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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