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8/3/2019 Lecture 2, Ch 12, Cell Cycle
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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures forBiology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 12
The Cell Cycle
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Overview: The Key Roles of Cell Division
The ability of organisms to reproduce bestdistinguishes living things from non-living matter
The continuity of life is based upon thereproduction of cells, or cell division
Cell division is integral part of cell cycle
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Types of cell division
Prokaryotes
Binary fission
Eukaryotes
Mitosis:
Growth, development & repair
Asexual reproduction (yields genetically identical cells)
Occurs in somatic (body) cells
Meiosis:
Sexual reproduction (yields genetically different cells with halfthe # of chromosomes)
Occurs in specific reproductive cells
Yields gametes (e.g., eggs & sperm) or spores
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Eukaryotic cell division consists of:
Mitosis, the division of the nucleus
Cytokinesis, the division of the cytoplasm
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Concept 12.1: Mitotic cell division results ingenetically identical daughter cells
Cells duplicate their genetic material before theydivide, ensuring that each daughter cell receivesan exact copy of the genetic material, DNA
A dividing cell duplicates its DNA, allocates thetwo copies to opposite ends of the cell, and onlythen splits into daughter cells
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Cellular Organization of the Genetic Material
A cells endowment of DNA (its genetic
information) is called its genome
DNA molecules* in a cell are packaged intochromosomes
*Prokaryotes-circular DNA
Eukaryotes-linear DNA
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Every eukaryotic species has a characteristicnumber of chromosomes in each cell nucleus
Somatic (non-reproductive) cells (normally) havetwo sets of chromosomes
Gametes (reproductive cells: sperm and eggs)(and spores) have half as many chromosomes assomatic cells
Eukaryotic chromosomes consist of chromatin, acomplex of DNA and protein that condensesduring cell division
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DNA associates with special proteins to form more stable
structure called chromosomes (different proteins inprokaryotes and eukaryotes, so chromosomes built different)
Chromosomes are found inside nucleus in eukaryotes
Human - 46 chromosomes, 23 pairs (1 set of 23 from egg, 1set of 23 from sperm)
Each chromosome contains many genes
Gene is a segment of DNA that is responsible for controllinga trait (e.g., coding for a specific protein)
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Human female karyotype
H l k
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Human male karyotype
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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 bedivided into subphases:
G1phase (first gap) S phase (synthesis)
G2phase (second gap)
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LE 12-5
G1
G2
S(DNA synthesis)
INTERPHASE
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Distribution of Chromosomes During Cell Division
In preparation for cell division, DNA is replicated
and the chromosomes condense
Each duplicated chromosome has two sisterchromatids, which separate during cell division
The centromere is the narrow waist of the
duplicated chromosome, where the twochromatids are most closely attached
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LE 12-4
Chromosomeduplication(including DNAsynthesis)
0.5 m
Centromere
Sisterchromatids
Separation
of sisterchromatids
Centromeres Sister chromatids
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Mitosis is conventionally divided into five phases:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis is well underway by late telophase
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1. Prophase
- Chromatin condenses, this causes the chromosomes tobegin to become visible
- Centrosomes separate, moving to opposite ends of thenucleus
- The centrosomes start to form a framework used toseparate the two sister chromatids called the mitoticspindle, that is made of microtubules
- Nucleolus disappears
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2. Prometaphase
- Nuclear envelope fragments
- Chromosomes become more condensed
- A kinetochore is formed at the centromere, the point wherethe sister chromatids are attached
- Microtubules attach at the kinetochores
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3. Metaphase
- Chromosomes align on an axis called the metaphase plate
- Note: the spindle consists of microtubules, one attached toeach chromosome
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4. Anaphase
- Each centromere splits making two chromatids free
- Each chromatid moves toward a pole
- Cell begins to elongate, caused by microtubules notassociated with the kinetochore
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5. Telophase
Formation of nuclear membrane and nucleolus
Short and thick chromosomes begin to elongate to formlong and thin chromatin
Formation of the cleavage furrow - a shallow groove inthe cell near the old metaphase plate
Cytokinesis = division of the cytoplasm
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Mitosis in an onion root
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Th Mit ti S i dl A Cl L k
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The Mitotic Spindle:A Closer Look
The mitotic spindle is an apparatus of
microtubules that controls chromosome movementduring mitosis
Assembly of spindle microtubules begins in the
centrosome, the microtubule organizing center
The centrosome replicates, forming twocentrosomes 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
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The spindle includes the centrosomes, the spindle
microtubules, and the asters
Some spindle microtubules attach to thekinetochores of chromosomes and move the
chromosomes to the metaphase plate
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Microtubules ChromosomesSisterchromatids
AsterCentrosome
Metaphaseplate
Kineto-chores
Kinetochoremicrotubules
0.5 m
Overlapping
nonkinetochoremicrotubules
1 mCentrosome
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In anaphase, sister chromatids separate and
move along the kinetochore microtubules towardopposite ends of the cell
The microtubules shorten by depolymerizing at
their kinetochore ends
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Chromosomemovement
Microtubule Motorprotein
Chromosome
Kinetochore
Tubulinsubunits
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Nonkinetochore microtubules from opposite polesoverlap and push against each other, elongatingthe cell
In telophase, genetically identical daughter nuclei
form at opposite ends of the cell
Cytokinesis: A Closer Look
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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
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Cleavage furrow
100 m
Contractile ring ofmicrofilaments
Daughter cells
Cleavage of an animal cell (SEM)
Mitosis
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1 m
Daughter cells
Cell plate formation in a plant cell (TEM)
New cell wallCell plate
Wall ofparent cell
Vesiclesformingcell plate
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Nucleus
Cell plateChromosomesNucleolus
Chromatin
condensing 10 m
Prophase. Thechromatin is condensing.The nucleolus isbeginning to disappear.Although not yet visiblein the micrograph, the
mitotic spindle is startingto form.
Prometaphase. Wenow see discretechromosomes; eachconsists of two identicalsister chromatids. Laterin prometaphase, the
nuclear envelope willfragment.
Metaphase. The spindle iscomplete, and thechromosomes, attachedto microtubules at theirkinetochores, are all atthe metaphase plate.
Anaphase. Thechromatids of eachchromosome haveseparated, and thedaughter chromosomesare moving to the ends of
the cell as theirkinetochore micro-tubules shorten.
Telophase. Daughternuclei are forming.Meanwhile, cytokinesishas started: The cellplate, which will dividethe cytoplasm in two, is
growing toward theperimeter of the parentcell.
Binary Fission
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Binary Fission
Prokaryotes (bacteria and archaea) reproduce by
a type of cell division called binary fission
In binary fission, the chromosome replicates(beginning at the origin of replication), and the two
daughter chromosomes actively move apart
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Origin ofreplication
Cell wall
Plasmamembrane
Bacterial
chromosome
E. colicell
Two copiesof origin
Chromosomereplication begins.Soon thereafter,one copy of the originmoves rapidly toward
the other end of the cell.
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Origin ofreplication
Cell wall
Plasma
membraneBacterialchromosome
E. colicell
Two copiesof origin
Chromosomereplication begins.Soon thereafter,
one copy of the originmoves rapidly towardthe other end of the cell.
Replication continues.One copy of the originis now at each end ofthe cell.
Origin Origin
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O i i fCell wall
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Origin ofreplication
Cell wall
Plasmamembrane
Bacterialchromosome
E. colicell
Two copies
of origin
Chromosome
replication begins.Soon thereafter,one copy of the originmoves rapidly towardthe other end of the cell.
Replication continues.
One copy of the originis now at each end ofthe cell.
Origin Origin
Replication finishes.The plasma membrane
grows inward, andnew cell wall isdeposited.
Two daughtercells result.
The Evolution of Mitosis
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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 andmitosis
LE 12-12Bacterial
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chromosome
Chromosomes
Microtubules
Prokaryotes
Dinoflagellates
Intact nuclearenvelope
Kinetochoremicrotubules
Kinetochoremicrotubules
Intact nuclearenvelope
Diatoms
Centrosome
Most eukaryotes
Fragments ofnuclear envelope
Concept 12.3: The cell cycle is regulated by a
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Concept 12.3: The cell cycle is regulated by amolecular control system
The frequency of cell division varies with the typeof cell and with cell happiness
These cell cycle differences result from regulationat the molecular level
The cell cycle appears to be driven by specificchemical signals present in the cytoplasm
The levels of these chemical signals are
influenced by biotic & abiotic factors
The Cell Cycle Control System
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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 clock has specific checkpoints where the cell
cycle stops until a go-ahead signal is received
For many cells, the G1 checkpoint seems to be themost important one
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G1 checkpoint
G1S
M
M checkpoint
G2 checkpoint
G2
Controlsystem
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G1
G1 checkpoint
G1
G0
If a cell receives a go-aheadsignal at the G1 checkpoint,the cell continues on in thecell cycle.
If a cell does not receive ago-ahead signal at the G1checkpoint, the cell exits thecell cycle and goes into G0, anondividing state.
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Degradedcyclin G2
checkpoint
Cdk
Cyclin isdegraded
MPFCyclin
Cdk
Molecular mechanisms that help regulate the cell cycle
Stop and Go Signs: Internal and External Signals at
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p g gthe Checkpoints An example of an internal signal is that
kinetochores not attached to spindlemicrotubules send a molecular signal thatdelays anaphase
Some external signals are growth factors,proteins released by certain cells that stimulateother cells to divide
For example, platelet-derived growth factor(PDGF) stimulates the division of humanfibroblast cells in culture
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Petriplate
p
Without PDGF
With PDGF
Without PDGF
With PDGF
10 mm
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Another example of external signals is density-
dependent inhibition, in which crowded cells stopdividing
Most animal cells also exhibit anchorage
dependence, in which they must be attached to asubstratum in order to divide
LE 12-18aCells anchor to dish surface and
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divide (anchorage dependence).
When cells have formed a completesingle layer, they stop dividing(density-dependent inhibition).
If some cells are scraped away, theremaining cells divide to fill the gap andthen stop (density-dependent inhibition).
25 mNormal mammalian cells
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Cancer cells exhibit neither density-dependentinhibition nor anchorage dependence
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Cancer cells do not exhibit
anchorage dependenceor density-dependent inhibition.
Cancer cells25 m
Loss of Cell Cycle Controls in Cancer Cells
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y
Cancer cells do not respond normally to the
bodys control mechanisms
Cancer cells form tumors, masses of abnormalcells within otherwise normal tissue
If abnormal cells remain at the original site, thelump is called a benign tumor
Malignant tumors invade surrounding tissues andcan metastasize, exporting cancer cells to otherparts of the body, where they may form secondarytumors