Download Viruses and Prokaryotes

Prokaryotes
Chapter 20
Figure 5.1 The Scale of Life
Figure 5.2 Why Cells Are Small
Figure 5.3 (1) Looking at Cells
Figure 5.3 (2) Looking at Cells
The Prokaryotes: Domain Bacteria and
Archaea
•
Endospores
–
–
–
Dehydrate cell
Chromosome encased in heavy, protective coat
Allows bacteria to remain dormant during unfavorable environmental conditions
Figure 5.4 A Prokaryotic Cell
Figure 5.5 Prokaryotic Flagella
Bacterial Genome and Its Replication
Replication fork
• The bacterial chromosome is
usually a circular DNA molecule
with few associated proteins
• Many bacteria also have
plasmids, smaller circular DNA
molecules that can replicate
independently of the
chromosome
• Bacterial cells divide by binary
fission
Origin of
replication
Termination
of replication
Mutation and Genetic Recombination as
Sources of Genetic Variation
• Rapid reproduction, mutation, and genetic
recombination contribute to the genetic diversity of
bacteria
• Since bacteria can reproduce rapidly, new mutations
quickly increase genetic diversity
• More genetic diversity arises by recombination of
DNA from two different bacterial cells
• Three processes bring bacterial DNA from different
individuals together:
– Transformation
– Transduction
– Conjugation
Mechanisms of Gene Transfer and Genetic
Recombination in Bacteria
• Three processes bring bacterial DNA from different
individuals together:
– Transformation
• Transformation is the alteration of a bacterial cell’s genotype
and phenotype by the uptake of naked, foreign DNA from the
surrounding environment
– Transduction
• phages carry bacterial genes from one host cell to another
– Conjugation
• Conjugation is the direct transfer of genetic material between
bacterial cells that are temporarily joined
• The transfer is one-way: One cell (“male”) donates DNA, and
its “mate” (“female”) receives the genes
Phage DNA
A+ B+
Sex pilus
A+ B+
Donor
cell
F plasmid
A+
Crossing
over
Bacterial chromosome
F+ cell
Mating
bridge
F+ cell
F– cell
F+ cell
Bacterial
chromosome
A+
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
A– B–
Recipient
cell
A+ B–
Recombinant cell
Implication: Lateral Gene Transfer
Complicates linear tree thinking!!
Figure 26.10 Lateral Gene Transfer Complicates Phylogenetic Relationships
Figure 26.1 The Three Domains of the Living World
Common Ancestor?
• Prokaryotic
• Genetic material was DNA
• DNA --> RNA --> Protein process in place
– Genetic code established
•
•
•
•
Circular chromosome
Operons
No introns
Heterotroph (glycolysis/fermentation)
Prokaryotic Classification
• Domain Bacteria vs Archaea
• Cell Wall composition
– Gram negative or gram positive
• Cell shape
• Mode of nutrition
• Molecular characteristics
– rRNA sequence comparisons
Why 3 Domains?
• Prokaryotes include Domains Bacteria and Archaea
– Archaea diverged from a prokaryotic lineage
• Archaea and Bacteria very distinct
– No peptidoglycan
– Branched hydrocarbons and ether linkages in cell membranes
– Unique rRNA sequences
– Archaea lineage lead to Domain Eukarya
• Archaea should share more ancestral traits with Eukarya than Bacteria
– Translation machinery more similar
– RNA polymerases more similar
• If left in single Kingdom, would result in Kingdom that was
paraphyletic
• This Kingdom ‘Prokaryote’ would not include all decendents (the
eukaryotes) of common ancestor
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Domain Bacteria
• Typical prokaryotes
– Include all gram positive and gram negative bacteria
– Cyanobacteria
Figure 26.2 Bacterial Cell Shapes
Figure 26.5 The Gram Stain and the Bacterial Cell Wall
•
Prokaryotic Nutrition
Dependence on oxygen
– Obligate anaerobes: die in presence of oxygen
– Facultative anaerobes: grow in either presence or absence of oxygen
– Aerobic: require constant supply of oxygen
•
Autotrophic
– Do NOT give off O2
• PS I only
• Bacteriochlorophyll
• Green sulfur and purple bacteria
– Anaerobic mud: CO2 + 2 H2S --> sugar + 2 S
– DO give off O2
• PS I and PS II
• Chlorophyll a (plants)
• Cyanobacteria
– Some Cyanobacteria also able to fix N2; probably first photoautotrophs of early Earth to
release oxygen
•
Heterotrophic
– Decomposers (saprotrophs)
– often capable of breaking down unusual materials
– Symbiotic bacteria
• Mutualistic, commensalistic, or parasitic
• Nitrogen-fixing bacteria, Rhizobium
Figure 26.9 Bacteriochlorophyll Absorbs Long-Wavelength Light
Cyanobacteria
• Gram negative
• Photosynthesize similar to plants
– First to introduce oxygen to atmosphere of early Earth
•
•
•
•
Unicellular or colonial
Many fix N2 - only require water CO2, N2 to grow!!!
Thylakoids
Lichens - symbiotic relationship of cyanobacteria with fungi
Figure 26.15 Cyanobacteria
Figure 26.19 Modes of Nutrition in the Proteobacteria
Figure 36.11 The Nitrogen Cycle
Figure 36.9 A Nodule Forms
Figure 36.8 Nitrogenase Fixes Nitrogen
Domain Archaea
• rRNA sequence comparisons and cell wall/membrane
composition distinguished them from Bacteria
– Carl Woese
• Archaea more closely related to Eukarya
– Share some ribosomal proteins not found in bacteria
– Initiate transcription in same manner
– Similar types of tRNA
Figure 26.22 Membrane Architecture in Archaea
Domain Archaea: Structure and Function
• Plasma membranes contain unusual lipids
– Glycerol linked to branched-chain hydrocarbons rather than
fatty acids
• No peptidoglycan in cell walls
• Unique habitats and metabolism