1. Chapter 12
The Cell Cycle

2. 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
Organisms develop from a single cell
Single-celled
eukaryote
Repair/replace
damaged cells

3. Continuity of Life is Based on Cell Division
2. 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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4. 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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5. Fig. 12-3
Figure 12.3 Eukaryotic chromosomes
20 µm

6. Gametes (n) have half as many chromosomes as somatic cells.
Somatic cells (2n) (body cells) have two sets (pairs) of chromosomes. A total of 46.
5. Muscle, nerve. blood
Gametes (n) have half as many chromosomes as somatic cells.
reproductive cells: sperm and eggs
23 chromosomes in human gamete
9. Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division.
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7. Distribution of Chromosomes During Eukaryotic Cell Division
11. chromosome- one DNA molecule and associated proteins
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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8. single DNA molecule 0.5 µm Chromosomes DNA molecules Chromo- some arm
Fig. 12-4
0.5 µm
Chromosomes
DNA molecules
single DNA
molecule
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

9. 13. Mitosis, the division of the nucleus
Cytokinesis, the division of the cytoplasm
14. Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell.
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10. 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

11. Phases of the Cell Cycle
17. 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. microtubule-organizing center
Replicates during interphase.
Move apart during prophase.
22. A structure of proteins associated with specific sections of chromosomal DNA at the centromere.

13. 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

14. 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

15. 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

16. Kinetochores help separate chromatids.
Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell, not attached to chromosomes.
Kinetochores help separate chromatids.
26. As chromosomes moved poleward, the microtubule segments on kinetochore side shorten while those on spindle side stayed the same.
For the Cell Biology Video Microtubules in Cell Division, go to Animation and Video Files.
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17. Cytokinesis: A Closer Look
27. In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow in the plasma membrane. A contractile ring of actin microfilaments beneath the furrow contracts and deepens the furrow.
In plant cells, a cell plate enlarges until its surrounding membrane fuses with the plasma membrane of a neighboring cell.
Vesicles from Golgi apparatus move to the middle of the cell and coalesce to form cell plate.
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18. 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)

19. Binary Fission
30. In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes move apart as the cell elongates. Plasma membrane grows inward.
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20. 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

21. The Evolution of Mitosis
Mode of reproduction: pro- binary fission, euk- mitosis
Number of chromosomes: pro- 1, euk- many
Shape of bacterial chromosome: pro circular, euk- linear
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22. Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control system
32. The concentration of certain cytoplasmic molecules controls cell cycle.
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23. The Cell Cycle Control System
33. The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received
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24. 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

25. Controlled by degradation of cyclin.
34. G1- 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.
Controlled by degradation of cyclin.
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26. 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.

27. G2- checks for DNA damage during synthesis
controlled by accumulation of cyclins
M- checks for problems in metaphase-anaphase transition, controlled by MPF
G0- nondividing state
most body cells are in this state, ex. nerve and liver

28. The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
Kinases must be activated by cyclins.
Changes in concentration of cyclin partner.
39. 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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29. (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

30. Stop and Go Signs: Internal and External Signals at the Checkpoints
40. Kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase. M checkpoint is not passed.
41. Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide.
PDGF molecules triggers a signal transduction pathway that allows cells to pass G1 checkpoint
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31. 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

32. External Signals
42. 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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33. 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

34. 44. - 45. 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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35. 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

36. radiation- damages DNA in cancer cells
chemotherapy drugs- interfere with specific steps of cell cycle

37. Various Cell Cycle Phases in onion root tip:

38. Fig. 12-UN3

39. Fig. 12-UN4

41. 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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42. 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.
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