Transcript The Cell Cycle

Chapter 12
The Cell Cycle
Lab 3
Mitosis and Meiosis
Mitosis
Division of the cell
nucleus.
Cytokinesis
Division of the cell
cytoplasm.
The Cell Cycle
 The
continuity of
life
 Is based upon
the
reproduction
of cells, or
cell division
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The Cell Cycle – Making an “EXACT copy”
 Unicellular organisms
 Reproduce by cell division
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The Cell Cycle
 Multicellular organisms depend on cell division for
 Development from a fertilized cell
 Growth
 Repair
Growth and development
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Tissue renewal
Cell Division
 Cell division results in genetically
identical daughter cells
 Cells duplicate their total genetic material
(genome)
 Before they divide, ensuring that each
daughter cell receives an exact copy of
the genetic material, DNA
Genetic Material
 The DNA molecules
in a cell
 Are packaged into
chromosomes
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Genetic Material
Eukaryotic chromosomes
 Consist of chromatin, a complex of DNA &
protein that condenses during cell division
 In animals
 Somatic cells have two sets of
chromosomes
 46 in humans
 Gametes (egg & sperm) have one set
of chromosomes
 23 in humans
DNA Replication
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 Each duplicated
chromosome
 Has two sister
chromatids,
which separate
during cell
division
 They are
attached by a
central area
called the
centromere
Chromosome
duplication
(including DNA
synthesis)
chromatids
centromere
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Separation
of sister
chromatids
Phases of the Cell Cycle
 The cell cycle consists of
INTERPHASE
 Interphase
 Mitotic phase
G1
S
(DNA synthesis)
G2
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Mitotic Phase
 Mitosis consists of five distinct phases
1.) Prophase
2.) Prometaphase
G2 OF
INTERPHASE
Centrosomes
Chromatin
(with centriole pairs)
(duplicated)
Nucleolus
Nuclear
Plasma
envelope membrane
PROPHASE
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids
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PROMETAPHASE
Fragments
Kinetochore
of nuclear
envelope
Nonkinetochore
microtubules
Kinetochore
microtubule
Mitotic Phase
3.) Metaphase
4.) Anaphase
5.) Telophase (usually followed by cytokinesis)
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Centrosome at Daughter
one spindle pole chromosomes
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TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nuclear
envelope
forming
Nucleolus
forming
Types of Microtubules
Kinetochore microtubules
 Attach to the kinetochores of
chromosomes and move the
chromosomes to the metaphase plate
Nonkinetechore microtubules from
opposite poles
 Overlap and push against each other,
elongating the cell
 Assembly of the spindle microtubules starts
in the centrosome.
 The centrosome (microtubule-organizing
center) of animals has a pair of centrioles at
the center, but the function of the centrioles is
somewhat undefined.
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 As mitosis starts, the two centrosomes are
located near the nucleus.
 As the spindle fibers grow from them, the
centrioles are pushed apart.
 By the end of prometaphase they develop
as the spindle poles at opposite ends of the
cell.
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 Each sister chromatid has a kinetochore
of proteins and chromosomal DNA at the
centromere.
 The kinetochores of the joined sister
chromatids face in opposite directions.
 During prometaphase,
some spindle
microtubules
attach to the
kinetochores.
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 When a chromosome’s kinetochore is
“captured” by microtubules, the
chromosome moves toward the pole from
which those microtubules come.
 When microtubules attach to the other pole,
this movement stops and a tug-of-war
ensues.
 Eventually, the chromosome settles
midway between the two poles of the cell,
the metaphase plate.
 Other microtubules from opposite poles
interact as well, elongating the cell.
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 One hypothesis for the movement of
chromosomes in anaphase is that motor
proteins at the kinetochore “walk” the
attached chromosome along the
microtubule toward the opposite pole.
 The excess microtubule sections
depolymerize.
Fig. 12.7a
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 Experiments
support the
hypothesis that
spindle fibers
shorten during
anaphase from
the end attached
to the
chromosome,
not the
centrosome.
Fig. 12.7b
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 Nonkinetichore microtubules are
responsible for lengthening the cell along
the axis defined by the poles.
 These microtubules interdigitate across the
metaphase plate.
 During anaphase motor proteins push
microtubules from opposite sides away from
each other.
 At the same time, the addition of new tubulin
monomers extends their length.
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Cytokinesis
In animal cells
 Cytokinesis occurs by a process known as
cleavage, forming a cleavage furrow
Cleavage furrow
Contractile ring of
microfilaments
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Daughter cells
Cytokinesis
 In plant cells, during cytokinesis
 A cell plate forms
Vesicles
forming
cell plate
1 µm
Wall of
patent cell Cell plate New cell wall
Daughter cells
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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
G1 checkpoint
Control
system
G1
M
G2
M checkpoint
G2 checkpoint
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S
Cell Cycle Control System
 The clock has specific checkpoints
 Where the cell cycle stops until a go-ahead signal is
received
G0
G1 checkpoint
G1
(a) If a cell receives a go-ahead signal at
the G1 checkpoint, the cell continues
on in the cell cycle.
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G1
(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.
Normal Cell Behavior
In density-dependent inhibition

Crowded cells stop dividing
Most animal cells exhibit anchorage dependence

In which they must be attached to a substratum to
divide
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
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Abnormal Cell Behavior
Cancer cells

Transformation is the process of a normal cell becoming
cancerous

Cancer cells exhibit neither density-dependent inhibition nor
anchorage dependence

Why cells transform is often a mystery but most likely genes
controlling the cell control system are involved
Cancer cells. Cancer cells usually
continue to divide well beyond a
single layer, forming a clump of
overlapping cells.
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Cancerous Cells
 Malignant tumors invade surrounding tissues
and can metastasize
 Exporting cancer cells to other parts of the body
where they may form secondary tumors
Lymph
vessel
Tumor
Blood
vessel
Glandular
tissue
1 A tumor grows from a
single cancer cell.
Cancer cell
2 Cancer cells invade
neighboring tissue.
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3 Cancer cells spread
through lymph and
blood vessels to
other parts of the body.
Metastatic
Tumor
4 A small percentage of
cancer cells may survive
and establish a new tumor
in another part of the body.
Thinking Question
Many cancer drugs (chemotherapy)
work by interfering with the production
or proper function of microtubules.
What specific effect would this have on
cell division and why is this beneficial in
the treatment of cancer? Common side
effects of chemotherapy are hair loss
and nausea. Why do you think this is
the case?
Key Points of Chapter 12
 Cell division results in two genetically
identical daughter cells
 The mitotic phase alternates with
interphase in the cell cycle
 The cell cycle is regulated by a
molecular control system