Prokaryotic Cell Structure and function (Part II)

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Prokaryotic Cell Structure and function (Part II). BIO3124 Lecture #3 (II). Flagella and Motility. 15-20 um long appendages extended through CW and anchored to CM Provide motility differently distributed on each cell type. Patterns of Flagella Distribution. - PowerPoint PPT Presentation

Transcript of Prokaryotic Cell Structure and function (Part II)

  • Prokaryotic Cell Structure and function (Part II) BIO3124Lecture #3 (II) *

  • Flagella and Motility 15-20 um long appendages extended through CW and anchored to CM Provide motility differently distributed on each cell type


  • Patterns of Flagella Distributionmonotrichous one flagellum usually polar (ie. flagellum at one end of cell)amphitrichous one flagellum at each end of celllophotrichous (tuft): cluster of flagella at one or both ends peritrichous spread over entire surface of cell*

  • Flagellar UltrastructureGram negative Gram positive *

  • Animation: Bacterial flagellum rotation mechanism*

  • The filamentextends from cell surface to the tiphollow, rigid cylindercomposed of the protein flagellinsome procaryotes have a sheath around filament, eg. Spirochetes*

  • Flagellum Synthesisan example of self-assemblycomplex process involving many genes and gene products new molecules of flagellin are transported through the hollow filament growth is from tip, not base*

  • 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 directionChemotaxis*

  • 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*

  • CCW and CW rotation of flagella*

  • Chemotaxis: molecular eventsRegulated by two-component signalingMajor proteinsMCPs: Methyl-accepting chemotaxis proteinsclustered at cell poles bind chemoattractants, receptor sensor and kinase (CheA/CheW), phosphorayte CheYCheY-P, a response regulator, increase the tumble frequencyOther regulatory proteinsCheR & CheB: reversible methylation or demethylation of MCPs desensitizes or sensitizes MCPsCheZ, dephosphorylation of CheY-P


  • *

  • Chemotaxis*

  • Single loop of double-stranded DNAAttached to cell envelopeNo membrane separates DNA from cytoplasm Replicates once for each cell divisionCompacted via supercoiling by topoisomerases I and II

    The Nucleoid*

  • Plasmidsusually small, closed circular DNA moleculesexist and replicate independently of chromosomehave relatively few genes presentgenes 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*

  • Cell division, or cell fission, requires highly coordinated growth and expansion of all the cells parts.Unlike eukaryotes, prokaryotes synthesize RNA and proteins continually while the cells DNA undergoes replication.Bacterial DNA replication is coordinated with the cell wall expansion and ultimately the separation of the two daughter cells.Cell Division*

  • 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*

  • Cell Division (Fission)Cell elongates as it growsAdds new wall at cell equatorDNA replicates to make 2 chromosomesDNA replicates bidirectionallyCan begin next replication before cell dividesCell undergoes septationUsually at equatorEach daughter has same shape*

  • Cell Division (Binary Fission)*

  • Coordination of leading and lagging strands*

  • Cytokinesis: Role of Cytoskeletal Proteinsprocess not well understoodprotein MreBsimilar to eucaryotic actindetermination of cell shape and movement of chromosomes to opposite cell polesprotein FtsZ,similar to eucaryotic tubulinZ ring formationMinCD proteininhibitor of FtsZ multimerizationOscillates between cell polesLocalizes the Z ring to the equatorial plane*

  • DivisomeCytokinesis & chromosome replication coordinately regulatedFts proteins form divisomeFtsA, ZipA: anchor Z ring to cytoplasmic membraneFtsK: coordinates septation with chromosome partitioningOthers: FtsI,L,N,Q,B,W and AmiC involved in PG synthesis


  • Gene ExpressionRNA Polymerase transcribes DNA to mRNARibosome translates RNA to ProteinProcesses occur simultaneously- This is aided by the signal recognition particle (SRP), which binds to the growing peptide.*

  • Special strutures

    Cyanobacteria have thylakoidsExtensively folded inner membraneContain chlorophyllAncestors of chloroplastsCarboxysomes fix carbonRubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase),use energy to make sugar Other bacterial photosynthetic pigmentsPurple membranes containing Bacteriorhodopsin among HalobacteriaPhycobilisome proteins collect light energy


  • Organic inclusion bodiesIntracellular deposits of materialGlycogen (sugar) for energyParahydroxy 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*

  • Inorganic inclusion bodiesPolyphosphate granulesalso called volutin granules or metachromatic granuleslinear polymers of phosphates, stored and used in DNA synthesissulfur granules: periplasmic or cytoplasmic, accumulated by sulfur bacteria Magnetosomes contain iron in the form of magnetite (Fe3O4) used to orient magnetotactic bacteria in magnetic fieldsReviews: Arash, SchulerIridescent sulfur granules*

    ****negatively stained flagella from E. coli; arrows indicate curved hooks and basal bodies******************