1

The Nucleoid

Eukaryotes have a membrane-bound nucleus.

Prokaryotes have a nucleoid region that extends throughout the cytoplasm.

Figure 3.26

2

•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 identical daughter cells

The Cell Division- an Overview

3

Procaryotic chromosomes are located in the nucleoid, an area in the cytoplasm

4

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

5

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

6

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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• 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.

DNA Replication

8

Figure 7.14

9

1010

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

11

Figure 3.32

12

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.

Septation Completes Cell Division

Figure 3.33

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The spatial orientation of septation has a key role in determining the shape and arrangement of cocci.

Septation Completes Cell Division

Figure 3.34

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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 crescent-shaped bacteria

The Bacterial Cytoskeleton

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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)

Specialized Structures of Phototrophs

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Storage granules

- Glycogen, PHB, and PHA, for energy

- Sulfur, for oxidation

Magnetosomes

- Membrane-embedded crystals of Fe3O4 fixation

- Orient the swimming of magnetotactic bacteria

Other Specialized Structures

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Pili or fimbriae are straight filaments of protein monomers called pilin.

Sex pili are used in conjugation.

Stalks are membrane-embedded extensions

of the cytoplasm.

- Tips secrete adhesion factors called holdfasts.

Cell Attachment

Figure 3.40

Figure 3.41

1818

Pili and Fimbriae

Fimbriae (fimbria)short, thin, hairlike, proteinaceous

appendagesup to 1,000/cell

mediate attachment to surfacessome (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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Prokaryotes that are motile generally swim by means of rotary flagella.

Rotary Flagella

Figure 3.42a

Peritrichous cells have flagella randomly distributed around the cell.

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.

Figure 3.43

Note: Flagella rotate either clockwise (CW) or counterclockwise (CCW) relative to the cell.

21

The filament

Extends from cell surface to the tip Hollow, rigid cylinder Composed of the protein flagellin. Some procaryotes have a sheath around filament

2222

The Hook and Basal Body

Hooklinks filament to

basal body

Basal bodyseries of rings that

drive flagellar motor

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Flagellar Ultrastructure

2424

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 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

Figure 3.42

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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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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.