Prokaryotic Cell Structure Prokaryotic Cell Structure and function and function (Part II)(Part II)
BIO3124Lecture #3 (II)
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Flagella and Motility
• 15-20 um long appendages• extended through CW and anchored to CM• Provide motility• differently distributed on each cell type
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Patterns of Flagella Distribution
monotrichous – one flagellum
usually polar (ie. flagellum at one
end of cell)
amphitrichous – one flagellum at
each end of cell
lophotrichous (tuft): cluster of
flagella at one or both ends
peritrichous – spread over entire
surface of cell3
Flagellar Ultrastructure
Gram negative Gram positive
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Animation: Bacterial flagellum rotation mechanism
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The filament
extends from cell surface to the tip
hollow, rigid cylinder
composed of the protein flagellin
some procaryotes have a sheath around filament, eg.
Spirochetes
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Flagellum Synthesis an example of
self-assembly complex
process involving many genes and gene products
new molecules of flagellin are transported through the hollow filament
growth is from tip, not base
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Chemotaxis is the movement of a bacterium in response to chemical gradients.
Attractants cause CCW rotation.- Flagella bundle together.- Push cell forward- “Run”
Repellents cause CW rotation.- Flagellar bundle falls apart. - “Tumble” = Bacterium briefly stops, then changes direction
Chemotaxis
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The alternating runs and tumbles cause a “random walk.”
- Receptors detect attractant concentrations.
- Sugars, amino acids
- Attractant concentration increases and prolongs run.
- This is termed a “biased random walk.”
- Causes a net movement of bacteria toward attractants (or away from repellents)
Chemotaxis
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CCW and CW rotation of flagella
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Chemotaxis: molecular events Regulated by two-component signalingMajor proteins MCPs: Methyl-accepting chemotaxis proteins
- clustered at cell poles bind chemoattractants, receptor sensor and kinase (CheA/CheW), phosphorayte CheY
CheY-P, a response regulator, increase the tumble frequency
Other regulatory proteins CheR & CheB: reversible methylation or demethylation
of MCPs desensitizes or sensitizes MCPs CheZ, dephosphorylation of CheY-P
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ChemotaxisChemotaxis
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Single loop of double-stranded DNA
Attached to cell envelope No membrane separates
DNA from cytoplasm
Replicates once for each cell division
Compacted via supercoiling by topoisomerases I and II
The Nucleoid
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Plasmids usually small, closed circular DNA molecules exist and replicate independently of
chromosome have relatively few genes present genes on plasmids are not essential to host but
may confer selective advantage (e.g., drug resistance)
classification of plasmids based on mode of existence and function eg. R-factors, F-plasmids and metabolic plasmids
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Cell division, or cell fission, requires highly coordinated growth and expansion of all the cell’s parts.
Unlike eukaryotes, prokaryotes synthesize RNA and proteins continually while the cell’s DNA undergoes replication.
Bacterial DNA replication is coordinated with the cell wall expansion and ultimately the separation of the two daughter cells.
Cell Division
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In prokaryotes, a circular chromosome begins to replicate at its origin, or ori site.
Two replication forks are generated, which proceed outward in both directions.- At each fork, DNA is synthesized by DNA
polymerase with the help of accessory proteins (the replisome).
As the termination site is replicated, the two forks separate from the DNA.
DNA Replication
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Cell Division (Fission) Cell elongates as it grows
Adds new wall at cell equator DNA replicates to make 2
chromosomesDNA replicates bidirectionallyCan begin next replication
before cell divides Cell undergoes septation
Usually at equatorEach daughter has same shape
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Cell Division (Binary Fission)
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Coordination of leading and lagging strandsCoordination of leading and lagging strands
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Cytokinesis: Role of Cytoskeletal Proteins process not well understood protein MreB
similar to eucaryotic actin determination of cell shape and
movement of chromosomes to opposite cell poles
protein FtsZ, similar to eucaryotic tubulin Z ring formation
MinCD protein inhibitor of FtsZ multimerization Oscillates between cell poles Localizes the Z ring to the equatorial
plane
FtsZ
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Divisome Cytokinesis &
chromosome replication coordinately regulated
Fts proteins form divisome
FtsA, ZipA: anchor Z ring to cytoplasmic membrane
FtsK: coordinates septation with chromosome partitioning
Others: FtsI,L,N,Q,B,W and AmiC involved in PG synthesis
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Gene Expression RNA Polymerase transcribes DNA to mRNA Ribosome translates RNA to Protein
Processes occur simultaneously
- This is aided by the signal recognition particle (SRP), which binds to the growing peptide.
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Special struturesSpecial strutures Cyanobacteria have thylakoids
Extensively folded inner membrane Contain chlorophyll Ancestors of chloroplasts
Carboxysomes fix carbon Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase),use
energy to make sugar
Other bacterial photosynthetic pigments Purple membranes containing Bacteriorhodopsin among
Halobacteria
Phycobilisome proteins collect light energy
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Organic inclusion bodies Intracellular deposits of material
Glycogen (sugar) for energy Parahydroxy butyrate (PHB), fatty acid
polymer for energy Carboxysomes,lipid energy-storage granules
Gas vacuoles
found in cyanobacteria and some other
aquatic procaryotes, provide buoyancy
aggregates of hollow cylindrical structures
called gas vesicles
Function: floatation to regulate O2 and light
intensity
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Inorganic inclusion bodiesInorganic inclusion bodies Polyphosphate granules
also called volutin granules or metachromatic granules
linear polymers of phosphates, stored and used in DNA synthesis
sulfur granules: periplasmic or cytoplasmic, accumulated by sulfur bacteria
Magnetosomes contain iron in the form of magnetite (Fe3O4)
used to orient magnetotactic bacteria in magnetic fields
Reviews: Arash, Schuler
Iridescent sulfur granules
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