Transcript The Cell Cycle Control System

Chapter 12
The Cell Cycle
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: The Key Roles of Cell Division
• The ability of organisms to reproduce best
distinguishes living things from non-living matter
• The continuity of life is based upon the
reproduction of cells, or cell division
• Cell division is integral part of cell cycle
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Types of cell division
•
Prokaryotes (no nucleus: bacteria – simple life forms)
–
•
Binary fission
Eukaryotes (contain a nucleus)
–
–
Mitosis:
•
Growth, development & repair
•
Asexual reproduction (yields genetically identical cells)
•
Occurs in somatic (body) cells
Meiosis:
•
Sexual reproduction (yields genetically different cells with half
the # 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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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 12.1: Mitotic cell division results in
genetically identical daughter cells
• Cells duplicate their genetic material before they
divide, ensuring that each daughter cell receives
an exact copy of the genetic material, DNA
• A dividing cell duplicates its DNA, allocates the
two copies to opposite ends of the cell, and only
then splits into daughter cells
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Cellular Organization of the Genetic Material
• A cell’s endowment of DNA (its genetic
information) is called its genome
• DNA molecules* in a cell are packaged into
chromosomes
*Prokaryotescircular DNA
Eukaryoteslinear DNA
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• Every eukaryotic species has a characteristic
number of chromosomes in each cell nucleus
• Somatic (non-reproductive) cells (normally) have
two sets of chromosomes
• Gametes (reproductive cells: sperm and eggs)
(and spores) 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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DNA associates with special proteins to form more stable
structure called chromosomes (different proteins in
prokaryotes and eukaryotes, so chromosomes built different)
Chromosomes are found inside nucleus in eukaryotes
Human - 46 chromosomes, 23 pairs (1 set of 23 from egg, 1
set of 23 from sperm)
Each chromosome contains many genes
Gene is a segment of DNA that is responsible for controlling
a trait (e.g., coding for a specific protein)
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Human female karyotype
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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 be
divided into subphases:
– G1 phase (“first gap”)
– S phase (“synthesis”)
– G2 phase (“second gap”)
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LE 12-5
INTERPHASE
G1
S
(DNA synthesis)
G2
Distribution of Chromosomes During 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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LE 12-4
0.5 µm
Chromosome
duplication
(including DNA
synthesis)
Centromere
Sister
chromatids
Separation
of sister
chromatids
Centromeres
Sister chromatids
• Mitosis is conventionally divided into five phases:
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
• Cytokinesis is well underway by late telophase
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
1. Prophase
-
Chromatin condenses, this causes the chromosomes to
begin to become visible
-
Centrosomes separate, moving to opposite ends of the
nucleus
-
The centrosomes start to form a framework used to
separate the two sister chromatids called the mitotic
spindle, that is made of microtubules
-
Nucleolus disappears
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
2. Prometaphase
- Nuclear envelope fragments
- Chromosomes become more condensed
- A kinetochore is formed at the centromere, the point where
the sister chromatids are attached
- Microtubules attach at the kinetochores
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3. Metaphase
- Chromosomes align on an axis called the metaphase plate
- Note: the spindle consists of microtubules, one attached to
each chromosome
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
4. Anaphase
- Each centromere splits making two chromatids free
- Each chromatid moves toward a pole
- Cell begins to elongate, caused by microtubules not
associated with the kinetochore
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
5. Telophase
•
Formation of nuclear membrane and nucleolus
•
Short and thick chromosomes begin to elongate to form
long and thin chromatin
•
Formation of the cleavage furrow - a shallow groove in
the cell near the old metaphase plate
•
Cytokinesis = division of the cytoplasm
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mitosis in an onion root
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The Mitotic Spindle: A Closer Look
• The mitotic spindle is an apparatus of
microtubules that controls chromosome movement
during mitosis
• 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
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• The spindle includes the centrosomes, the spindle
microtubules, and the asters
• Some spindle microtubules attach to the
kinetochores of chromosomes and move the
chromosomes to the metaphase plate
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 12-7
Aster
Microtubules
Sister
chromatids
Chromosomes
Centrosome
Metaphase
plate
Kinetochores
Overlapping
nonkinetochore
microtubules
Centrosome
1 µm
Kinetochore
microtubules
0.5 µm
• 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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LE 12-8b
Chromosome
movement
Microtubule
Motor
protein
Chromosome
Kinetochore
Tubulin
subunits
• Nonkinetochore microtubules from opposite poles
overlap and push against each other, elongating
the cell
• In telophase, genetically identical daughter nuclei
form at opposite ends of the cell
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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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LE 12-9a
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Daughter cells
Cleavage of an animal cell (SEM)
Mitosis
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LE 12-9b
Vesicles
forming
cell plate
Wall of
parent cell
Cell plate
1 µm
New cell wall
Daughter cells
Cell plate formation in a plant cell (TEM)
LE 12-10
Nucleus
Nucleolus
Chromatin
condensing
Prophase. The
chromatin is condensing.
The nucleolus is
beginning to disappear.
Although not yet visible
in the micrograph, the
mitotic spindle is starting
to form.
Chromosomes
Prometaphase. We
now see discrete
chromosomes; each
consists of two identical
sister chromatids. Later
in prometaphase, the
nuclear envelope will
fragment.
Cell plate
Metaphase. The spindle is
complete, and the
chromosomes, attached
to microtubules at their
kinetochores, are all at
the metaphase plate.
Anaphase. The
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends of
the cell as their
kinetochore microtubules shorten.
10 µm
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will divide
the cytoplasm in two, is
growing toward the
perimeter of the parent
cell.
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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LE 12-11_1
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Two copies
of origin
Bacterial
chromosome
LE 12-11_2
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Replication continues.
One copy of the origin
is now at each end of
the cell.
Bacterial
chromosome
Two copies
of origin
Origin
Origin
LE 12-11_3
Cell wall
Origin of
replication
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Replication continues.
One copy of the origin
is now at each end of
the cell.
Replication finishes.
The plasma membrane
grows inward, and
new cell wall is
deposited.
Two daughter
cells result.
Plasma
membrane
Bacterial
chromosome
Two copies
of origin
Origin
Origin
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 © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 12-12
Bacterial
chromosome
Prokaryotes
Chromosomes
Microtubules
Intact nuclear
envelope
Dinoflagellates
Kinetochore
microtubules
Intact nuclear
envelope
Diatoms
Kinetochore
microtubules
Centrosome
Fragments of
nuclear envelope
Most eukaryotes
Concept 12.3: The cell cycle is regulated by a
molecular control system
• The frequency of cell division varies with the type
of cell and with cell “happiness”
• These cell cycle differences result from regulation
at the molecular level
• The cell cycle appears to be driven by specific
chemical signals present in the cytoplasm
• The levels of these chemical signals are
influenced by biotic & abiotic factors
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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 the
most important one
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 12-14
G1 checkpoint
Control
system
G1
M
M checkpoint
G2 checkpoint
G2
S
LE 12-15
G0
G1 checkpoint
G1
If a cell receives a go-ahead
signal at the G1 checkpoint,
the cell continues on in the
cell cycle.
G1
If a cell does not receive a
go-ahead signal at the G1
checkpoint, the cell exits the
cell cycle and goes into G0, a
nondividing state.
LE 12-16b
Cdk
Degraded
cyclin
G2
Cdk
checkpoint
Cyclin is
degraded
MPF
Cyclin
Molecular mechanisms that help regulate the cell cycle
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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LE 12-17
Scalpels
Petri
plate
Without PDGF
With PDGF
Without PDGF
With PDGF
10 mm
• Another example of external signals is densitydependent 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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LE 12-18a
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 (density-dependent inhibition).
Normal mammalian cells
25 µm
• Cancer cells exhibit neither density-dependent
inhibition nor anchorage dependence
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LE 12-18b
Cancer cells do not exhibit
anchorage dependence
or density-dependent inhibition.
25 µm
Cancer cells
Loss of Cell Cycle Controls in Cancer Cells
• Cancer cells do not respond normally to the
body’s control mechanisms
• 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 © 2005 Pearson Education, Inc. publishing as Benjamin Cummings