1. 9
The Cell Cycle

2. The Mitotic Spindle: A Closer Look
The mitotic spindle is a structure made of microtubules and associated proteins
It controls chromosome movement during mitosis
In animal cells, assembly of spindle microtubules begins in the centrosome, a type of microtubule organizing center
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3. The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase
An aster (radial array of short microtubules) extends from each centrosome
The spindle includes the centrosomes, the spindle microtubules, and the asters
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4. During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes
Kinetochores are protein complexes that assemble on sections of DNA at centromeres
At metaphase, the centromeres of all the chromosomes are at the metaphase plate, an imaginary structure at the midway point between the spindle’s two poles
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5. Video: Mitosis Spindle
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6. Aster Centrosome Sister chromatids Metaphase plate (imaginary) Kineto-
Figure 9.8
Aster
Centrosome
Sister
chromatids
Metaphase plate
(imaginary)
Kineto-
chores
Microtubules
Chromosomes
Overlapping
nonkinetochore
microtubules
Figure 9.8 The mitotic spindle at metaphase
Kinetochore
microtubules
1 m
0.5 m
Centrosome
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7. Microtubules Chromosomes 1 m Centrosome Figure 9.8-1
Figure The mitotic spindle at metaphase (part 1: TEM)
1 m
Centrosome
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8. Kinetochores Kinetochore microtubules 0.5 m Figure 9.8-2
Figure The mitotic spindle at metaphase (part 2: kinetochore TEM)
0.5 m
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9. The microtubules shorten by depolymerizing at their kinetochore ends
In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell
The microtubules shorten by depolymerizing at their kinetochore ends
Chromosomes are also “reeled in” by motor proteins at spindle poles, and microtubules depolymerize after they pass by the motor proteins
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10. Experiment Results Conclusion Kinetochore Spindle pole Mark Chromosome
Figure 9.9
Experiment
Results
Kinetochore
Spindle
pole
Conclusion
Mark
Chromosome
movement
Kinetochore
Figure 9.9 Inquiry: At which end do kinetochore microtubules shorten during anaphase?
Motor
protein
Tubulin
subunits
Microtubule
Chromosome
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11. Experiment Kinetochore Spindle pole Mark Figure 9.9-1
Figure Inquiry: At which end do kinetochore microtubules shorten during anaphase? (part 1: experiment)
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12. Results Conclusion Chromosome movement Kinetochore Motor Tubulin
Figure 9.9-2
Results
Conclusion
Chromosome
movement
Figure Inquiry: At which end do kinetochore microtubules shorten during anaphase? (part 2: results and conclusion)
Kinetochore
Motor
protein
Tubulin
subunits
Microtubule
Chromosome
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13. Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell
At the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated parent cell
Cytokinesis begins during anaphase or telophase, and the spindle eventually disassembles
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14. Cytokinesis: A Closer Look
In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow
In plant cells, a cell plate forms during cytokinesis
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15. Animation: Cytokinesis
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16. Video: Cytokinesis and Myosin
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17. (a) Cleavage of an animal cell (SEM)
Figure 9.10
(a) Cleavage of an animal cell (SEM)
(b) Cell plate formation in a plant cell (TEM)
100 m
Vesicles
forming
cell plate
Wall of
parent cell
1 m
Cleavage furrow
Cell
plate
New
cell wall
Figure 9.10 Cytokinesis in animal and plant cells
Contractile ring of
microfilaments
Daughter cells
Daughter cells
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18. 100 m Cleavage furrow Figure 9.10-1
Figure Cytokinesis in animal and plant cells (part 1: cleavage of animal cell, SEM)
100 m
Cleavage furrow
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19. Vesicles Wall of forming parent cell 1 m cell plate Figure 9.10-2
Figure Cytokinesis in animal and plant cells (part 2: cell plate formation in plant cell, TEM)
Vesicles
forming
cell plate
Wall of
parent cell
1 m
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20. Chromosomes condensing Nucleus Nucleolus Chromosomes 10 m Prophase
Figure 9.11
Chromosomes
condensing
Nucleus
Nucleolus
Chromosomes
10 m
Prophase
Prometaphase
Cell plate
Figure 9.11 Mitosis in a plant cell
Metaphase
Anaphase
Telophase
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21. Nucleus Chromosomes condensing Nucleolus 10 m Prophase Figure 9.11-1
Figure Mitosis in a plant cell (part 1: prophase)
10 m
Prophase
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22. Chromosomes 10 m Prometaphase Figure 9.11-2
Figure Mitosis in a plant cell (part 2: prometaphase)
10 m
Prometaphase
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23. Figure
Figure Mitosis in a plant cell (part 3: metaphase)
10 m
Metaphase
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24. Figure
Figure Mitosis in a plant cell (part 4: anaphase)
10 m
Anaphase
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25. Cell plate 10 m Telophase Figure 9.11-5
Figure Mitosis in a plant cell (part 5: telophase)
10 m
Telophase
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26. Binary Fission in Bacteria
Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission
In E. coli, the single chromosome replicates, beginning at the origin of replication
The two daughter chromosomes actively move apart while the cell elongates
The plasma membrane pinches inward, dividing the cell into two
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27. Origin of replication Cell wall Plasma membrane E. coli cell
Figure 9.12-s1
Origin of
replication
Cell wall
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Bacterial
chromosome
Two copies
of origin
Figure 9.12-s1 Bacterial cell division by binary fission (step 1)
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28. Origin of replication Cell wall Plasma membrane E. coli cell
Figure 9.12-s2
Origin of
replication
Cell wall
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Bacterial
chromosome
Two copies
of origin
Origin
Origin
One copy of the origin
is now at each end of
the cell.
Figure 9.12-s2 Bacterial cell division by binary fission (step 2)
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29. Origin of replication Cell wall Plasma membrane E. coli cell
Figure 9.12-s3
Origin of
replication
Cell wall
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Bacterial
chromosome
Two copies
of origin
Origin
Origin
One copy of the origin
is now at each end of
the cell.
Figure 9.12-s3 Bacterial cell division by binary fission (step 3)
Replication finishes.
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30. Origin of replication Cell wall Plasma membrane E. coli cell
Figure 9.12-s4
Origin of
replication
Cell wall
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Bacterial
chromosome
Two copies
of origin
Origin
Origin
One copy of the origin
is now at each end of
the cell.
Figure 9.12-s4 Bacterial cell division by binary fission (step 4)
Replication finishes.
Two daughter
cells result.
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31. The Evolution of Mitosis
Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission
Certain protists (dinoflagellates, diatoms, and some yeasts) exhibit types of cell division that seem intermediate between binary fission and mitosis
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32. (b) Diatoms and some yeasts
Figure 9.13
Chromosomes
Microtubules
Intact nuclear
envelope
(a) Dinoflagellates
Kinetochore
microtubule
Figure 9.13 Mechanisms of cell division
Intact nuclear
envelope
(b) Diatoms and some yeasts
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