diversity
• Prokaryotes are divided into two
domains: bacteria and archaea
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 27.1: Structural and functional
adaptations contribute to prokaryotic success
• Most prokaryotes are unicellular, although
some species form colonies
• Most prokaryotic cells are much smaller than
eukaryotic cells (1/10th the size)
• Prokaryotic cells have a variety of shapes
• The three most common shapes of prokaryotes
are: spheres (cocci), rods (bacilli), and spirals
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 27-2
1 µm
(a) Spherical
(cocci)
2 µm
(b) Rod-shaped
(bacilli)
5 µm
(c) Spiral
Cell-Surface Structures
• Prokaryotes have cell walls which maintains
cell shape, provides physical protection, and
prevents the cell from bursting in a hypotonic
environment
• Eukaryote cell walls are made of cellulose or
chitin
• Bacterial cell walls contain peptidoglycan, a
network of sugar polymers cross-linked by
polypeptides
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Using the Gram stain, scientists classify many
bacterial species into:
•
Gram-positive: simpler walls with more
peptidoglycan
• Gram-negative: less peptidoglycan and an
outer membrane that can be toxic. They are
more likely to be antibiotic resistant
• Many antibiotics target peptidoglycan and
damage bacterial cell walls
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 27-3
Carbohydrate portion
of lipopolysaccharide
Peptidoglycan
Cell
wall
Cell
layer
wall
Outer
membrane
Peptidoglycan
layer
Plasma membrane
Plasma membrane
Protein
Protein
Grampositive
bacteria
(a) Gram-positive: peptidoglycan traps
crystal violet.
Gramnegative
bacteria
20 µm
(b) Gram-negative: crystal violet is easily rinsed away,
revealing red dye.
Fig. 27-4
•A polysaccharide or protein layer called a capsule covers many prokaryotes
200 nm
Capsule
•Some prokaryotes have fimbriae (also called
attachment pili), which allow them to stick to their
substrate or other individuals in a colony
•Sex pili are longer than fimbriae and allow
prokaryotes to exchange DNA
Fimbriae
200 nm
Motility
• Most motile bacteria propel themselves by
flagella that are structurally and functionally
different from eukaryotic flagella
• In a heterogeneous environment, many
bacteria exhibit taxis, the ability to move
toward or away from certain stimuli
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 27-6
Flagellum
Filament
50 nm
Cell wall
Hook
Basal apparatus
Plasma
membrane
Internal and Genomic Organization
• Prokaryotic cells usually lack complex
compartmentalization
• Some prokaryotes do have specialized
membranes that perform metabolic functions
• The prokaryotic genome is circular and has less
DNA than the eukaryotic genome
•
DNA is not surrounded by a membrane and that is
located in a nucleoid region
• Some species of bacteria also have smaller rings
of DNA called plasmids
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 27-8
Chromosome
Plasmids
1 µm
Reproduction and Adaptation
• Prokaryotes reproduce quickly by binary fission
and can divide every 1–3 hours
• Prokaryotes can evolve rapidly because of their
short generation times
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 27-10
EXPERIMENT
Daily serial transfer
0.1 mL
(population sample)
RESULTS
1.8
Fitness relative
to ancestor
Figure 27.10 Can
prokaryotes
evolve rapidly in
response to
environmental
change?
New tube
(9.9 mL
growth
medium)
Old tube
(discarded
after
transfer)
1.6
1.4
1.2
1.0
0
5,000
10,000
15,000
Generation
20,000
Concept 27.2: Rapid reproduction, mutation, and
genetic recombination promote genetic diversity in
prokaryotes
• Prokaryotes have considerable genetic
variation
• Three factors contribute to this genetic
diversity:
1. Rapid reproduction
2. Mutation
3. Genetic recombination
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
1. Rapid Reproduction and Mutation
• Prokaryotes reproduce by binary fission, and
offspring cells are generally identical
• Mutation rates during binary fission are low, but
because of rapid reproduction, mutations can
accumulate rapidly in a population
• High diversity from mutations allows for rapid
evolution
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
2. Genetic Recombination
• Prokaryotic DNA from different individuals can
be brought together by 3 mechanisms:
1. Transformation: prokaryote takes up DNA
form its environment
2. Transduction: viruses transfer genes between
prokaryotes
3. Conjugation: genes are directly transferred
from one prokaryote to another
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 27-11-4
Phage DNA
A+ B+
A+ B+
Donor
cell
Figure 27.11
Transduction
A+
Recombination
A+
A– B–
Recipient
cell
A+ B–
Recombinant cell
Conjugation and Plasmids
• In Conjugation, sex pili allow cells to connect
and pull together for DNA transfer
• A piece of DNA called the F factor is required
for the production of sex pili
• The F factor can exist as a separate plasmid or
as DNA within the bacterial chromosome
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 27-12
Sex pilus
1 µm
The F Factor as a Plasmid
• Cells containing the F plasmid function as
DNA donors during conjugation
• Cells without the F factor function as DNA
recipients during conjugation
• The recipient becomes a recombinant
bacterium, with DNA from two different cells
• The F factor is transferable during conjugation
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 27-13
Figure 27.13 Conjugation and recombination in
E. coli
F plasmid
Bacterial chromosome
F+ cell
F+ cell
Mating
bridge
F– cell
F+ cell
Bacterial
chromosome
(a) Conjugation and transfer of an F plasmid
Hfr cell
A+
A+
A+
F factor
F– cell
A+
A–
Recombinant
F– bacterium
A–
A–
(b) Conjugation and transfer of part of an Hfr bacterial chromosome
A+
A–
A+
R Plasmids and Antibiotic Resistance
• R plasmids carry genes for antibiotic
resistance
• Antibiotics select for bacteria with genes that
are resistant to the antibiotics
• Antibiotic resistant strains of bacteria are
becoming more common
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings