Download - BIOL 101 Chp 12 The Cell Cycle
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BIOL 101 General Biology I
Chp 12The Cell Cycle
Rob SwatskiAssociate Professor of Biology
HACC – York Campus
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Overview: The Key Roles of Cell Division
• The ability of organisms to reproduce best
distinguishes living things from nonliving things
• The continuation of life is based on cell
reproduction (cell division)
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Fig. 12-1
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• In unicellular organisms, division of 1 cell
reproduces the entire organism
• Multicellular organisms use cell division for:
– Development from a fertilized cell
– Growth
– Repair
• Cell division is a vital part of the cell cycle, the
life of a cell from formation to its own division
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Fig. 12-2a
100 µm
(a) Reproduction
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Fig. 12-2b
200 µm
(b) Growth and development
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Fig. 12-2c
20 µm
(c) Tissue renewal
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Concept 12.1: Cell division results in geneticallyidentical daughter cells
• Most cell division
results in daughter cells
with identical genetic
information, DNA
• A special type of
division (...?...)
produces nonidentical
daughter cells
(gametes)
- sperm & egg cells
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Cellular Organization of the Genetic Material
• All the DNA in a cell makes up the cell’s
genome
• A genome can consist of one DNA molecule
(common in prokaryotes) or many DNA
molecules (common in eukaryotes)
• DNA molecules in a cell are packaged into
chromosomes
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Fig. 12-3
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• Every eukaryotic species has a characteristic
chromosome number in each cell nucleus
• Somatic cells (nonreproductive body cells)
have 2 sets of chromosomes
• Gametes (reproductive cells: sperm & eggs)
have half the number of chromosomes as
somatic cells
• Eukaryotic chromosomes consist of
chromatin, a complex of DNA & protein that
condenses during cell division
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Distribution of Chromosomes During Eukaryotic Cell Division
• In preparation for cell division, DNA is
replicated & the chromosomes condense
• Each duplicated chromosome has 2 sister
chromatids, which separate during cell
division
• The centromere is the narrow “waist” of the
duplicated chromosome, where the 2
chromatids are most closely attached
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Fig. 12-40.5 µm Chromosomes
Chromosomeduplication(including DNAsynthesis)
Chromo-some arm
Centromere
Sisterchromatids
DNA molecules
Separation ofsister chromatids
Centromere
Sister chromatids
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• Eukaryotic cell division consists of:
– Mitosis: division of the nucleus
– Cytokinesis: division of the cytoplasm
• Gametes are produced by another type of cell
division called meiosis
• Meiosis produces nonidentical daughter cells
that have only 1 set of chromosomes (half as
many as the parent cell)
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Phases of the Cell Cycle
• The cell cycle consists of:
– Interphase: cell growth & copying of
chromosomes in preparation for cell division
– Mitotic (M) phase: mitosis & cytokinesis
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Fig. 12-5
S(DNA synthesis)
G1
G2
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Fig. 12-UN1
Telophase andCytokinesis
Anaphase
Metaphase
Prometaphase
Prophase
MITOTIC (M) PHASE
Cytokinesis
Mitosis
SG1
G2
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• Interphase (90% of the cell cycle) is divided
into subphases:
– G1 phase (“first gap” or “growth”)
– S phase (“synthesis”)
– G2 phase (“second gap” or “growth”)
• The cell grows during all 3 phases, but
chromosomes are copied only during the S
phase
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• Mitosis is divided into 5 phases:
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
• Cytokinesis is well underway by late telophase
BioFlix: Mitosis
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Fig. 12-6a
Prophase PrometaphaseG2 of Interphase
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Fig. 12-6b
PrometaphaseProphaseG2 of Interphase
Nonkinetochoremicrotubules
Fragmentsof nuclearenvelope
Aster CentromereEarly mitoticspindle
Chromatin(duplicated)
Centrosomes(with centriolepairs)
Nucleolus Nuclearenvelope
Plasmamembrane
Chromosome, consistingof two sister chromatids
Kinetochore Kinetochoremicrotubule
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Fig. 12-6c
Metaphase Anaphase Telophase and Cytokinesis
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Fig. 12-6d
Metaphase Anaphase Telophase and Cytokinesis
Cleavage
furrow
Nucleolus
forming
Metaphase
plate
Centrosome at
one spindle poleSpindle
Daughter
chromosomesNuclear
envelope
forming
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The Mitotic Spindle: A Closer Look
• The mitotic spindle is made of microtubules
(MT’s) that control chromosome movement
during mitosis
• During prophase, assembly of spindle MT’s
begins in the centrosome, the MT organizing
center
• The centrosome replicates & then each
migrates to opposite ends of the cell, as
spindle MT’s grow out from them
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• An aster (a star-like array of short MT’s)
extends from each centrosome
• The spindle includes the centrosomes, the
spindle MT’s, & the asters
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• During
Prometaphase, some
spindle MT’s attach to
the kinetochores of
chromosomes & start
to move the
chromosomes
• At Metaphase, the
chromosomes are all
lined up at the
metaphase plate, the
midway point between
the spindle’s 2 poles
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Fig. 12-7
Microtubules Chromosomes
Sisterchromatids
Aster
Metaphaseplate
Centrosome
Kineto-chores
Kinetochoremicrotubules
Overlappingnonkinetochoremicrotubules
Centrosome 1 µm
0.5 µm
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Fig. 12-UN4
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• In Anaphase, sister chromatids separate & move
along the kinetochore MT’s toward opposite ends
of the cell
• The MT’s shorten by depolymerizing at their
kinetochore ends
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Fig. 12-8a
Kinetochore
Spindlepole
Mark
EXPERIMENT
RESULTS
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Fig. 12-8b
Kinetochore
MicrotubuleTubulinSubunits
Chromosome
Chromosomemovement
Motorprotein
CONCLUSION
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• Nonkinetochore MT’s from opposite poles
overlap & push against each other, which
elongates the cell
• In Telophase, genetically identical daughter
nuclei form at opposite ends of the cell
- opposite of Prophase
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Cytokinesis: A Closer Look
• In animal cells, Cytokinesis occurs by a
process known as cleavage
- forms a cleavage furrow
• In plant cells, a cell plate forms during
cytokinesis
Animation: Cytokinesis
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Cleavage furrow
Fig. 12-9a
100 µm
Daughter cells
(a) Cleavage of an animal cell (SEM)
Contractile ring ofmicrofilaments
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Rat Kangaroo
kidney cells
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Fig. 12-9b
Daughter cells
(b) Cell plate formation in a plant cell (TEM)
Vesiclesformingcell plate
Wall ofparent cell
New cell wallCell plate
1 µm
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Fig. 12-10
Chromatincondensing
Metaphase Anaphase TelophasePrometaphase
Nucleus
Prophase1 2 3 54
Nucleolus Chromosomes Cell plate10 µm
Video: Sea Urchin (Time Lapse)
Video: Animal Mitosis
Plant Mitosis
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Fig. 12-10a
Nucleus
Prophase1
Nucleolus
Chromatin
condensing
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Fig. 12-10b
Prometaphase2
Chromosomes
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Fig. 12-10c
Metaphase3
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Fig. 12-10d
Anaphase4
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Fig. 12-10e
Telophase5
Cell plate10 µm
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Fig. 12-UN2
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Binary Fission
• Prokaryotes (bacteria & archaea) reproduce by
another type of cell division called binary
fission
• In binary fission, the single chromosome
replicates (beginning at the origin of
replication), & the 2 daughter chromosomes
actively move apart
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Fig. 12-11-1
Origin ofreplication
Two copiesof origin
E. coli cellBacterialchromosome
Plasmamembrane
Cell wall
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Fig. 12-11-2
Origin ofreplication
Two copiesof origin
E. coli cellBacterialchromosome
Plasmamembrane
Cell wall
Origin Origin
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Fig. 12-11-3
Origin ofreplication
Two copiesof origin
E. coli cellBacterialchromosome
Plasmamembrane
Cell wall
Origin Origin
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Fig. 12-11-4
Origin ofreplication
Two copiesof origin
E. coli cellBacterialchromosome
Plasmamembrane
Cell wall
Origin Origin
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The Evolution of Mitosis
• Since prokaryotes evolved before eukaryotes,
mitosis probably evolved from binary fission
• Some protists display types of cell division that
appear intermediate between binary fission &
mitosis
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Fig. 12-12ab
Bacterial
chromosome
Chromosomes
Microtubules
(a) Bacteria
(b) Dinoflagellates
Intact nuclearenvelope
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Fig. 12-12cd
Kinetochoremicrotubule
(c) Diatoms and yeasts
Kinetochoremicrotubule
(d) Most eukaryotes
Fragments of nuclear envelope
Intact nuclearenvelope
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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 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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Fig. 12-13
Experiment 1 Experiment 2
EXPERIMENT
RESULTS
S G1 M G1
M MSS
When a cell in theS phase was fused with a cell in G1, the G1
nucleus immediatelyentered the Sphase—DNA was synthesized.
When a cell in theM phase was fused with a cell in G1, the G1
nucleus immediatelybegan mitosis—aspindle formed andchromatin condensed,even though the
chromosome had notbeen duplicated.
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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 & external controls
• The clock has specific checkpoints where the
cell cycle stops until a go-ahead signal is
received
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Fig. 12-14
SG1
M checkpoint
G2M
Controlsystem
G1 checkpoint
G2 checkpoint
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• 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 & divide
• If the cell does not receive the go-ahead signal,
it will exit the cycle & switch into a nondividing
state called the G0 phase
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Fig. 12-15
G1
G0
G1 checkpoint
(a) Cell receives a go-aheadsignal
G1
(b) Cell does not receive ago-ahead signal
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The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
• Two types of regulatory proteins are involved in
cell cycle control: cyclins & cyclin-dependent
kinases (Cdks)
• The activity of cyclins & Cdks fluctuates during
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
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Fig. 12-17a
Time
(a) Fluctuation of MPF activity and cyclin concentration duringthe cell cycle
Cyclinconcentration
MPF activity
M M MSSG1 G1 G1G2 G2
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Fig. 12-17b
Cyclin isdegraded
Cdk
MPF
CdkG2
checkpoint
Degradedcyclin
Cyclin
(b) Molecular mechanisms that help regulate the cell cycle
Cyclin
accu
mu
latio
n
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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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Fig. 12-18
Petriplate
Scalpels
Cultured fibroblasts
Without PDGFcells fail to divide
With PDGFcells prolifer-ate
10 µm
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• Another example of external signals 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
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Fig. 12-19
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
(a) Normal mammalian cells (b) Cancer cells
25 µm25 µm
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Loss of Cell Cycle Controls in Cancer Cells
• Cancer cells do not respond normally to the
body’s control mechanisms
- they do not display either density-dependent
inhibition nor anchorage dependence
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• Cancer cells may not need growth factors to grow
& 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
Loss of Cell Cycle Controls in Cancer Cells
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Dividing Prostate Cancer Cells
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• A normal cell is converted to a cancerous cell
by a process called transformation
• 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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Schwannoma Tumor (benign)
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Fig. 12-20
Tumor
A tumor grows
from a single
cancer cell.
Glandulartissue
Lymphvessel
Bloodvessel
Metastatictumor
Cancercell
Cancer cells
invade neigh-
boring tissue.
Cancer cells spreadto other parts ofthe body.
Cancer cells maysurvive and
establish a new
tumor in another
part of the body.
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