Transcript Document

Chapter 12
The Cell Cycle
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: The Key Roles of Cell Division
• The ability of organisms to reproduce best
distinguishes living things from nonliving matter
• The continuity of life is based upon the
reproduction of cells, or cell division
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
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LE 12-2
Amoeba-single
celled eukaryote
100 µm
Reproduction
Sand dollar
embryo after
fertilized egg
divided 200 µm
Growth and development
Dividing bone
marrow cells
give rise to new
blood cells 20 µm
Tissue renewal
Concept 12.1: 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
• A dividing cell duplicates its DNA, allocates the
two copies to opposite ends of the cell, and only
then splits into daughter cells
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Cellular Organization of the Genetic Material
• A cell’s endowment of DNA (its genetic
information) is called its genome
-Typical human cell is about 2 m of DNA– 250,000
times greater than the cell’s diameter
• DNA molecules in a cell are packaged into
chromosomes (Gr chroma, color and soma, body)
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• Every eukaryotic species has a characteristic
number of chromosomes in each cell nucleus
• Somatic (nonreproductive) cells have two sets of
chromosomes
-Humans 46 c’somes made up two 23 sets
• Gametes (reproductive cells: sperm and eggs)
have half as many chromosomes as somatic cells
• Eukaryotic chromosomes consist of chromatin, a
complex of DNA and protein that condenses
during cell division
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LE 12-3
25 µm
Distribution of Chromosomes During Cell Division
• In preparation for cell division, DNA is replicated
and the chromosomes condense
• Each duplicated chromosome has two sister
chromatids, which separate during Anaphase of
cell division
-Cohesions: adhesive protein complexes
attached along the lengths of the sister chromatids
• The centromere is the narrow “waist” of the
duplicated chromosome, where the two
chromatids are most closely attached
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0.5 µm
1. Before
duplication each
c’some has a
single DNA mc
Chromosome
duplication
(including DNA
synthesis)
2. Once replicated
c’some consists
of 2 chromatids.
Each chromatid
containing a copy
of DNA
3. Separation of
the 2 chromatids
into c’some and
into two daughter
cells
Centromere
Sister
chromatids
Separation
of sister
chromatids
Centromeres
Sister chromatids
• Eukaryotic cell division consists of:
– Mitosis, the division of 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
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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
• To Flemming, it appeared that the cell simply grew
larger between one cell division and the next
• Now we know that many critical events occur
during this stage in a cell’s life
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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)
• Interphase (about 90% of the cell cycle) can be
divided into subphases:
– G1 phase (“first gap”)
– S phase (“synthesis”)
– G2 phase (“second gap”)
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LE 12-5
INTERPHASE
G1
90% of the cycle
Grows & copies c’some
S
(DNA synthesis)
Cell Grows
G2
Continues to
grow and
prepares for
M-phase
C’somes
duplicated
• Mitosis is conventionally divided into five phases:
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
• Cytokinesis is well underway by late telophase
[Animations and videos listed on slide following figure]
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LE 12-6ca
G2 OF INTERPHASE
PROPHASE
PROMETAPHASE
LE 12-6da
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Video: Animal Mitosis
Video: Sea Urchin (time lapse)
Animation: Mitosis (All Phases)
Animation: Mitosis Overview
Animation: Late Interphase
Animation: Prophase
Animation: Prometaphase
Animation: Metaphase
Animation: Anaphase
Animation: Telophase
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The Mitotic Spindle: A Closer Look
• The mitotic spindle is an apparatus of microtubules
that controls chromosome movement during mitosis
• Assembly of spindle microtubules begins in the
centrosome, the microtubule organizing center
• The centrosome replicates, forming two
centrosomes that migrate to opposite ends of the
cell, as spindle microtubules grow out from them
-G2 phase
• An aster (a radial array of short microtubules)
extends from each centrosome -Prophase
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• The spindle includes the centrosomes, the spindle
microtubules, and the asters
• Some spindle microtubules attach to the
kinetochores of chromosomes and move the
chromosomes to the metaphase plate
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LE 12-7
Aster
Microtubules
Sister
chromatids
Chromosomes
Centrosome
Metaphase
plate
Kinetochores
Overlapping
nonkinetochore
microtubules
Centrosome
1 µm
Kinetochore
microtubules
0.5 µm
• 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
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As c’some move poleward, c’some are walked along a MT as
the MT depolymerizes at its kinetochore end, releasing tubulin
subunits.
Chromosome
movement
Microtubule
Motor
protein
Kinetochore
Tubulin
subunits
Chromosome
Pg 235 Figure 12.8 Borisy’s lab at University of
Wisconsin 1987
• 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
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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
Animation: Cytokinesis
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LE 12-9a
Contractile ring:
composed of
actin & myosin
filaments (pulling
of drawstring)
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Daughter cells
Cleavage of an animal cell (SEM)
Cleavage
furrow
deepens until
cell is pinched
off producing
two id cells
LE 12-9b
Vesicles derived
from Golgi
Apparatus move
along MT 
middle of cell
coalescecell
plate
Cell plate
enlarges until
it fuses with
plasma
membrane
2 new
daughter
cells
Vesicles
forming
cell plate
Wall of
parent cell
Cell plate
1 µm
New cell wall
Daughter cells
Cell plate formation in a plant cell (TEM)
LE 12-10
Nucleus
Nucleolus
Chromatin
condensing
Prophase. The
chromatin is condensing.
The nucleolus is
beginning to disappear.
Although not yet visible
in the micrograph, the
mitotic spindle is starting
to form.
Chromosomes
Prometaphase. We
now see discrete
chromosomes; each
consists of two identical
sister chromatids. Later
in prometaphase, the
nuclear envelope will
fragment.
Cell plate
Metaphase. The spindle is
complete, and the
chromosomes, attached
to microtubules at their
kinetochores, are all at
the metaphase plate.
Anaphase. The
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends of
the cell as their
kinetochore microtubules shorten.
10 µm
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will divide
the cytoplasm in two, is
growing toward the
perimeter of the parent
cell.
Binary Fission
• Prokaryotes (bacteria and archaea) reproduce by
a type of cell division called binary fission (division
in half)
• In binary fission, the chromosome replicates
(beginning at the origin of replication), and the two
daughter chromosomes actively move apart
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Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Two copies
of origin
Bacterial
chromosome
LE 12-11_2
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Replication continues.
One copy of the origin
is now at each end of
the cell.
Bacterial
chromosome
Two copies
of origin
Origin
Origin
LE 12-11_3
Cell wall
Origin of
replication
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Replication continues.
One copy of the origin
is now at each end of
the cell.
Replication finishes.
The plasma membrane
grows inward, and
new cell wall is
deposited.
Two daughter
cells result.
Plasma
membrane
Bacterial
chromosome
Two copies
of origin
Origin
Origin
The Evolution of Mitosis
• Since prokaryotes evolved before eukaryotes,
mitosis probably evolved from binary fission
• Certain protists exhibit types of cell division that
seem intermediate between binary fission and
mitosis
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LE 12-12
Bacterial
chromosome
Prokaryotes
Chromosomes
Microtubules
Intact nuclear
envelope
Dinoflagellates
Kinetochore
microtubules
Intact nuclear
envelope
Diatoms
Kinetochore
microtubules
Centrosome
Fragments of
nuclear envelope
Most eukaryotes
Concept 12.3: The cell cycle is regulated by a
molecular control system
• The frequency of cell division varies with the type
of cell
-liver cells maintain the ability to divide but will
when appropriate (damage)
-nerve and muscle cells do not divide in
mature humans
• These cell cycle differences result from regulation
at the molecular level
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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
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LE 12-13
Experiment 1
Cell cycle
is driven
by
specific
molecular
signals
present in
the
cytoplasm
Experiment 2
S
G1
M
G1
S
S
M
M
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.
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 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
-triggers and coordinates key events in the cell
cycle
• The clock has specific checkpoints where the cell
cycle stops until a go-ahead signal is received
-signals are transmitted within the cell by _? __
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LE 12-14
G1 checkpoint
Control
system
G1
M
M checkpoint
G2 checkpoint
Restriction Point
S
G2
System is subject
to regulation at
various
checkpoints (3)
• For many cells, the G1 checkpoint seems to be the
most important one
• 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
-most of the human body cells are in G0 phase
-external clues such as GF released during
injuiry can call back cells from the G0 phase
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LE 12-15
G0
G1 checkpoint
G1
If a cell receives a go-ahead
signal at the G1 checkpoint,
the cell continues on in the
cell cycle.
G1
If a cell does not receive a
go-ahead signal at the G1
checkpoint, the cell exits the
cell cycle and goes into G0, a
nondividing state.
The Cell Cycle Clock: Cyclins and
Cyclin-Dependent Kinases
• Two types of regulatory proteins are involved in
cell cycle control: cyclins and cyclin-dependent
kinases (Cdks)
-remember from Ch 11 the role of protein kinases?
-Cyclins (proteins): cycling fluctuating concentrating
in the cell
-Cdks; the activity rises and falls with changes in the
concentration of its cyclin partner
• The activity of cyclins and Cdks fluctuates during
the cell cycle
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LE 12-16a
MPF triggers the cell’s passage from G2 into M-phase
M
G1
S
G2
M
G1
S
G2
M
MPF activity
Cyclin
Time
Fluctuation of MPF activity and cyclin concentration
during the cell cycle
LE 12-16b
1. Synthesis of
cyclin begins in
late S phase G2
5. Conditions favor
degradation of cyclin &
Cdk component of MPF
is recycled
Cdk
Degraded
cyclin
4. During
anaphase,
cyclin of MPF is
degraded,
terminating the
M-phase
G2
Cdk
checkpoint
Cyclin is
degraded
MPF
Cyclin
3. MPF promotes Mphase by phsphryltng
proteins
2. Accumled
cyclin and
Cdk mc’s
produce MPF
to pass G2 
mitosis
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
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LE 12-17
Scalpels
Petri
plate
Without PDGF
With PDGF
Without PDGF
With PDGF
10 mm
• Another 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
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LE 12-18a
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).
Normal mammalian cells
25 µm
• Cancer cells exhibit neither density-dependent
inhibition nor anchorage dependence
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LE 12-18b
Cancer cells do not exhibit
anchorage dependence
or density-dependent inhibition.
25 µm
Cancer cells
Loss of Cell Cycle Controls in Cancer Cells
• Cancer cells do not respond normally to the
body’s control mechanisms
• 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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LE 12-19
Lymph
vessel
Tumor
Blood
vessel
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.
Metastatic
tumor
A small percentage
of cancer cells may
survive and establish
a new tumor in another
part of the body.