2.5 cell division

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2.5 CELL DIVISION
2.5.6 STATE THAT GROWTH,
EMBRYONIC DEVELOPMENT, TISSUE
REPAIR AND ASEXUAL
REPRODUCTION INVOLVE MITOSIS
OVERVIEW: THE KEY ROLES OF CELL DIVISION
The ability of organisms to produce more of their own
kind best distinguishes living things from nonliving
matter
 The continuity of life is based on the reproduction of
cells, or cell division

© 2011 Pearson Education, Inc.


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
© 2011 Pearson Education, Inc.
FIGURE 12.2
100 m
(a) Reproduction
200 m
(b) Growth and
development
20 m
(c) Tissue renewal
CONCEPT 12.1: MOST CELL DIVISION RESULTS IN
GENETICALLY IDENTICAL DAUGHTER CELLS
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

© 2011 Pearson Education, Inc.
2.5.5 EXPLAIN HOW MITOSIS
PRODUCES TWO GENETICALLY
IDENTICAL NUCLEI
CONCEPT 12.1: MOST CELL DIVISION RESULTS IN
GENETICALLY IDENTICAL DAUGHTER CELLS
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

© 2011 Pearson Education, Inc.
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

© 2011 Pearson Education, Inc.
FIGURE 12.3
20 m
Eukaryotic chromosomes consist of chromatin, a
complex of DNA and protein that condenses during cell
division
 Every eukaryotic species has a characteristic number of
chromosomes in each cell nucleus
 Somatic cells (nonreproductive cells) have two sets of
chromosomes
 Gametes (reproductive cells: sperm and eggs) have half
as many chromosomes as somatic cells

© 2011 Pearson Education, Inc.
DISTRIBUTION OF CHROMOSOMES DURING
EUKARYOTIC CELL DIVISION
In preparation for cell division, DNA is replicated and
the chromosomes condense
 Each duplicated chromosome has two sister
chromatids (joined copies of the original chromosome),
which separate during cell division
 The centromere is the narrow “waist” of the duplicated
chromosome, where the two chromatids are most closely
attached

© 2011 Pearson Education, Inc.
FIGURE 12.4
Sister
chromatids
Centromere
0.5 m
During cell division, the two sister chromatids of each
duplicated chromosome separate and move into two
nuclei
 Once separate, the chromatids are called chromosomes

© 2011 Pearson Education, Inc.
FIGURE 12.5-1
Chromosomes
1
Chromosomal
DNA molecules
Centromere
Chromosome
arm
FIGURE 12.5-2
Chromosomes
1
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Chromosome duplication
(including DNA replication)
and condensation
2
Sister
chromatids
FIGURE 12.5-3
Chromosomes
1
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Chromosome duplication
(including DNA replication)
and condensation
2
Sister
chromatids
Separation of sister
chromatids into
two chromosomes
3

Eukaryotic cell division consists of


Mitosis, the division of the genetic material in the nucleus
Cytokinesis, the division of the cytoplasm
Gametes are produced by a variation of cell division
called meiosis
 Meiosis yields nonidentical daughter cells that have only
one set of chromosomes, half as many as the parent cell

© 2011 Pearson Education, Inc.
2.5.1 OUTLINE THE STAGES IN
THE CELL CYCLE, INCLUDING
INTERPHASE (G1, S, G 2),
MITOSIS AND CYTOKINESIS
CONCEPT 12.2: THE MITOTIC PHASE ALTERNATES
WITH INTERPHASE IN THE CELL CYCLE

In 1882, the German anatomist Walther Flemming
developed dyes to observe chromosomes during mitosis
and cytokinesis
© 2011 Pearson Education, Inc.
PHASES OF THE CELL CYCLE

The cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)
 Interphase (cell growth and copying of chromosomes in
preparation for cell division)

© 2011 Pearson Education, Inc.

Interphase (about 90% of the cell cycle) can be divided
into subphases
G1 phase (“first gap”)
 S phase (“synthesis”)
 G2 phase (“second gap”)


The cell grows during all three phases, but chromosomes
are duplicated only during the S phase
© 2011 Pearson Education, Inc.
FIGURE 12.6
INTERPHASE
G1
S
(DNA synthesis)
G2
2.5.3 STATE THAT INTERPHASE
IS AN ACTIVE PERIOD IN THE
LIFE OF A CELL WHEN MANY
METABOLIC REACTIONS OCCUR:
Including protein synthesis, DNA replication and
an increase in the number of mitochondria and/or
chloroplasts







The cell specialises to a particular function in a
process called differentiation.
Through gene expression and protein synthesis there
is a specialisation of cell structure and function.
During this interphase the cell carries out this
specialist function.
The length of the interphase varies from one type of
cell to another.
G1 follows cytokinesis. The cell is involved in the
synthesis of various proteins which allow the cell to
specialise.
S-phase involves the replication of DNA molecules
which takes place prior to the phases of mitosis.
G2 preparation for the phases of mitosis which
involves the replication of mitochondria and in the
case of plants, the chloroplast.
2.5.4 DESCRIBE THE EVENTS THAT
OCCUR IN THE FOUR PHASES OF
MITOSIS (PROPHASE, METAPHASE,
ANAPHASE AND TELOPHASE

Mitosis is conventionally divided into five phases





Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis overlaps the latter stages of mitosis
 Mitosis and Cytokinesis animation
 Mitosis video

© 2011 Pearson Education, Inc.
10 m
FIGURE 12.7
G2 of Interphase
Centrosomes
(with centriole pairs)
Nucleolus
Chromatin
(duplicated)
Nuclear
envelope
Plasma
membrane
Prophase
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids
Prometaphase
Fragments
of nuclear
envelope
Kinetochore
Metaphase
Nonkinetochore
microtubules
Kinetochore
microtubule
Anaphase
Metaphase
plate
Spindle
Centrosome at
one spindle pole
Telophase and Cytokinesis
Cleavage
furrow
Daughter
chromosomes
Nuclear
envelope
forming
Nucleolus
forming
THE MITOTIC SPINDLE: A CLOSER LOOK
The mitotic spindle is a structure made of
microtubules that controls chromosome movement
during mitosis
 In animal cells, assembly of spindle microtubules begins
in the centrosome, the microtubule organizing center
 The centrosome replicates during interphase, forming
two centrosomes that migrate to opposite ends of the cell
during prophase and prometaphase

© 2011 Pearson Education, Inc.
An aster (a radial array of short microtubules) extends
from each centrosome
 The spindle includes the centrosomes, the spindle
microtubules, and the asters

© 2011 Pearson Education, Inc.
During prometaphase, some spindle microtubules attach
to the kinetochores of chromosomes and begin to move
the chromosomes
 Kinetochores are protein complexes associated with
centromeres
 At metaphase, the chromosomes are all lined up at the
metaphase plate, an imaginary structure at the
midway point between the spindle’s two poles

© 2011 Pearson Education, Inc.
FIGURE 12.8
Aster
Centrosome
Sister
chromatids
Metaphase
plate
(imaginary)
Microtubules
Chromosomes
Kinetochores
Centrosome
1 m
Overlapping
nonkinetochore
microtubules
Kinetochore
microtubules
0.5 m
FIGURE 12.8A
Kinetochores
Kinetochore
microtubules
0.5 m
FIGURE 12.8B
Microtubules
Chromosomes
Centrosome
1 m
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

© 2011 Pearson Education, Inc.
FIGURE 12.9
Figure 12.9
Inquiry: At
which end do
kinetochore
microtubules
shorten during
anaphase?
EXPERIMENT
Kinetochore
Spindle
pole
Mark
RESULTS
CONCLUSION
Microtubule
Chromosome
movement
Motor protein
Chromosome
Kinetochore
Tubulin
subunits
FIGURE 12.9A
EXPERIMENT
Kinetochore
Spindle
pole
Mark
RESULTS
FIGURE 12.9B
CONCLUSION
Microtubule
Chromosome
movement
Motor protein
Chromosome
Kinetochore
Tubulin
subunits
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
 Cytokinesis begins during anaphase or telophase and the
spindle eventually disassembles

© 2011 Pearson Education, Inc.
CYTOKINESIS: A CLOSER LOOK
In animal cells, cytokinesis occurs by a process known as
cleavage, forming a cleavage furrow
 In plant cells, a cell plate forms during cytokinesis

© 2011 Pearson Education, Inc.
Animation: Cytokinesis
Right-click slide / select ”Play”
© 2011 Pearson Education, Inc.
FIGURE 12.10
(a) Cleavage of an animal cell (SEM)
100 m
Cleavage furrow
Contractile ring of
microfilaments
(b) Cell plate formation in a plant cell (TEM)
Vesicles
forming
cell plate
Wall of parent cell
Cell plate
1 m
New cell wall
Daughter cells
Daughter cells
FIGURE 12.10A
(a) Cleavage of an animal cell (SEM)
Cleavage furrow
Contractile ring of
microfilaments
100 m
Daughter cells
FIGURE 12.10B
(b) Cell plate formation in a plant cell (TEM)
Vesicles Wall of parent cell
forming
cell plate
Cell plate
1 m
New cell wall
Daughter cells
FIGURE 12.10C
Cleavage
furrow
100 m
FIGURE 12.10D
Vesicles Wall of parent cell
forming
cell plate
1 m
FIGURE 12.11
Nucleus
Chromatin
condensing
Nucleolus
1 Prophase
Chromosomes
2 Prometaphase 3 Metaphase
Cell plate
4 Anaphase
10 m
5 Telophase
FIGURE 12.11A
Nucleus
Chromatin
condensing
Nucleolus
10 m
1 Prophase
FIGURE 12.11B
Chromosomes
10 m
2 Prometaphase
FIGURE 12.11C
10 m
3 Metaphase
FIGURE 12.11D
10 m
4 Anaphase
FIGURE 12.11E
Cell plate
10 m
5 Telophase
2.5.2 STATE THAT TUMORS
(CANCERS) ARE THE RESULT OF
UNCONTROLLED CELL DIVISION AND
THAT THESE CAN OCCUR IN ANY
ORGAN OR TISSUE
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 differences result from regulation at the molecular
level
 Cancer cells manage to escape the usual controls on the
cell cycle

© 2011 Pearson Education, Inc.
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

© 2011 Pearson Education, Inc.
FIGURE 12.14
EXPERIMENT
Experiment 1
Inquiry:
Do
molecular
signals in
S
the
cytoplasm RESULTS
regulate
the cell
cycle?
S
G1
S
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.
Experiment 2
M
M
G1
M
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.
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

© 2011 Pearson Education, Inc.
FIGURE 12.15
G1 checkpoint
Control
system
G1
M
G2
M checkpoint
G2 checkpoint
S
For many cells, the G1 checkpoint seems to be the most
important
 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

© 2011 Pearson Education, Inc.
FIGURE 12.16
G0
G1 checkpoint
G1
(a) Cell receives a go-ahead
signal.
G1
(b) Cell does not receive a
go-ahead signal.
THE CELL CYCLE CLOCK: CYCLINS AND CYCLINDEPENDENT KINASES
Two types of regulatory proteins are involved in cell cycle
control: cyclins and cyclin-dependent kinases
(Cdks)
 Cdks activity fluctuates during the cell cycle because it is
controled by cyclins, so named because their
concentrations vary with 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

© 2011 Pearson Education, Inc.
FIGURE 12.17
M
G1
S G2
M
G1 S
G2
M
G1
MPF activity
Cyclin
concentration
Time
(a) Fluctuation of MPF activity and cyclin concentration
during the cell cycle
Cdk
Degraded
cyclin
Cyclin is
degraded
G2
Cdk
checkpoint
MPF
Cyclin
(b) Molecular mechanisms that help regulate the cell cycle
FIGURE 12.17A
M
G 1 S G2
M
G1 S
G2
M
G1
MPF activity
Cyclin
concentration
Time
(a) Fluctuation of MPF activity and cyclin concentration
during the cell cycle
FIGURE 12.17B
Cdk
Degraded
cyclin
Cyclin is
degraded
G2
Cdk
checkpoint
MPF
Cyclin
(b) Molecular mechanisms that help regulate the cell cycle
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

© 2011 Pearson Education, Inc.
FIGURE 12.18
1 A sample of human
connective tissue is
cut up into small
pieces.
Scalpels
The effect of plateletderived growth factor
(PDGF) on cell
division.
Petri
dish
2 Enzymes digest
the extracellular
matrix, resulting in
a suspension of
free fibroblasts.
3 Cells are transferred to
culture vessels.
Without PDGF
4 PDGF is added
to half the
vessels.
With PDGF
10 m
A clear example of external signals is densitydependent 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

© 2011 Pearson Education, Inc.
FIGURE 12.19
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
20 m
20 m
(a) Normal mammalian cells
(b) Cancer cells
LOSS OF CELL CYCLE CONTROLS IN CANCER CELLS
Cancer cells do not respond normally to the body’s
control mechanisms
 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

© 2011 Pearson Education, Inc.



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A normal cell is converted to a cancerous cell by a
process called transformation
Cancer cells that are not eliminated by the immune
system form tumors, masses of abnormal cells within
otherwise normal tissue
If abnormal cells remain only 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 additional tumors
© 2011 Pearson Education, Inc.
FIGURE 12.20
Tumor
Lymph
vessel
Blood
vessel
Glandular
tissue
Cancer
cell
1 A tumor grows
from a single
cancer cell.
Metastatic
tumor
2 Cancer
cells invade
neighboring
tissue.
3 Cancer cells spread
through lymph and
blood vessels to
other parts of the
body.
4 Cancer cells
may survive
and establish
a new tumor
in another part
of the body.

Recent advances in understanding the cell cycle
and cell cycle signaling have led to advances in
cancer treatment
© 2011 Pearson Education, Inc.
FIGURE 12.21
FIGURE 12.UN01
P
G1
S
Cytokinesis
Mitosis
G2
MITOTIC (M) PHASE
Prophase
Telophase and
Cytokinesis
Prometaphase
Anaphase
Metaphase
FIGURE 12.UN02

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