Chapter 9 The Cell Cycle
Overview: The Key Roles of Cell Division
Key Concepts
• The continuity of life is based on the reproduction of
cells, or cell division
9.1 Most cell division results in genetically identical cells
9.2 The mitotic phase alternates with interphase in the cell cycle
9.3 The eukaryotic cell cycle is regulated by a molecular control
system
How do dividing cells distribute chromosomes to daughter cells?
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The functions of cell division
100
m
(a) Reproduction
50
m
(b) Growth and
development
20
m
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Life span of a cell
•
•
•
•
•
Skin cells: 1-34 days
Stomach lining cells: 2 days
Red blood cells: 120 days
Bone cells: 25-30 years
Nerve cells & Heart cells cannot be
replaced
(c) Tissue renewal
I
 Multicellular organisms depend on cell division for:
- Development from a fertilized cell
- Growth and development
- Repair of damaged tissue
- Replacing old cells
 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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Concept 9.1: Most cell division results in
genetically identical daughter cells
• Cells duplicate their genetic material
– Before they divide, ensuring that each daughter cell
receives an exact copy of the genetic material, DNA
• Most cell division results in daughter cells with
identical genetic information, DNA
• The exception is meiosis, a special type of division that
can produce sperm and egg cells
Eukaryotic Chromatin & Chromosomes
- Genetic material in eukaryotes is packaged in the nucleus into
discrete chromosomes
- Every eukaryotic species has a characteristic number of
chromosomes in each cell nucleus
- Eukaryotic chromosomes consist of chromatin, a complex of
DNA and protein
• Condensed & Visible when cell division is occurring
• Otherwise, called chromatin, the uncondensed form
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Fig. 16-21a
Fig. 16-21b
Chromatid
(700 nm)
30-nm fiber
Nucleosome
(10 nm in diameter)
DNA
double helix
(2 nm in diameter)
Loops
Scaffold
H1
DNA, the double helix
300-nm fiber
Histone tail
Histones
Histones
Nucleosomes, or “beads
on a string” (10-nm fiber)
Replicated
chromosome
(1,400 nm)
30-nm fiber
Chromosomes in Human cells
Looped domains
(300-nm fiber)
Metaphase
chromosome
Chromosomes in Human cells
In animals
Somatic cells (non reproductive cells) have two sets of
chromosomes, 46 chromosomes in a human somatic cell
- 44 Autosomes + XY in males
- 44 Autosomes +XX in females
SOMATIC CELL
- All Body cells, except gametes
-
GAMETE
reproductive cells; sperm & egg
-
-
23 chromosomes
46 chromosomes
- Diploid: Two sets of 23 (2n)
- Haploid: One set of 23 (n)
- Produced by mitosis
- Produced by meiosis (chapter 10)
Gametes (reproductive cells: sperm cells & egg cells) have one set
of chromosomes, 23 in a human gamete cell
- A Sperm cell in Males has 22 Autosomes +X or
22 Autosomes + Y
- An Egg cell in Females has 22 Autosomes + X
http://passel.unl.edu/Image/siteImages/SomaticGameteCellLG.gif
2
Figure 12.5-3
Chromosomes
A highly condensed, duplicated human chromosome (SEM)
1
The centromere is the narrow “waist” of the duplicated
chromosome, where the two chromatids are most closely
attached
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Chromosome
duplication
Sister
chromatids
2
Sister
chromatids
Centromere
0.5 m
Separation of
sister chromatids
During cell division, the
two sister chromatids of
each duplicated
chromosome separate
and move into two nuclei
Once separate, the
chromatids are called
chromosomes
3
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Distribution of Chromosomes During Eukaryotic
Cell Division
In preparation for cell division
- DNA is replicated and the chromatin condense to form
chromosomes
Each duplicated chromosome
Phases of the Cell Cycle
The mitotic phase alternates with interphase in the cell
cycle
1. Interphase, including cell growth and copying of
chromosomes in preparation for cell division
2. Mitotic (M) phase, including mitosis and
cytokinesis
- Has two sister chromatids, which separate during cell
division
- The two sister chromatids are held together by
centromere
Interphase
Interphase (about 90% of the cell cycle) can be divided into subphases
- G1 phase (“first gap”)
- S phase (“synthesis” Each chromosome now consists of 2 chromatids)
- G2 phase (“second gap”)
The cell grows during all three phases, but chromosomes are
duplicated(replicated) only during the S phase.
Interphase (about 90% of the cell cycle) can be
divided into subphases
• G1 phase: synthesis of proteins & organelles
• S phase: synthesis of DNA
- Each chromosome now consists of 2 chromatids
• G2 phase: synthesis of proteins & organelles
• G0 phase: cells that do not divide and exit the cell cycle
• The cell grows during all three phases, but
chromosomes are duplicated only during the S phase
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Cell Division
Mitosis is conventionally divided into five phases
Eukaryotic cell division consists of
- Prophase
- Mitosis, the division of the nucleus
Sometimes grouped together as “Prophase”
- Prometaphase
- Cytokinesis, the division of the cytoplasm
- Metaphase
In meiosis
- Anaphase
- Gamete cells are produced after a reduction in
chromosome number
Meiosis yields non identical daughter cells that have
half as many chromosomes as the parent cell
- Telophase
Cytokinesis overlaps the last stages of mitosis
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MITOSIS
INTERPHASE
G2 of Interphase
Centrosomes
(with centriole
pairs)
Prophase/Prometaphase
Prophase
Chromosomes
(duplicated,
uncondensed)
Early mitotic
Centromere
spindle
Aster
 Chromatin condenses to form chromosomes
 The nucleoli and nuclear envelop disappears
 Mitotic spindle begin to form. Microtubules extend from
centrosomes and attach to Kinetochore (region on the
chromosomes near the centromere)
 The centrosomes move away from each other
Nucleolus Nuclear
envelope
Plasma
membrane
Two sister chromatids
of one chromosome
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MITOSIS
Prometaphase
Fragments
of nuclear
envelope
Metaphase
Nonkinetochore
microtubules
Metaphase
plate
Metaphase
• Metaphase is the longest phase of mitosis.
• Chromosomes are fully condensed and most visible at
this stage
• The sister chromatids are arranged at the metaphase
plate, or the equator of the cell, an imaginary plane,
equidistant from the spindle’s two poles.
Kinetochore
Kinetochore
microtubules
Spindle
Centrosome at
one spindle pole
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Anaphase
Telophase
• The sister chromatids separate and become full-fledged
chromosomes
• Each freed chromatid (now referred to as a chromosome or
daughter chromosome) is pulled toward the opposite pole of the
cell
• The two sets of chromosomes have reached the opposite poles of
the cell
• The chromosomes decondense.
• The mitotic spindle disappears
• Two daughter nuclei begin to form, one at each pole
• The nuclear envelope and nucleolus reappear
• Nonkinetochore microtubules from opposite poles overlap and
push against each other, elongating the cell
• *Mitosis, the division of one nucleus into two genetically
identical nuclei, is now complete
MITOSIS
Anaphase
CONCEPT CHECK
Telophase and
Cytokinesis
Cleavage
furrow
Nucleolus
forming
WHICH of the following happen during mitosis?
A.
B.
C.
D.
Daughter
chromosomes
condensing of the chromosomes
synthesis of DNA
formation of the spindle
uncoupling of the chromatids at the
centromeres
Nuclear
envelope
forming
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THINK PAIR SHARE
In Humans:
Before S phase
How many chromosomes?
How many chromatids?
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
After S phase
How many chromosomes?
How many chromatids?
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Aster
The mitotic spindle at metaphase
Centrosome
Sister
chromatids
Metaphase plate
(imaginary)
 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
Kinetochores
Microtubules
Chromosomes
Overlapping
nonkinetochore
microtubules
Kinetochore
microtubules
1 m
0.5 m
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Centrosome
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Figure 9.9
At which end do kinetochore microtubules shorten during anaphase?
Experiment
- 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
Results
Kinetochore
Spindle
pole
Conclusion
Mark
- Chromosomes are also “reeled in” by motor proteins at spindle
poles, and microtubules depolymerize after they pass by the motor
proteins
Chromosome
movement
Kinetochore
Motor
Microtubule protein
Chromosome
Tubulin
subunits
At anaphase, the chromosome movement is correlated
with kinetochore microtubules shortening at their
kinetochore ends and not at the spindle pole ends.
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Animation: Cytokinesis
 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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Cytokinesis in animal and plant cells
Cytokinesis: A Closer Look
(a) Cleavage of an animal cell
(SEM)
(b) Cell plate formation in a plant cell
(TEM)
In animal cells, cytokinesis occurs by a process known as cleavage,
forming a cleavage furrow
In plant cells, a cell plate forms during cytokinesis
Cleavage furrow
Contractile ring of
microfilaments
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Mitosis in a plant cell
Nucleus
Chromosomes
Nucleolus condensing
Ed
ti
100 m
Vesicles Wall of
forming parent cell
Cell
cell plate
plate
Daughter cells
1 m
New
cell wall
Daughter cells
I
Bacterial cell division by binary fission
Chromosomes
Origin of
replication
Chromosome
replication begins.
E. coli cell
Two copies
of origin
Cell wall
Plasma
membrane
Bacterial
chromosome
10 m
Prometaphase
Cell plate
Prophase
One copy of the origin
is now at each end of
the cell.
Origin
Origin
Replication finishes.
Metaphase
Anaphase
Two daughter
cells result.
Telophase
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Binary Fission in Bacteria
The Evolution of Mitosis
Prokaryotes (bacteria and archaea) reproduce by a type of cell
division called binary fission
- Since prokaryotes evolved before eukaryotes, mitosis
probably evolved from binary fission
- In E. coli, the single chromosome replicates, beginning at the
origin of replication
- Certain protists (dinoflagellates, diatoms, and some
yeasts) exhibit types of cell division that seem
intermediate between binary fission and mitosis
- 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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Concept Check
All of the following happen during mitosis except
______________________ .
CONCEPT Check
CHECK
Concept
Which of the following reproduce by binary
fission?
A. bacteria
A. condensing of the chromosomes
B. synthesis of DNA
C. formation of the spindle
D. uncoupling of the chromatids at the
centromeres
The eukaryotic cell cycle is regulated by a molecular
control system
B. archaea
C. chloroplasts
D. mitochondria
E. All of these choices are correct.
Do molecular signals in the cytoplasm regulate the cell cycle?
Experiment
Experiment 1
Experiment 2
- The frequency of cell division varies with the type of cell
- These differences result from regulation at the molecular
level
S
G1
S
S
M
G1
Results
- Cancer cells manage to escape the usual controls on the
cell cycle
G1 nucleus
immediately
entered S phase
and DNA was
synthesized.
M
M
G1 nucleus began
mitosis without
chromosome
duplication.
Conclusion: Molecules present in the cytoplasm control the progression to
S and M phases.
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Evidence for Cytoplasmic Signals
- The cell cycle is driven by specific signaling molecules
present in the cytoplasm
- Some evidence for this hypothesis comes from
experiments with cultured mammalian cells
- Cells at different phases of the cell cycle were fused to
form a single cell with two nuclei at different stages
- Cytoplasmic signals from one of the cells could cause the
nucleus from the second cell to enter the “wrong” stage
of the cell cycle
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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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Mechanical analogy for the cell cycle control system
G1 checkpoint
 For many cells, the G1 checkpoint seems to be the most
important
Control
system
G1
M
 If a cell receives a go-ahead signal at the G1 checkpoint, it
will usually complete the S, G2, and M phases and divide
S
 If the cell does not receive the go-ahead signal, it
will exit the cycle, switching into a nondividing state
called the G0 phase
G2
M checkpoint
G2 checkpoint
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The clock has specific checkpoints
• Cell receives a go-ahead signal at G1 it will go through G1, S, G2
and Mitosis and divide
• Cells enter G0 phase if it does not receive the go-ahead signal
G1 checkpoint
G1
G1
M
G2
M
G0
G2
M
checkpoint
Anaphase
G1
Without go-ahead signal, cell
enters G0.
G1
With go-ahead signal, cell
continues cell cycle.
(a) G1 checkpoint
Prometaphase
G2
checkpoint
Metaphase
Without full chromosome
attachment, stop signal is
received.
With full chromosome
attachment, go-ahead signal is
received.
(b) M checkpoint
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Scalpels
Scalpels
 The cell cycle is regulated by a set of regulatory proteins and
protein complexes including kinases and proteins called cyclins
 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
AAsample
sampleofof
human
humanconnective
connective
tissue
tissueisiscut
cut
up
upinto
intosmall
small
pieces.
pieces.
Petri
Petri
dish
dish
Enzymes digest
the extracellular
matrix, resulting
in a suspension of
free fibroblasts.
Cells are transferred
to culture vessels.
Without PDGF
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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
PDGF is added to
half the vessels.
With PDGF
Cultured fibroblasts
(SEM)
10 m
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Pearson
Education,
Inc.
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Pearson
Education,
Inc.
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Pearson
Education,
Inc.
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2016
Pearson
Education,
Inc.
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Figure 9.18
 Another example of external signals is density-dependent
inhibition, in which crowded cells stop dividing
Anchorage dependence: cells
require a surface for division
 Most animal cells also exhibit anchorage dependence, in which
they must be attached to a substratum in order to divide
Density-dependent inhibition:
cells form a single layer
 Cancer cells exhibit neither density-dependent inhibition nor
anchorage dependence
Density-dependent inhibition:
cells divide to fill a gap and
then stop
20 m
(a) Normal mammalian cells
20 m
(b) Cancer cells
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Loss of Cell Cycle Controls in Cancer Cells
 A normal cell is converted to a cancerous cell by a process called
transformation
Cancer cells do not respond to signals that normally regulate the cell
cycle
 Cancer cells that are not eliminated by the immune system form
tumors, masses of abnormal cells within otherwise normal tissue
Cancer cells do not need growth factors to grow and divide
 If abnormal cells remain only at the original site, the lump is called
a benign tumor
- 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
 Malignant tumors invade surrounding tissues and undergo
metastasis, exporting cancer cells to other parts of the body, where
they may form additional tumors
 How a tumor suppressor gene blocks the cell cycle
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5 m
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 Recent advances in understanding the cell cycle and cell cycle
signaling have led to advances in cancer treatment
The growth and metastasis of
a malignant breast tumor
Breast cancer cell
(colorized SEM)
 Medical treatments for cancer are becoming more “personalized”
to an individual patient’s tumor
Metastatic
tumor
Lymph
vessel
Tumor
Blood
vessel
Cancer
cell
Glandular
tissue
A tumor grows
from a single
cancer cell.
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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.
Henrietta Lacks
HeLa cancer cells
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THINK/PAIR/SHARE
Summary of key concepts:
mitotic phase and interphase
P
G1
Cytokinesis
Mitosis
You get an knee scrape and your skin cells are
getting ready to divide and repair the damage. One of
these skin cells has gone through G2 phase and is
about to enter Mitosis.
S
G2
MITOTIC (M)
PHASE
1. How many chromosomes does it contain?
2. How many chromatids does it contain?
Prophase
Telophase
and
Cytokinesis
Prometaphase
Anaphase
Metaphase
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You should now be able to
1.
2.
3.
4.
5.
6.
7.
8.
Describe the structure 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
Role of kinetochore microtubules and nonkinetochore
microtubules
Compare cytokinesis in animals and plants
What is binary fission?
Difference between normal cells and cancer cells
Distinguish between benign, malignant, and metastatic
tumors
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