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Chapter 12
The Cell Cycle
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Biology, Seventh Edition
Neil Campbell and Jane Reece
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 12.1 Chromosomes in a dividing cell
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Figure 12.2 The functions of cell division
100 µm
(a) Reproduction. An amoeba,
a single-celled eukaryote, is
dividing into two cells. Each
new cell will be an individual
organism (LM).
200 µm
20 µm
(b) Growth and development.
(c) Tissue renewal. These dividing
This micrograph shows a
bone marrow cells (arrow) will
sand dollar embryo shortly after
give rise to new blood cells (LM).
the fertilized egg divided, forming
two cells (LM).
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Figure 12.3 Eukaryotic chromosomes
50 µm
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Figure 12.4 Chromosome duplication and
distribution during cell division
0.5 µm
A eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Centrometers
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Sister
chromatids
Sister chromatids
Figure 12.5 The cell cycle
INTERPHASE
S
(DNA synthesis)
G1
G2
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Figure 12.6 Exploring The Mitotic Division of an Animal Cell
G2 OF INTERPHASE
Centrosomes
(with centriole pairs)
Nucleolus
Chromatin
(duplicated)
Nuclear
Plasma
envelope membrane
PROPHASE
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids
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PROMETAPHASE
Fragments
of nuclear
envelope
Kinetochore
Nonkinetochore
microtubules
Kinetochore
microtubule
G2 of Interphase
• A nuclear envelope bounds
the nucleus.
• The nucleus contains one or
more nucleoli (singular,
nucleolus).
• Two centrosomes have
formed by replication of a
single centrosome.
• In animal cells, each
centrosome features two
centrioles.
• Chromosomes, duplicated
during S phase, cannot be
seen individually because
they have not yet condensed.
The light micrographs show dividing
lung cells from a newt, which has 22
chromosomes in its somatic cells
(chromosomes appear blue,
microtubules green, intermediate
filaments red). For simplicity, the
drawings show only four
chromosomes.
Prophase
• The chromatin fibers become
more tightly coiled, condensing
into discrete chromosomes
observable with a light
microscope.
• The nucleoli disappear.
• Each duplicated chromosome
appears as two identical sister
chromatids joined together.
• The mitotic spindle begins to form.
It is composed of the centrosomes
and the microtubules that extend
from them. The radial arrays of
shorter microtubules that extend
from the centrosomes are called
asters (“stars”).
• The centrosomes move away from
each other, apparently propelled
by the lengthening microtubules
between them.
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Prometaphase
• The nuclear envelope
fragments.
• The microtubules of the
spindle can now invade the
nuclear area and interact
with the chromosomes,
which have become even
more condensed.
• Microtubules extend from
each centrosome toward
the middle of the cell.
• Each of the two chromatids
of a chromosome now has
a kinetochore, a
specialized protein
structure located at the
centromere.
• Some of the microtubules
attach to the kinetochores,
becoming “kinetochore
microtubules.” These
kinetochore microtubules
jerk the chromosomes back
and forth.
• Nonkinetochore
microtubules interact with
those from the opposite
pole of the spindle.
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Centrosome at Daughter
one spindle pole chromosomes
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TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nuclear
envelope
forming
Nucleolus
forming
Metaphase
• Metaphase is the longest stage of
mitosis, lasting about 20 minutes.
• The centrosomes are now at
opposite ends of the cell.
•The chromosomes convene on the
metaphase plate, an imaginary
plane that is equidistant between
the spindle’s two poles. The
chromosomes’ centromeres lie on
the metaphase plate.
• For each chromosome, the
kinetochores of the sister
chromatids are attached to
kinetochore microtubules coming
from opposite poles.
• The entire apparatus of
microtubules is called the spindle
because of its shape.
Anaphase
• Anaphase is the shortest stage of
mitosis, lasting only a few minutes.
• Anaphase begins when the two sister
chromatids of each pair suddenly part.
Each chromatid thus becomes a fullfledged chromosome.
• The two liberated chromosomes begin
moving toward opposite ends of the cell,
as their kinetochore microtubules
shorten. Because these microtubules are
attached at the centromere region, the
chromosomes move centromere first (at
about 1 µm/min).
• The cell elongates as the
nonkinetochore microtubules lengthen.
• By the end of anaphase, the two ends of
the cell have equivalent—and
complete—collections of chromosomes.
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Telophase
• Two daughter nuclei begin to
form in the cell.
• Nuclear envelopes arise from
the fragments of the parent
cell’s nuclear envelope and
other portions of the
endomembrane system.
• The chromosomes become
less condensed.
• Mitosis, the division of one
nucleus into two genetically
identical nuclei, is now
complete.
Cytokinesis
• The division of the cytoplasm
is usually well underway by
late telophase, so the two
daughter cells appear shortly
after the end of mitosis.
• In animal cells, cytokinesis
involves the formation of a
cleavage furrow, which
pinches the cell in two.
Figure 12.7 The mitotic spindle at metaphase
Aster
Sister
chromatids
Centrosome
Metaphase
Plate
Kinetochores
Overlapping
nonkinetochore
microtubules
Kinetochores microtubules
0.5 µm
Microtubules
Chromosomes
Centrosome
1 µm
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Figure 12.8 Inquiry During anaphase, do kinetochore microtubules
shorten at their spindle pole ends or their kinetochore ends?
EXPERIMENT
1 The microtubules of a cell in early anaphase were labeled with a fluorescent dye
that glows in the microscope (yellow).
Kinetochore
Spindle
pole
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2 A Laser was used to mark the kinetochore mircotubles by eliminating the fluorescnce in a region between
one spindle pole and the chromosomes. As anaphase proceeded, researches monitored the changes
in the lengths of the microtubles on either side of the mark.
Mark
RESULTS
As the chromosomes moved toward the poles, the microtubule segments on the
kinetochore side of the laser mark shortened, while those on the spindle pole side stayed the
same length.
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CONCLUSION
This experiment demonstrated that during anaphase, kinetochore microtubules shorten
at their kinetochore ends, not at their spindle pole ends. This is just one of the experiments supporting the
hypothesis that during anaphase, a chromosome tracks along a microtubule as the microtubule
depolymerizes at its kinetochore end, releasing tubulin subunits
Chromosome
movement
Microtubule
Kinetochore
Tubulin
subunits
Motor
protein
Chromosome
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Figure 12.9 Cytokinesis in animal and plant cells
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Vesicles
forming
cell plate
Wall of
patent cell
1 µm
Cell plate
New cell wall
Daughter cells
Daughter cells
(a) Cleavage of an animal cell (SEM)
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(b) Cell plate formation in a plant cell (SEM)
Figure 12.10 Mitosis in a plant cell
Chromatine
Nucleus
Nucleolus condensing
1 Prophase.
The chromatin
is condensing.
The nucleolus is
beginning to
disappear.
Although not
yet visible
in the micrograph,
the mitotic spindle is
staring to from.
Chromosome
Metaphase. The
2 Prometaphase.
3
4
spindle is complete,
We now see discrete
and the chromosomes,
chromosomes; each
attached to microtubules
consists of two
at their kinetochores,
identical sister
are all at the metaphase
chromatids. Later
plate.
in prometaphase, the
nuclear envelop will
fragment.
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5
Anaphase. The
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends
of cell as their
kinetochore
microtubles shorten.
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will
divided the cytoplasm
in two, is growing
toward the perimeter
of the parent cell.
Figure 12.11 Bacterial cell division (binary fission)
Origin of
replication
Cell wall
Plasma
Membrane
E. coli cell
1 Chromosome replication begins.
Soon thereafter, one copy of the
origin moves rapidly toward the other
end of the cell.
2 Replication continues. One copy of
the origin is now at each end of
the cell.
3 Replication finishes. The plasma
membrane grows inward, and
new cell wall is deposited.
4 Two daughter cells result.
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Two copies
of origin
Origin
Bacterial
Chromosome
Origin
Figure 12.12 A hypothetical sequence for the
evolution of mitosis
(a) Prokaryotes. During binary fission, the origins of the
daughter chromosomes move to opposite ends of the cell.
The mechanism is not fully understood, but proteins may
anchor the daughter chromosomes to specific sites on the
plasma membrane.
(b) Dinoflagellates. In unicellular protists called dinoflagellates,
the nuclear envelope remains intact during cell division, and
the chromosomes attach to the nuclear envelope. Microtubules
pass through the nucleus inside cytoplasmic tunnels,
reinforcing the spatial orientation of the nucleus, which then
divides in a fission process reminiscent of bacterial division.
(c) Diatoms. In another group of unicellular protists, the
diatoms, the nuclear envelope also remains intact during
cell division. But in these organisms, the microtubules
form a spindle within the nucleus. Microtubules separate
the chromosomes, and the nucleus splits into two
daughter nuclei.
(d) Most eukaryotes. In most other eukaryotes, including
plants and animals, the spindle forms outside the
nucleus, and the nuclear envelope breaks down during
mitosis. Microtubules separate the chromosomes, and
the nuclear envelope then re-forms.
Bacterial
chromosome
Chromosomes
Microtubules
Intact nuclear
envelope
Kinetochore
microtubules
Intact nuclear
envelope
Kinetochore
microtubules
Centrosome
Fragments of
nuclear envelope
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Figure 12.13 Inquiry Are there molecular signals in
the cytoplasm that regulate the cell cycle?
EXPERIMENTS
In each experiment, cultured mammalian cells at two different phases of the cell cycle were induced to fuse.
Experiment 1
S
Experiment 2
G1
M
S
M
G1
RESULTS
S
When a cell in the S
phase was fused with
a cell in G1, the G1 cell
immediately entered the
S phase—DNA was
synthesized.
CONCLUSION
M
When a cell in the M phase
was fused with a cell in G1, the
G1 cell immediately began mitosis—
a spindle formed and chromatin
condensed, even though the
chromosome had not been duplicated.
The results of fusing cells at two different phases of the cell cycle suggest that molecules present in the
cytoplasm of cells in the S or M phase control the progression of phases.
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Figure 12.14 Mechanical analogy for the cell cycle
control system
G1 checkpoint
Control
system
G1
M
M checkpoint
G2 checkpoint
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G2
S
Figure 12.15 The G1 checkpoint
G0
G1 checkpoint
G1
(a) If a cell receives a go-ahead signal at
the G1 checkpoint, the cell continues
on in the cell cycle.
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G1
(b) If a cell does not receive a go-ahead
signal at the G1checkpoint, the cell
exits the cell cycle and goes into G0, a
nondividing state.
(a) Fluctuation of MPF activity and
cyclin concentration during
the cell cycle
Relative Concentration
Figure 12.16 Molecular control of the cell cycle at
the G2 checkpoint
G1 S G2 M
MPF activity
G 1 S G2 M
Cyclin
Time
(b) Molecular mechanisms that
help regulate the cell cycle
1 Synthesis of cyclin begins in late S
phase and continues through G2.
Because cyclin is protected from
degradation during this stage, it
accumulates.
5 During G1, conditions in
the cell favor degradation
of cyclin, and the Cdk
component of MPF is
recycled.
Cdk
Degraded
Cyclin
Cyclin is
degraded
4 During anaphase, the cyclin component
of MPF is degraded, terminating the M
phase. The cell enters the G1 phase.
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G2
Cdk
checkpoint
MPF
Cyclin
2 Accumulated cyclin molecules
combine with recycled Cdk molecules, producing enough molecules
of MPF to pass the G2 checkpoint and
initiate the events of mitosis.
3 MPF promotes mitosis by phosphorylating
various proteins. MPF‘s activity peaks during
metaphase.
Figure 12.17 Inquiry Does platelet-derived growth factor (PDGF)
stimulate the division of human fibroblast cells in culture?
EXPERIMENT
1
Scalpels
A sample of connective tissue was cut up
into small pieces.
Petri
plate
2
Enzymes were used to digest the extracellular matrix,
resulting in a suspension of free fibroblast cells.
3
Cells were transferred to sterile culture vessels
containing a basic growth medium consisting of
glucose, amino acids, salts, and antibiotics (as a
precaution against bacterial growth). PDGF was
added to half the vessels. The culture vessels
were incubated at 37°C.
With PDGF
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Without PDGF
RESULTS
a In a basic growth medium
without PDGF (the control),
cells failed to divide.
Without PDGF
b In a basic growth medium
plus PDGF, cells proliferated.
The SEM shows cultured
fibroblasts.
With PDGF
10 µm
CONCLUSION This experiment confirmed that PDGF stimulates the division
of human fibroblast cells in culture.
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Figure 12.18 Density-dependent inhibition and
anchorage dependence of cell division
(a) Normal mammalian cells. The
availability of nutrients, growth
factors, and a substratum for
attachment limits cell
density to a single layer.
Cells anchor to dish surface and
divide (anchorage dependence).
When cells have formed a complete
single layer, they stop dividing
(density-dependent inhibition).
If some cells are scraped away, the
remaining cells divide to fill the gap and
then stop (density-dependent inhibition).
25 µm
Cancer cells do not exhibit
anchorage dependence or
density-dependent inhibition.
(b) Cancer cells. Cancer cells usually
continue to divide well beyond a
single layer, forming a clump of
overlapping cells.
25 µm
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Figure 12.19 The growth and metastasis of a
malignant breast tumor
Lymph
vessel
Tumor
Blood
vessel
Glandular
tissue
1 A tumor grows from a
single cancer cell.
2 Cancer cells invade
neighboring tissue.
Cancer cell
3 Cancer cells spread
through lymph and
blood vessels to
other parts of the body.
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Metastatic
Tumor
4 A small percentage of
cancer cells may survive
and establish a new tumor
in another part of the body.
Unnumbered Figure p. 235
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1. Using the data in the table below, the best conclusion
concerning the difference between the S phases for beta
and gamma is that
Minutes Spent in Cell Cycle Phases
Cell Type
G1
S
G
M
2
Beta
Delta
18 24 12 16
100
0
0
0
Gamma
18 48 14 20
a. gamma contains more DNA than beta.
b. beta and gamma contain the same amount of DNA.
c.
beta contains more RNA than gamma.
d. gamma contains 48 times more DNA and RNA than beta.
e. beta is a plant cell and gamma is an animal cell.
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2. Cytokinesis usually, but not always, follows
mitosis. If a cell completed mitosis but not
cytokinesis, what would be the result?
a. a cell with a single large nucleus
b. a cell with high concentrations of actin
and myosin
c. a cell with two abnormally small nuclei
d. a cell with two nuclei
e. a cell with two nuclei but with half the
amount of DNA
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3. Taxol is an anticancer drug extracted from the Pacific
yew tree. In animal cells, taxol disrupts microtubule
formation by binding to microtubules and accelerating
their assembly from the protein precursor, tubulin.
Surprisingly, this stops mitosis. Specifically, taxol
must affect
a. the fibers of the mitotic spindle.
b. anaphase.
c. formation of the centrioles.
d. chromatid assembly.
e. the S phase of the cell cycle.
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4. Movement of the chromosomes during
anaphase would be most affected by a drug
that
a. reduced cyclin concentrations.
b. increased cyclin concentrations.
c. prevented elongation of microtubules.
d. prevented shortening of microtubules.
e. prevented attachment of the microtubules to
the kinetochore.
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5. Measurements of the amount of DNA per nucleus
were taken on a large number of cells from a
growing fungus. The measured DNA levels
ranged from 3 to 6 picograms per nucleus. In
which stage of the cell cycle was the nucleus with
6 picograms of DNA?
a. G0
b. G1
c. S
d. G2
e. M
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6. A group of cells is assayed for DNA content
immediately following mitosis and is found to
have an average of 8 picograms of DNA per
nucleus. Those cells would have __________
picograms at the end of the S phase and
__________ picograms at the end of G2.
a. 8 ... 8
b. 8 ... 16
c. 16 ... 8
d. 16 ... 16
e. 12 ... 16
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7. A particular cell has half as much DNA as
some of the other cells in a mitotically active
tissue. The cell in question is most likely in
a. G1.
b. G2.
c. prophase.
d. metaphase.
e. anaphase.
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8. In some organisms, mitosis occurs without
cytokinesis occurring. This will result in
a. cells with more than one nucleus.
b. cells that are unusually small.
c. cells lacking nuclei.
d. destruction of chromosomes.
e. cell cycles lacking an S phase.
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10. The rhythmic changes in cyclin concentration
in a cell cycle are due to
a. its increased production once the restriction
point is passed.
b. the cascade of increased production once its
enzyme is phosphorylated by MPF.
c. its degradation, which is initiated by
active MPF.
d. the correlation of its production with the
production of Cdk.
e. the binding of the growth factor PDGF.
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
11. A cell in which of the following phases would
have the least amount of DNA?
a. G0
b. G2
c. prophase
d. metaphase
e. anaphase
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings