1. 9
The Cell Cycle

2. Overview: The Key Roles of Cell Division
The continuity of life is based on the reproduction of cells, called cell division or mitosis
The process by which a single parent cell divides to become 2 identical daughter cells
Prokaryotic cells divide by binary fission (which is mitosis in bacteria).
Eukaryotic cells divide by mitosis and cytokinesis
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3. Mitotic Cell Division Genetic make-up of an organism
Eukaryotes
Prokaryotes
Genome large and linear
Genome small and circular
DNA in nucleus
DNA in nucleoid region of cytoplasm
Eukaryotic cells divide by mitosis (nuclear division) and cytokinesis (cytoplasmic division). Together known as mitotic cell division.
To understand how cell division in eukaryotes differs, it is important to first recall that the genome in eukaryotes is large and linear and the genetic material in a eukaryote is separated from the other cellular components because it is in the nucleus of the cell.
Functions:
Reproduction of cells
Growth & development
Tissue renewal

4. Functions of Mitosis (a) Reproduction 100 m 50 m
Figure 9.2
(a) Reproduction
100 m
50 m
(b) Growth and development
Functions of Mitosis
Figure 9.2 The functions of cell division
20 m
(c) Tissue renewal
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5. THE HUMAN LIFE CYCLE Multicellular adults: diploid
2n = 46, total number chromosomes in humans
Exception: reproductive cells (gametes) in testes and ovaries
Sperm & egg: each is haploid, n = 23

6. Chromosomes and Chromatin
Thread-like structures in eukaryotic nuclei that package your genes
Chemically made of 40% DNA & 60% histone proteins
Visible only during mitosis & meiosis
Each species has a characteristic #
Human somatic/body cells: 46 total (23 prs)
22 pr homologous chromosomes
1 pr sex chromosomes
Human gametes: 23 only
Chromatin
Long, thinner fiber that shortens to become chromosome
Occurs long and thin during Interphase
Same as chromosome, but not visible

7. Chromatin 20 m Figure 9.3 Figure 9.3 Eukaryotic chromosomes
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8. Karyotype of Chromosomes
In eukaryotic cells, DNA is organized with histones and other proteins into chromatin, which can be looped and packaged to form the structures we know as chromosomes. The portrait formed by the number and shapes of chromosomes representative of a species is called its karyotype.
Most cells of the human body contain 46 chromosomes (23 pairs). Cells from horses have 64 chromosomes, and cells from corn have 20.
The human karyotype here shows 22 pairs of homologous chromosomes
Numbered 1−22 from the longest to the shortest chromosome
and 1 pair of sex chromosomes.
Homologous chromosomes carry the same set of genes, one from the mother and one from the father. The sex chromosomes are X and Y; two X chromosomes are a female and an X and Y chromosome is a male.
Photomicrograph of somatic cell metaphase chromosomes

9. Distribution of Chromosomes During Eukaryotic Cell Division
In preparation for cell division, DNA is replicated and the chromosomes condense/shorten
Each duplicated chromosome has two sister chromatids, joined identical copies of the original chromosome
The centromere is where the two chromatids are most closely attached
Sister chromatids
Centromere
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10. Chromosome duplication
Figure 9.5-s3
Chromosomal
Chromosomes
DNA molecules
Centromere
Chromosome
arm
Chromosome duplication
Sister
chromatids
Figure 9.5-s3 Chromosome duplication and distribution during cell division (step 3)
Separation of sister
chromatids
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11. Cell Cycle (Eukaryotes)
A generation of the cell:
M phase: mitosis and cytokinesis
Alternates with Interphase
G1 phase
S phase
G2 phase
G0 phase
With few exceptions (RBC), most cells undergo cell cycle
Duration varies with cell type
Many repetitions of cell growth 20 – 50 x
Cell division in eukaryotes proceeds through a number of steps that make up the cell cycle. The cell cycle consists of two distinct stages:
M phase: the time during which the parent cell divides into two daughter cells
Interphase: the time between two successive M phases

12. Interphase
Interphase: 90% of cell cycle: growth & metabolic activities
G1 phase: preparation for DNA synthesis
S phase: DNA synthesis into sister chromatids
G2 phase: preparation for M and C
G0 phase: cells not actively dividing (either permanently- brain, nerves, skeletal muscles or cardiac muscles; or temporarily- liver)
Interphase lasts about 10−14 hours. During this stage, the cell makes many preparations for division. These preparations include replication of the DNA in the nucleus and an increase in the cell size so that each daughter cell receives sufficient amounts of cytoplasmic and membrane components.
G1 phase: The first “gap” phase, where the size and protein content of the cell increases in preparation for the S phase. During this phase, many regulatory proteins are made and activated.
S phase: The “synthesis” phase, where the entire DNA content in the nucleus of the cell is replicated.
G2 phase: The second “gap phase,” where the cell prepares for mitosis and cytokinesis.
G0 phase: Distinguished from the G1 phase because there is no active preparation taking place for cell division. This phase is present in cell types that do not actively divide. Cells in this phase would include liver cells, nerve cells, and those that form the lens of the eye.

13. I N T E R PHASE S G1 (DNA synthesis) Cytokinesis G2 Mitosis MIT (M)
Figure 9.6 I N T E R
PHASE S G1
(DNA synthesis)
Cytokinesis G2
Mitosis
Figure 9.6 The cell cycle
OTIC
MIT
(M)
PHA SE
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14. Cell Cycle and Mitotic Phases
Figure 9.UN02
Cell Cycle and Mitotic Phases N T E R P I H A S E G1 S
Cytokinesis
Mitosis G2
MITOTIC (M)
PHASE
Prophase
Telophase
and
Cytokinesis
Figure 9.UN02 Summary of key concepts: mitotic phase and interphase
Prometaphase
Anaphase
Metaphase
Mitotic Phases: PPMAT/C
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15. G2 of Interphase Prophase Prometaphase 10 m Centrosomes
Figure 9.7-1
10 m
G2 of Interphase
Prophase
Prometaphase
Centrosomes
(with centriole
pairs)
Chromosomes
(duplicated,
uncondensed)
Early mitotic
spindle
Centromere
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Aster
Figure Exploring mitosis in an animal cell (part 1: G2 of interphase through prometaphase)
Nucleolus
Nuclear
envelope
Plasma
membrane
Two sister chromatids
of one chromosome
Kinetochore
Kinetochore
microtubules
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16. Telophase and Cytokinesis
Figure 9.7-2
10 m
Metaphase
Anaphase
Telophase and
Cytokinesis
Metaphase
plate
Cleavage
furrow
Nucleolus
forming
Figure Exploring mitosis in an animal cell (part 2: metaphase through cytokinesis)
Nuclear
envelope
forming
Spindle
Centrosome at
one spindle pole
Daughter
chromosomes
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17. G2 of Interphase Prophase Centrosomes (with centriole pairs)
Figure 9.7-3
G2 of Interphase
Prophase
Centrosomes
(with centriole
pairs)
Chromosomes
(duplicated,
uncondensed)
Early mitotic
spindle
Centromere
Aster
Figure Exploring mitosis in an animal cell (part 3: G2 of interphase and prophase)
Nucleolus
Nuclear
envelope
Plasma
membrane
Two sister chromatids
of one chromosome
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18. 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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19. Prometaphase Metaphase Fragments of nuclear envelope Nonkinetochore
Figure 9.7-4
Prometaphase
Metaphase
Fragments
of nuclear
envelope
Nonkinetochore
microtubules
Metaphase
plate
Figure Exploring mitosis in an animal cell (part 4: prometaphase and metaphase)
Kinetochore
Kinetochore
microtubules
Spindle
Centrosome at
one spindle pole
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20. Telophase and Cytokinesis
Figure 9.7-5
Anaphase
Telophase and
Cytokinesis
Cleavage
furrow
Nucleolus
forming
Figure Exploring mitosis in an animal cell (part 5: anaphase, telophase and cytokinesis)
Nuclear
envelope
forming
Daughter
chromosomes
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21. 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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22. 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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23. Cytokinesis (a) Cleavage of an animal cell (SEM)
Figure 9.10
Cytokinesis
(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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24. Binary Fission in Prokaryotes
Figure 9.12-s4
Binary Fission in Prokaryotes
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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25. Checkpoints of 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 timing device of a washing machine
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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26. G1 checkpoint seems to be the most important G1 checkpoint
Figure 9.15
G1 checkpoint seems to be the most important
G1 checkpoint
Control
system S G1 G2 M
Figure 9.15 Mechanical analogy for the cell cycle control system
M checkpoint
G2 checkpoint
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27. Figure
G1 Checkpoint
G1 checkpoint G0 G1 G1
Figure Two important checkpoints (part 1: G1 checkpoint)
Without go-ahead signal, cell
enters G0.
With go-ahead signal, cell
continues cell cycle.
(a) G1 checkpoint
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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28. Figure
G2 and M Checkpoints G1 G1 M G2 M G2 M
checkpoint G2
checkpoint
Anaphase
Figure Two important checkpoints (part 2: M checkpoint)
Prometaphase
Metaphase
Without full chromosome
attachment, stop signal is
received.
With full chromosome
attachment, go-ahead signal is
received.
(b) M checkpoint
Anaphase does not begin if any kinetochores remain unattached to spindle microtubules
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29. Density-Dependent Inhibition
An example of an external signal 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
Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence
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30. Anchorage dependence: cells require a surface for division
Figure 9.18
Anchorage dependence: cells
require a surface for division
Density-dependent inhibition:
cells form a single layer
Density-dependent inhibition:
cells divide to fill a gap and
then stop
Figure 9.18 Density-dependent inhibition and anchorage dependence of cell division
20 m
20 m
(a) Normal mammalian cells
(b) Cancer cells
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31. Loss of Cell Cycle Controls in Cancer Cells
Cancer cells do not respond to signals that normally regulate the cell cycle
Cancer cells do 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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32. Malignant Tumors are Cancer
Cancer cells that are not eliminated by the immune system form tumors or masses of abnormal cells within otherwise normal tissue
If abnormal cells remain only at the original site, the lump is called a benign tumor
A normal cell converted to a cancerous cell is called transformation
Malignant tumors invade surrounding tissues and undergo metastasis, exporting cancer cells to other parts of the body, where they may form additional tumors
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33. Breast cancer cell (colorized SEM) Metastatic tumor Lymph vessel Tumor
Figure 9.19
5 m
Breast cancer cell
(colorized SEM)
Metastatic
tumor
Lymph
vessel
Tumor
Blood
vessel
Figure 9.19 The growth and metastasis of a malignant breast tumor
Glandular
tissue
Cancer
cell
A tumor grows
from a single
cancer cell.
Cancer cells
invade
neighboring
tissue.
Cancer cells spread
through lymph and
blood vessels to
other parts of the
body.
A small percentage
of cancer cells may
metastasize to
another part of the
body.
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