Lecture III.1. Bacteria and Archaea. - Biology Learning Center

35 Pages·2016·2.17 MB·English

Lecture III.1.
Bacteria and Archaea.

Prokaryotic and eukaryotic cells compared. Not every eukaryotic cell
exhibits all features shown, e.g., animal and fungal cells lack plastids;
fungal and most plant cells lack undulipodia, etc.

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Prokaryotes.

 Small; mostly unicellular.

 No nucleus.

 DNA organized into

1. Circular chromosomes.

2. Plasmids – exchanged
during conjugation or
Plasmid transmission when
taken up from envt.
bacterial cells divide. Top. Inde-

pendently. Bottom. As part of
 No mitosis; no meiosis. Re- the bacterial chromosome fol-
produce by binary fission. lowing integration.

 No membrane-enclosed organelles – mitochondria, chloro-
plasts, etc. But, bacteria do possess

1. Cytoskeleton proteins – some with eukaryotic counter-
parts – so-called ESPs (Lecture II.4).

2. Protein-encased organelles such as carboxysomes that
increase rate of photosynthesis.
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Components of the Caulobacter Cytoskeleton. Caulobacter
cells have homologs of each of the three major eukaryotic
cytoskeletal systems. FtsZ localizes to the division plane
and regulates cell division. MreB localizes to a
dynamically contracting and expanding spiral and
regulates cell shape, polarity & chromosome
segregation. CreS (crescentin) localizes to Caulobacter’s
inner curvature and regulates cell shape.

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Carboxysomes concentrate CO , thereby facillitating its uptake by
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RuBisCO. Left. Schematic representation of bacterial carbon
fixation. Inorganic carbon is pumped into the cytosol as bicarbonate
−
ion (𝑯𝑪𝑶 ) and enters the carboxysome through small pores. The
𝟑
carboxysome contains carbonic anhydrase (CA), which converts
bicarbonate into CO , and RuBisCO, which catalyzes the entry of
2
CO into the Calvin cycle, the latter via reaction with ribulose-1,5-
2
bisphosphate (RuBP). CO concentration within the carboxysome is
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elevated by relative impermiability of the protein coat to CO . Right.
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Schematic showing polyhedral structure of the protein coat. Colors
correspond to different sheet paralogous proteins (products of
duplicated genes) and interior enzymes RuBisCO and CA. Despite
the similarity of the protein coat to the coats of viruses, there is no
evidence as of this writing for phylogenetic relationships (Falkner et
al. 2017. Nanoscale. 9: 10662-10673). From Samborska and Kimber
(2012).
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The entry of 𝐂𝐎 into the Calvin cycle is catalyzed by the enzyme
𝟐
RuBisCO. As the cycle turns, extra G3P molecules (bottom) are pro-
duced. These three carbon sugars are subsequently converted into
glucose (6 carbon sugar), which plants store as starch.

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Two Prokaryote Domains: Archaea and Bacteria.

 Based on gene sequence of small ribosomal subunit.

1. Large and small subunits collaborate to assemble pro-
teins (translation).

2. Highly conserved.

3. Three Domain theory re-
quires that observations
consistent with symbio- Translation entails passage of
mRNA between the large and
genic origin of Eukarya
small ribosomal subunits,
the result of lateral
gene transfer.
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Characters Shared by All Three Domains.

 Genetic

1. DNA the hereditary molecule.

2. Semi-conservative DNA replication – daughter mole-
cules consist of one new strand; one old.

3. Transcription and translation: DNA  RNA  proteins.

4. “Universal” genetic code.

 Metabolic

1. Use of high-energy compounds such as ATP.

2. Production of ATP by glycolysis – anaerobic respiration.

 Structural

1. Ribosomes

2. Plasma membranes and flagella – but see below.

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Table I. The Three Domains Compared.

Character Bacteria Archaea Eukarya
Archaea Similar to Bacteria
Multicellular Rarely No Many
Nuclear Mem-
Rarely No Yes
brane
Circular Chromo-
Most Yes No
some
Membrane–en-
No No Yes
closed organelles
Sex No1 No1 Common
Archaea Similar to Eukarya
Histones associ-
No Yes Yes
ated with DNA
Initiator tRNA FMet2 Methionine Methionine
Archaea Unique
Cell Membrane See accompanying figure
Most include Cellulose
Cell Wall No peptidoglycan
peptidoglycan or chitin
Flagella Grow from tip Grow from base Entirely different

1. DNA transfer and recombination. For Archaea, see Bernstein and Bern-
stein. 2017. Pp. 103-117. In, Witzany (Ed.) Biocommunication of Archaea.
Springer)

2. N-formylmethionine – methionine (sulfur containing amino acid) to which
a (CHO) group has been added.

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Archaea Present a Mix of Characters.

 Similar to Bacteria:
1. Unicellular.
2. No nuclear envelope
3. Circular chromosome
4. Absence of organelles

 Similar to Eukarya:
1. Histones associated with DNA.
2. Translation initiated by methionine (Met).

 Unique:
1. Plasma membrane structure.
2. Cell wall composition.
3. Flagellar structure and development – grow from base
(archaea) vs. tip (bacteria).

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Archaeal DNA is wrapped around histone tetramers. Shown
here are electron micrographs taken of archaeal DNA in the
presence of increasing concentrations of archaeal histone.
From Marc et al. (2002).
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