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Chap 27
Prokaryotes and the Origins of Metabolic Diversity
Current nucleotide analysis of a specific RNA molecule called 16S has
classified life on Earth into 3 lineages or Domains:
i.
Eukarya
ii.
Bacteria
iii. Archaea
The purpose of this chapter is to compare and contrast the prokaryotes, the
bacteria and Archaea.
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Figure 27.2 The three domains of life
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Major Groups of Prokaryotes
Spirochetes
Extreme
Methanogens
Thermophiles
Extreme
Halophiles
Proteobacteria
Cyanobacteria
Bacteria
Archaea
Universal Ancestor
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Domain Archaea
Structural Differences
•
Cell Wall
• A:
only protein
• B:
peptidoglycans
Gram positive and gram negative: stain acts on the
peptidoglycans and gram positive bacteria have more
peptidoglycans so pick up more stain.
• E:
plants have polysaccharides; animals have no cell walls and
fungi have chitin
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Figure 27.5x Gram-positive and gram-negative bacteria
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Figure 27.5 Gram-positive and gram-negative bacteria
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•
Cell Membrane
• B & E: straight chain fatty acids connected to glycerol
• A: HC’s linked to glycerol molecules (no fatty acids)
Molecular Biology Comparisons
•
Organization of the genome
• similar to bacteria
• chromosome is a single piece of circular DNA
• plasmids are present
• thermal denaturation resistance: provided by the presence of a high
salt concentration and DNA binding proteins
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•
DNA Replication
• DNA polymerases resemble eukaryotic polymerases in their
primary protein sequence.
• some proofreading that occurs during replication
• topoisomerases and restriction restriction enzymes are present.
•
Transcription
• RNA polymerase is more closely related to euk. polymerase in
terms of genes. There are several kinds of RNA polymerases and
therefore several genes.
• E. coli RNA polymerase (bacterial) can initiate transcription
whereas the archaea RNA polymerase cannot when in vitro
(transcription factors are needed.)
• Archaea promoter regions are rich in A’s and T’s like the TATA box
of eukaryotes.
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•
Gene Organization
• Introns have been found in rRNA and tRNA genes of archeal
genomes.
• functionally related genes are often organized in operon-like
structures.
Reproduction
•
Prokaryotes reproduce asexually by binary fission
•
Neither mitosis nor meiosis occurs (as it does sin eukaryotes)
• Three Mechanisms of Gene Transfer (not sexual)
•
1.
Transformation
2.
Conjugation: direct transfer by sex pili
3.
Transduction: viral transfer from one prokaryote to another.
Endospores: resistant cells that can withstand most high heat.
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Figure 27.x1 Prokaryotic conjugation
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Table 27.2 A Comparison of the Three Domains of Life
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Archaea Diversity
Extremophiles: a general term to describe the “extreme” environments in
which the archaea inhabit
•
Methanogens
• methane or marsh gas is produced
• CO2 + H2 produces CH4 + H2O
• they hate oxygen
• strict anaerobes
• they are decomposers at sewage plants, inhabit the guts of
herbivores (cattle), termites.
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•
Extreme Halophiles
• Great Lake, Dead Sea
• Some tolerate the high salt, others require it.
• They photosynthesize using a pigment called bacteriorhodopsin.
•
Extreme Thermophiles
• most enjoy the warm of a 60 – 80o C environment
• some will obtain energy from sulfur compounds
• hot springs and thermal vents
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Figure 27.10 An anthrax endospore
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Evolution of Photosynthesis
Very first prokaryotes were heterotrophs. That is, they required an organic
compound such as glucose to make energy.
•
Photoheterotrophs: obtain carbon from some organic compound but
make ATP with the help of light.
•
Chemoheterotroph: breakdown organic compounds for ATP and to
obtain their carbon. Most widely for of nutrition in prokaryotes
Glycolysis was probably the first metabolic pathway used to make ATP.
•
The fact that almost every modern organism has this pathway supports
the idea that glycolysis was one of the earliest pathways to make
ATP.
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What selection pressure would have begun to favor some sort of organism that
could make its own energy?
•
The ability to make their own food required the:
• harnessing of the sun to make ATP
• creation of reducing power too convert CO2 into organic cpds.
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Evidence the PS evolved early on
1.
Many different prokaryotes possess this ability. The researches think
because of the complexity of PS, that it developed in a common
ancestor.
2.
PS may have been present and then lost in organisms related to those
that still are photosynthetic.
3.
Cyanobacteria
•
These are the only autotrophic prokaryotes that release oxygen
with the splitting of water.
•
Cyanobacteria have been shown to be a major part of
environment when oxygen was present
•
Fossils of prokaryotes, some 3.5 billion years old, look like
modern cyanobacteria
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Evidence the PS evolved early on
4.
5.
The complexity of PS
•
Recall that it requires two photosystems
•
Some prokaryotes use a single photosytem involving hydrogen
sulfide. The H2S gets split instead of water.
The evolution of cyanobacteria with their oxygen production changed
the entire environment from a reducing one to an oxidizing one.
•
The use of oxygen in respiration increases the efficiency of
breaking down fats and carbos.
•
Electron transport chains and proton gradient mechanisms as well
as ATP synthesis proteins are thought to have evolved from PS
machinery.
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Table 27.1 Major Nutritional Modes
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