Transcript Cell structure and function

The Nucleoid
Eukaryotes have a
membrane-bound
nucleus.
Prokaryotes have a
nucleoid region that
extends throughout
the cytoplasm.
Figure 3.26
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The Cell Division- an Overview
•Bacteria reproduce by a type of cell division called binary fission.
•Most of the genetic information in bacteria is found on a single circular
chromosome which consists of DNA and proteins.
•The bacterial cell replicates its chromosome before the process of fission
takes place
•Cell division in bacteria is controlled by the FtsZ, a collection of about a
dozen proteins that collect around the site of division. The cell wall and
plasma membrane starts growing transversely from near the middle of the
dividing cell.
•The two copies of the chromosome remain attached to the membrane
and the membrane simply grows between the two attached sites. After the
cell has grown to about twice its normal size, the membrane pinches
inward and a cell wall develops. The parent has been divided into two
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identical daughter cells
Procaryotic
chromosomes
are located in the
nucleoid, an area
in the cytoplasm
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The E. coli nucleoid
appears as clear
regions that exclude
the ribosome and
contain the DNA
strands.
Figure 3.27
The nucleoid forms about
50 loops or domains.
Within each domain, the
DNA is supercoiled by
DNA-binding proteins.
Figure 3.28
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Cell Division
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.
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Chromosome Replication and
Partitioning
• Most procaryotic chromosomes are circular
• Origin of replication – site at which replication
begins
• Terminus – site at which replication is terminated
• Replisome – group of proteins needed for DNA
synthesis; parent DNA spools through the
replisome as replication occurs
• MreB – an actin homolog plays role in
determination of cell shape
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DNA Replication
•
In prokaryotes, a circular chromosome begins to
replicate at its origin, or “ori” site.
• Two replications 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.
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Figure 7.14
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DNA Replication in Rapidly Growing
Cells
Cell cycle completed in 20 minutes
-40 minutes for DNA replication
-20 minutes for septum formation and cytokinesis
Look at timing-how can this happen?
Second, third or fourth round of replication can
begin before first round of replication is
completed
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Figure 3.32
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Septation Completes Cell Division
Replication of the termination site triggers growth of
the dividing partition, or septum.
The septum grows inward, at last constricting and
sealing off the two daughter cells.
Figure 3.33
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Septation Completes Cell Division
The spatial
orientation of
septation has
a key role in
determining
the shape and
arrangement
of cocci.
Figure 3.34
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The Bacterial Cytoskeleton
Shape-determining proteins
- FtsZ = Forms a “Z ring”
in spherical cells
- MreB = Forms a coil
inside rod-shaped cells
- CreS “Crescentin” =
Forms a polymer along
the inner side of crescentshaped bacteria
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Specialized Structures of Phototrophs
Thylakoids = Extensively folded intracellular
membranes
Carboxysomes = Polyhedral bodies packed with
the enzyme Rubisco for CO2 fixation
Gas vesicles = To increase buoyancy (found in
aquatic bacteria)
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Other Specialized Structures
Storage granules
- Glycogen, PHB, and
PHA, for energy
- Sulfur, for oxidation
Magnetosomes
- Membrane-embedded
crystals of Fe3O4 fixation
- Orient the swimming of
magnetotactic bacteria
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Cell Attachment
Pili or fimbriae are straight
filaments of protein
monomers called pilin.
Sex pili are used in
conjugation.
Stalks are membraneembedded extensions
of the cytoplasm.
- Tips secrete adhesion
factors called holdfasts.
Figure 3.40
Figure 3.41
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Pili and Fimbriae
Fimbriae (fimbria)
short, thin, hairlike, proteinaceous
appendages
up to 1,000/cell
mediate attachment to surfaces
some (type IV fimbriae) required for twitching
motility or gliding motility that occurs in
some bacteria
Sex pili (pilus)
similar to fimbriae except longer, thicker, and
less numerous (1-10/cell)
required for mating
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Rotary Flagella
Prokaryotes that are motile generally swim by
means of rotary flagella.
Peritrichous cells have
flagella randomly
distributed around the cell.
Figure 3.42a
Lophotrichous cells have flagella at the
end(s).
Monotrichous cells have a single flagellum.
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Each flagellum is a spiral filament of protein
monomers called flagellin.
The filament is rotated by a motor driven by
the proton motive force.
Note: Flagella
rotate either
clockwise
(CW) or
counterclock
wise (CCW)
relative to
the cell.
Figure 3.43
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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
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The Hook and Basal Body
Hook
links filament to
basal body
Basal body
series of rings that
drive flagellar
motor
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Flagellar Ultrastructure
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The Mechanism of Flagellar
Movement
Flagellum rotates like a propeller:
-Counterclockwise rotation causes
forward motion (run).
-Clockwise rotation disrupts run
causing a tumble (twiddle)
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Chemotaxis
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
Figure 3.42
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Chemotaxis
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)
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Figure 3.44
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Chapter Summary
The DNA of prokaryotes is organized into loops in
the nucleoid.
● Most bacteria divide by binary fission.
- Cell growth and DNA replication are coordinated.
● Bacteria may have specialized structures,
including thylakoids, storage granules, and
magnetosomes.
● Pili and stalks are used for attachment.
● Flagella are rotary appendages used for
movement and chemotaxis.
●
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