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Chapter 12: Mitosis
Cell Division
• The continuity of life– Is based upon the reproduction of cells, or cell
division
Figure 12.1
Where it all began…You started as a cell smaller than a period at the end of a sentence…
• Going from egg to baby…. the original fertilized egg has to divide…
and divide…and divide…
and divide…
Getting from there to here…
• For reproduction – asexual reproduction
• one-celled organisms
• For growth– from fertilized egg to
multi-celled organism
• For repair & renewal– replace cells that die
from normal wear & tear or from injury
Why do cells divide?
amoeba
• The DNA molecules in a cell– Are packaged into chromosomes
50 µm
Figure 12.3
• Eukaryotic chromosomes– Consist of chromatin, a complex of DNA and protein that
condenses during cell division
• In animals– Somatic cells
• have two sets of chromosomes (46 chromosomes in humans, 23 pairs)
• Body cells
– Gametes • have one set of chromosomes (23 chromosomes in humans)
• Sex cells
Binary Fission
– The bacterial chromosome replicates
– The two daughter chromosomes actively move apart
E. coli cell
OriginOrigin
Chromosome replication begins.Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.
1
Replication continues. One copy ofthe origin is now at each end of the cell.
2
Replication finishes. The plasma membrane grows inward, andnew cell wall is deposited.
3
Two daughter cells result.4
Cell cycle• Cell has a “life cycle”
cell is formed from a mitotic division
cell grows & maturesto divide again
cell grows & matures to never divide again
G1, S, G2, M G1G0
epithelial cells,blood cells,stem cells
liver cells
brain / nerve cellsmuscle cells
• Eukaryotic cell division consists of– Mitosis, the division of the nucleus– Cytokinesis, the division of the cytoplasm
AND…INTERPHASE
G1
S(DNA synthesis)
G2Cyto
kines
is
Mito
sis
MITOTIC(M) PHASE
Figure 12.5
Interphase• 90% of cell life cycle
– cell doing its “everyday job”• synthesize proteins/enzymes, etc
– prepares for duplication if triggered
I ’m working here!
Time to divide& multiply!
• Interphase can be divided into subphases– G1 phase
– S phase
– G2 phase
Interphase• Divided into 3 phases:
– G1 = 1st Gap (Growth)• cell doing its “everyday job”• cell grows
– S = DNA Synthesis• copies chromosomes
– G2 = 2nd Gap (Growth)• prepares for division • cell grows (more)• produces organelles,
proteins, membranes
G0
signal to
divide
double-strandedmitotic humanchromosomes
Mitotic Chromosome
Duplicated chromosome 2 sister chromatids narrow at centromeres contain identical
copies of original DNAhomologouschromosomes
homologouschromosomes
sister chromatidshomologous = “same information”single-stranded
double-stranded
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 where the two chromatids are most closely attached
Mitosis • Dividing cell’s DNA between
2 daughter nuclei– “dance of the chromosomes”
• 4 phases– prophase– metaphase– anaphase– telophase
Overview of mitosis
interphase prophase (pro-metaphase)
metaphase anaphase telophase
cytokinesis
I.P.M.A.T.
Prophase Chromatin condenses
– visible chromosomes – chromatids
• Protein fibers cross cell to form mitotic spindle– Made of microtubules/microfilaments – coordinates movement of
chromosomes
• Centrioles move to opposite poles of cell – animal cell only
• Nucleolus disappears• Nuclear membrane breaks down
The Mitotic Spindle• The mitotic spindle is an apparatus of
microtubules that controls chromosome movement during mitosis
• During prophase, 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
• An aster (a radial array of short microtubules) extends from each centrosome
• The spindle includes the centrosomes, the spindle microtubules, and the asters
Fig. 12-7
Microtubules Chromosomes
Sisterchromatids
Aster
Metaphaseplate
Centrosome
Kineto-chores
Kinetochoremicrotubules
Overlappingnonkinetochoremicrotubules
Centrosome 1 µm
0.5 µm
Transition to Metaphase • Prometaphase
– spindle fibers attach to centromeres
• creating kinetochores (region where microtubules attach)
• connect centromeres to centrosomes
– chromosomes begin moving
Metaphase • Chromosomes align along
middle of cell– metaphase plate
• meta = middle plane
– spindle fibers coordinate movement
– helps to ensure chromosomes separate properly
• so each new nucleus receives only 1 copy of each chromosome
Anaphase • Sister chromatids separate at
kinetochores – move to opposite poles
– pulled at centromeres
– pulled by motor proteins “walking”along microtubules
• actin, myosin
Fig. 12-8b
Kinetochore
MicrotubuleTubulinSubunits
Chromosome
Chromosomemovement
Motorprotein
CONCLUSION
Telophase• Chromosomes arrive at
opposite poles– daughter nuclei form – nucleoli form– chromosomes disperse
• no longer visible under light microscope
• Spindle fibers disperse• Cytokinesis begins
– cell division
green = key features
Cytokinesis• Animals
– constriction belt of actin microfilaments around equator of cell
• cleavage furrow forms (shallow groove near old metaphase plate)
• splits cell in two• like tightening a draw
string
Mitosis in whitefish blastula
Mitosis in plant cell
Cytokinesis in Plants• Plants
– cell plate forms• vesicles line up at
equator– derived from Golgi
• vesicles fuse to form 2 cell membranes
– new cell wall laid down between membranes
• new cell wall fuses with existing cell wall
Cytokinesis in Animals
(play Cells Alive movies here)
(play Thinkwell movies here)
• Mitosis in a plant cell
1 Prophase. The chromatinis condensing. The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is staring to from.
Prometaphase.We now see discretechromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelop will fragment.
Metaphase. The spindle is complete,and the chromosomes,attached to microtubulesat their kinetochores, are all at the metaphase plate.
Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomesare moving to the ends of cell as their kinetochoremicrotubles shorten.
Telophase. Daughternuclei are forming. Meanwhile, cytokinesishas started: The cellplate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell.
2 3 4 5
NucleusNucleolus
ChromosomeChromatinecondensing
Figure 12.10
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• The cell cycle is regulated by a molecular control system
• Chemical signals in cytoplasm • The frequency of cell division
– Varies with the type of cell (skin vs liver vs brain)
• These cell cycle differences– Result from regulation at the molecular level
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
Control system
G2 checkpoint
M checkpoint
G1 checkpoint
G1
S
G2M
• The clock has specific checkpoints– Where the cell cycle stops until a go-ahead signal is
received
G1 checkpoint
G1G1
G0
(a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.
(b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.
G0 phase: a non-dividing state
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 12-15
G1
G0
G1 checkpoint
(a) Cell receives a go-ahead signal
G1
(b) Cell does not receive a go-ahead signal
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)
• The activity of cyclins and Cdks fluctuates during the cell cycle
The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
• Protein Kinases: enzymes that activate or inactivate other proteins by phosphorylating them
(give go ahead at G1 and G2 checkpoints)
- Cyclins are proteins that cyclically fluctuate concentration
The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
• MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase
*M phase promoting factor
The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
• Most of the time kinases in inactive form• Must be attached to a CYCLIN (protein) to be active
• Cyclin-dependent kinases (Cdks)– Activity rises and falls with partner– Ex: MPF triggers the cell’s passage past the G2
checkpoint into M phase– When cyclins are present with MPF, it phosphorylates a
variety of proteins, initiating mitosis
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
Cyclin
accu
mu
latio
n
• Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide
• 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 (extracellular matrix)
Fig. 12-19
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
(a) Normal mammalian cells (b) Cancer cells25 µm25 µm
• Cancer cells– Exhibit neither density-dependent inhibition nor
anchorage dependence
25 µm
Cancer cells do not exhibitanchorage dependence or density-dependent inhibition.
Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells.
(b)
Figure 12.18 B
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
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Fig. 12-UN2
Fig. 12-UN5
Mitosis in plant cell