cell reproduction and the division of the nucleus and cytoplasm
TRANSCRIPT
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Cell reproduction
and the division of
the NUCLEUS and CYTOPLASM
The Mitosis chapter (Includes: Cancer)
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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 on the reproduction of cells, or cell division
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Fig. 12-1
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In unicellular organisms, division of one cell reproduces the entire organism
Multicellular organisms depend on cell division for:Development from a fertilized cellGrowthRepair
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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Fig. 12-2
100 µm 200 µm 20 µm
(a) Reproduction (b) Growth and development
(c) Tissue renewal
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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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Cell division results in genetically identical daughter cells
Most cell division results in daughter cells with identical genetic information, DNA
A special type of division produces nonidentical daughter cells (gametes, or sperm and egg cells)
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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
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Fig. 12-3
20 µm
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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
Eukaryotic chromosomes consist of chromatin, a complex of DNA and 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 and the chromosomes condense
Each duplicated chromosome has two sister chromatids, which separate during cell division
The centromere is the narrow “waist” of the duplicated chromosome, where the two 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, the division of the nucleusCytokinesis, 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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Phases of the Cell CycleThe cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)Interphase (cell growth and copying of
chromosomes in preparation for cell division)
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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
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Fig. 12-5
S(DNA synthesis)
MITOTIC(M) PHASE
Mito
sis
Cytokinesis
G1
G2
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Mitosis is conventionally divided into five phases:ProphasePrometaphaseMetaphaseAnaphaseTelophase
Cytokinesis is well underway by late telophase
BioFlix: Mitosis
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Fig. 12-6
G2 of Interphase
Centrosomes(with centriolepairs)
Chromatin(duplicated)
Nucleolus Nuclearenvelope
Plasmamembrane
Early mitoticspindle
Aster Centromere
Chromosome, consisting of two sister chromatids
Prophase Prometaphase
Fragmentsof nuclearenvelope
Nonkinetochoremicrotubules
Kinetochore Kinetochoremicrotubule
Metaphase
Metaphaseplate
Spindle Centrosome atone spindle pole
Anaphase
Daughterchromosomes
Telophase and Cytokinesis
Cleavagefurrow
Nucleolusforming
Nuclearenvelopeforming
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Prophase
Fig. 12-6a
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
Cleavagefurrow
Nucleolusforming
Metaphaseplate
Centrosome atone spindle pole
SpindleDaughterchromosomes
Nuclearenvelopeforming
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An aster (a radial array of short microtubules) extends from each centrosome
• The spindle includes the centrosomes, the spindle microtubules, and the asters
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During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes
At metaphase, the chromosomes are all lined up at the metaphase plate, the midway point between the spindle’s two 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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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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Fig. 12-8EXPERIMENT
Kinetochore
RESULTS
CONCLUSION
Spindlepole
Mark
Chromosomemovement
Kinetochore
MicrotubuleMotorprotein
Chromosome
Tubulinsubunits
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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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Cytokinesis: A Closer LookIn 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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Cleavage furrow
Fig. 12-9a
100 µm
Daughter cells
(a) Cleavage of an animal cell (SEM)
Contractile ring ofmicrofilaments
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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
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Fig. 12-10a
Nucleus
Prophase1
Nucleolus
Chromatincondensing
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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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Binary FissionProkaryotes (bacteria and archaea) reproduce
by a type of cell division called binary fissionIn binary fission, the chromosome replicates
(beginning at the origin of replication), and the two daughter chromosomes actively move apart
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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 Cell Cycle Control SystemThe 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
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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 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
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Fig. 12-15
G1
G0
G1 checkpoint
(a) Cell receives a go-ahead signal
G1
(b) Cell does not receive a go-ahead signal
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Fig. 12-17
M G1S G2
M G1S G2
M G1
MPF activity
Cyclinconcentration
Time(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle
Degradedcyclin
Cdk
G 1S
G 2
M
CdkG2
checkpointCyclin isdegraded
CyclinMPF
(b) Molecular mechanisms that help regulate the cell cycle
Cy
clin
ac
cu
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 cells25 µm25 µm
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Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence
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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 factorThey may convey a growth factor’s signal without
the presence of the growth factorThey may have an abnormal cell cycle control
system
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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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Fig. 12-20
Tumor
A tumor growsfrom a singlecancer cell.
Glandulartissue
Lymphvessel
Bloodvessel
Metastatictumor
Cancercell
Cancer cellsinvade neigh-boring tissue.
Cancer cells spreadto other parts ofthe body.
Cancer cells maysurvive andestablish a newtumor in anotherpart of the body.
1 2 3 4
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Fig. 12-UN2