1. Chapter 12
The Cell Cycle

2. Multicellular organisms depend on cell division for:
Continuity of Life is Based on Cell Division
In unicellular organisms, division of one cell reproduces the entire organism.
Multicellular organisms depend on cell division for:
Development from a fertilized cell
Growth
Repair
Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division.
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3. Functions of Cell Division
Fig. 12-2
Functions of Cell Division
100 µm
200 µm
20 µm
(a) Reproduction
(b) Growth and
development
(c) Tissue renewal
Repair
Figure 12.2 The functions of cell division

4. Concept 12.1: Mitotic Cell Division results in genetically identical daughter cells
Most cell division is mitotic and results in daughter cells with identical genetic information, DNA.
A special type of meiotic cell division produces nonidentical daughter cells (gametes, or sperm and egg cells).
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5. Cellular Organization of the Genetic Material
All the DNA in a cell constitutes the cell’s genome.
A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells)
DNA molecules in a cell are packaged into chromosomes.
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6. Fig. 12-3
Figure 12.3 Eukaryotic chromosomes
20 µm

7. Somatic cells (2n) (body cells) have two sets (pairs) of chromosomes.
Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.
Somatic cells (2n) (body cells) have two sets (pairs) of chromosomes.
Gametes (n) (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells.
Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division.
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8. Distribution of Chromosomes During Eukaryotic Cell Division
In preparation for cell division, DNA is replicated and the chromosomes condense.
Each duplicated chromosome has two identical sister chromatids, which separate during cell division.
The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached.
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9. 0.5 µm Chromosomes DNA molecules Chromo- some arm Chromosome
Fig. 12-4
0.5 µm
Chromosomes
DNA molecules
Chromo-
some arm
Chromosome
duplication
(including DNA
synthesis)
Centromere
Sister
chromatids
Figure 12.4 Chromosome duplication and distribution during cell division
Separation of
sister chromatids
Centromere
Sister chromatids

10. Eukaryotic cell division consists of:
Mitosis, the division of the nucleus
Cytokinesis, the division of the cytoplasm
Gametes (n) are produced by a variation of cell division called meiosis = reduction division.
Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell.
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11. Phases of the Cell Cycle
The cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)
Interphase:
G1 normal cell growth
S copying of chromosomes
G2 growth in preparation for cell division.
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12. S (DNA synthesis) G1 Cytokinesis G2 Mitosis
Fig. 12-5
INTERPHASE S
(DNA synthesis) G1
Cytokinesis G2
Mitosis
Figure 12.5 The cell cycle
MITOTIC
(M) PHASE

13. Mitosis is conventionally divided into five phases:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis is well underway by late telophase.
For the Cell Biology Video Myosin and Cytokinesis, go to Animation and Video Files.
BioFlix: Mitosis
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14. Chromosome, consisting of two sister chromatids
Fig. 12-6
G2 of Interphase
Prophase
Prometaphase
Metaphase
Anaphase
Telophase and Cytokinesis
Centrosomes
(with centriole
pairs)
Chromatin
(duplicated)
Early mitotic
spindle
Aster
Centromere
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Metaphase
plate
Cleavage
furrow
Nucleolus
forming
Figure 12.6 The mitotic division of an animal cell
Daughter
chromosomes
Nucleolus
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
Kinetochore
microtubule
Spindle
Centrosome at
one spindle pole
Nuclear
envelope
forming

15. Chromosome, consisting of two sister chromatids
Fig. 12-6b
G2 of Interphase
Prophase
Prometaphase
Centrosomes
(with centriole
pairs)
Chromatin
(duplicated)
Early mitotic
spindle
Aster
Centromere
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Figure 12.6 The mitotic division of an animal cell
Nucleolus
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
Kinetochore
microtubule

16. Telophase and Cytokinesis
Fig. 12-6d
Metaphase
Anaphase
Telophase and Cytokinesis
Metaphase
plate
Cleavage
furrow
Nucleolus
forming
Figure 12.6 The mitotic division of an animal cell
Daughter
chromosomes
Nuclear
envelope
forming
Spindle
Centrosome at
one spindle pole

17. The Mitotic Spindle: A Closer Look
The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis.
During prophase, assembly of spindle microtubules begins in the centrosome, the MTOC microtubule organizing center.
The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them.
An aster (a radial array of short microtubules) extends from each centrosome.
For the Cell Biology Video Spindle Formation During Mitosis, go to Animation and Video Files.
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18. During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes.
At metaphase, the chromosomes are all lined up at the metaphase plate, the midway point between the spindle’s two poles.
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19. Centrosome Fig. 12-7 Aster Sister chromatids Microtubules Chromosomes
Metaphase
plate
Kinetochores
Centrosome
1 µm
Figure 12.7 The mitotic spindle at metaphase
Overlapping
nonkinetochore
microtubules
Kinetochore
microtubules
0.5 µm

20. 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.
For the Cell Biology Video Microtubules in Anaphase, go to Animation and Video Files.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

21. EXPERIMENT RESULTS CONCLUSION Kinetochore Spindle pole Mark Chromosome
Fig. 12-8
EXPERIMENT
Kinetochore
Spindle
pole
Mark
RESULTS
Figure 12.8 At which end do kinetochore microtubules shorten during anaphase?
CONCLUSION
Chromosome
movement
Kinetochore
Motor
protein
Tubulin
subunits
Microtubule
Chromosome

22. EXPERIMENT Kinetochore Spindle pole Mark RESULTS Fig. 12-8a
Figure 12.8 At which end do kinetochore microtubules shorten during anaphase?
RESULTS

23. Chromosome movement Kinetochore Tubulin Motor Subunits Microtubule
Fig. 12-8b
CONCLUSION
Chromosome
movement
Kinetochore
Tubulin
Subunits
Motor
protein
Microtubule
Figure 12.8 At which end do kinetochore microtubules shorten during anaphase?
Chromosome

24. Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell.
In telophase, genetically identical daughter nuclei form at opposite ends of the cell.
For the Cell Biology Video Microtubules in Cell Division, go to Animation and Video Files.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

25. Cytokinesis: A Closer Look
In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow in the plasma membrane.
In plant cells, a cell plate forms from Golgi vesicles (membrane) during cytokinesis.
Animation: Cytokinesis
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26. Fig. 12-9
Cytokinesis
Vesicles
forming
cell plate
Wall of
parent cell
1 µm
100 µm
Cleavage furrow
Cell plate
New cell wall
Figure 12.9 Cytokinesis in animal and plant cells
Contractile ring of
microfilaments
Daughter cells
Daughter cells
(a) Cleavage of an animal cell (SEM)
(b) Cell plate formation in a plant cell (TEM)

27. 10 µm Fig. 12-10 Plant Cell Mitotic Division Nucleus Chromatin
condensing
10 µm
Nucleolus
Chromosomes
Cell plate
Figure Mitosis in a plant cell 1
Prophase 2
Prometaphase 3
Metaphase 4
Anaphase 5
Telophase

28. Binary Fission
Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission.
In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes move apart as the cell elongates.
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29. Binary Fission Cell wall Origin of replication Plasma membrane
E. coli cell
Bacterial
chromosome
Two copies
of origin
Origin
Origin
Figure Bacterial cell division by binary fission

30. The Evolution of Mitosis
Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission.
Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis.
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31. (b) Protists: Dinoflagellates = algae
Fig
Bacterial
chromosome
(a) Bacteria
Chromosomes
Microtubules
Intact nuclear
envelope
(b) Protists: Dinoflagellates = algae
Kinetochore
microtubule
Intact nuclear
envelope
Figure A hypothetical sequence for the evolution of mitosis
(c) Diatoms and yeasts
Kinetochore
microtubule
Fragments of
nuclear envelope
(d) Most eukaryotes

32. Bacterial chromosome (a) Bacteria Chromosomes Microtubules
Fig ab
Bacterial
chromosome
(a) Bacteria
Chromosomes
Microtubules
Figure A hypothetical sequence for the evolution of mitosis
Intact nuclear
envelope
(b) Dinoflagellates

33. Kinetochore microtubule Intact nuclear envelope (c) Diatoms and yeasts
Fig cd
Kinetochore
microtubule
Intact nuclear
envelope
(c) Diatoms and yeasts
Kinetochore
microtubule
Figure A hypothetical sequence for the evolution of mitosis
Fragments of
nuclear envelope
(d) Most eukaryotes

34. The frequency of cell division varies with the type of cell.
Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control system
The frequency of cell division varies with the type of cell.
These cell cycle differences result from regulation at the molecular level.
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35. Evidence for Cytoplasmic Signals
The cell cycle appears to be driven by specific chemical signals present in the cytoplasm.
Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei.
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36. EXPERIMENT RESULTS Experiment 1 Experiment 2 S G1 M G1 S S M M
Fig
EXPERIMENT
Experiment 1
Experiment 2 S G1 M G1
RESULTS S S M M
When a cell in the
S phase was fused
with a cell in G1, the G1
nucleus immediately
entered the S
phase—DNA was
synthesized.
When a cell in the
M phase was fused with
a cell in G1, the G1
nucleus immediately
began mitosis—a
spindle formed and
chromatin condensed,
even though the
chromosome had not
been duplicated.
Figure Do molecular signals in the cytoplasm regulate the cell cycle?

37. The Cell Cycle Control System
The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock.
The cell cycle control system is regulated by both internal and external controls
The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received
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38. G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint
Fig
G1 checkpoint
Control
system S G1 G2 M
Figure Mechanical analogy for the cell cycle control system
M checkpoint
G2 checkpoint

39. For many cells, the G1 checkpoint seems to be the most important one.
If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide.
If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase.
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40. G0 G1 G1 G1 checkpoint (b) Cell does not receive a
Fig G0
G1 checkpoint
Figure The G1 checkpoint G1 G1
Cell receives a go-ahead
G1 checkpoint.
(b) Cell does not receive a
go-ahead signal --> exit.

41. The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks).
The activity of cyclins and Cdks fluctuates during the cell cycle.
MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase.
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42. M G1 S G2 M G1 S G2 M G1 Fig. 12-17 MPF activity Cyclin concentration
Time
(a) Fluctuation of MPF activity and cyclin concentration during
the cell cycle G1 S
Cdk
Figure Molecular control of the cell cycle at the G2 checkpoint
Cyclin accumulation M
Degraded
cyclin G2 G2
Cdk
Cyclin is
degraded
checkpoint
Cyclin
MPF
(b) Molecular mechanisms that help regulate the cell cycle

43. (a) Fluctuation of MPF activity and cyclin concentration during
Fig a M G1 S G2 M G1 S G2 M G1
MPF activity
Cyclin
concentration
Figure Molecular control of the cell cycle at the G2 checkpoint
Time
(a) Fluctuation of MPF activity and cyclin concentration during
the cell cycle

44. (b) Molecular mechanisms that help regulate the cell cycle
Fig b G1 S
Cdk
Cyclin accumulation M
Degraded
cyclin G2 G2
checkpoint
Cdk
Figure Molecular control of the cell cycle at the G2 checkpoint
Cyclin is
degraded
Cyclin
MPF
(b) Molecular mechanisms that help regulate the cell cycle

45. Stop and Go Signs: Internal and External Signals at the Checkpoints
An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase.
Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide.
For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture.
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46. Scalpels Petri plate Without PDGF cells fail to divide With PDGF
Fig
Scalpels
Petri
plate
Without PDGF
cells fail to divide
Figure The effect of a growth factor on cell division
With PDGF
cells prolifer-
ate
Cultured fibroblasts
10 µm

47. External Signals
Besides growth factors, another example of external signals is density-dependent inhibition, in which crowded cells stop dividing
Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide
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48. Density-dependent inhibition
Fig
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
Figure Density-dependent inhibition and anchorage dependence of cell division
25 µm
25 µm
(a) Normal mammalian cells
(b) Cancer cells

49. Loss of Cell Cycle Controls in Cancer Cells
Cancer cells do not respond normally to the body’s control mechanisms.
Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence.
Cancer cells may not need growth factors to grow and divide:
They may make their own growth factor
They may convey a growth factor’s signal without the presence of the growth factor
They may have an abnormal cell cycle control system.
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50. A normal cell is converted to a cancerous cell by a process called transformation.
Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue.
If abnormal cells remain at the original site, the lump is called a benign tumor.
Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors.
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51. Cancer: Does NOT obey cell cycle control signals
Lymph
vessel
Tumor
Blood
vessel
Cancer
cell
Glandular
tissue
Metastatic
tumor 1
A tumor grows
from a single
cancer cell. 2
Cancer cells
invade neigh-
boring tissue. 3
Cancer cells spread
to other parts of
the body. 4
Cancer cells may
survive and
establish a new
tumor in another
part of the body.
Figure The growth and metastasis of a malignant breast tumor

52. Review G1 S Cytokinesis Mitosis G2 MITOTIC (M) PHASE Prophase
INTERPHASE G1 S
Cytokinesis
Mitosis G2
MITOTIC (M) PHASE
Prophase
Telophase and
Cytokinesis
Prometaphase
Anaphase
Metaphase

53. Various Cell Cycle Phases in onion root tip:

54. Fig. 12-UN3

55. Fig. 12-UN4

57. You should now be able to:
Describe the structural organization of the prokaryotic genome and the eukaryotic genome.
List the phases of the cell cycle; describe the sequence of events during each phase.
List the phases of mitosis and describe the events characteristic of each phase.
Draw or describe the mitotic spindle, including centrosomes, kinetochore microtubules, nonkinetochore microtubules, and asters.
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58. Compare cytokinesis in animals and plants.
Describe the process of binary fission in bacteria and explain how eukaryotic mitosis may have evolved from binary fission.
Explain how the abnormal cell division of cancerous cells escapes normal cell cycle controls.
Distinguish between benign, malignant, and metastatic tumors.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings