Download The Cell Cycle and Cell Division

7
The Cell Cycle and Cell
Division
Chapter 7 The Cell Cycle and Cell Division
Key Concepts
• 7.1 Different Life Cycles Use Different Modes of
Cell Reproduction
• 7.2 Both Binary Fission and Mitosis Produce
Genetically Identical Cells
• 7.3 Cell Reproduction Is Under Precise Control
• 7.4 Meiosis Halves the Nuclear Chromosome
Content and Generates Diversity
• 7.5 Programmed Cell Death Is a Necessary
Process in Living Organisms
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
The lifespan of an organism is linked to cell
reproduction—usually called cell division.
Organisms have two basic strategies for
reproducing themselves:
• Asexual reproduction
• Sexual reproduction
Cell division is also important in growth and
repair of tissues.
Figure 7.1 The Importance of Cell Division (Part 1)
Figure 7.1 The Importance of Cell Division (Part 2)
Figure 7.1 The Importance of Cell Division (Part 3)
Figure 7.2 Asexual Reproduction on a Large Scale
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
•Gamete
•Somatic Cell
•Chromosome
•Homologous
chromosome
•Haploid(N)
•Diploid (2N)
•Fertilization
•Zygote
Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
Alternation of generations—most plants, some
protists; meiosis gives rise to haploid spores
Spores divide by mitosis to form the haploid
generation (gametophyte).
Gametophyte forms gametes by mitosis.
Gametes then fuse to form diploid zygote
(sporophyte), which in turn produces haploid
spores by meiosis.
Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 2)
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
Diplontic life cycle—animals and some plants;
gametes are the only haploid stage
A mature organism is diploid and produces
gametes by meiosis.
Gametes fuse to form diploid zygote; zygote
divides by mitosis to form mature organism.
Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 3)
Figure 7.4 Prokaryotic Cell Division – Binary Fission
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Cytokinesis begins after chromosome
segregation by a pinching in of the plasma
membrane—protein fibers form a ring.
As the membrane pinches in, new cell wall
materials are synthesized resulting in
separation of the two cells.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Eukaryotic cells divide by mitosis followed by
cytokinesis.
Replication of DNA occurs as long strands are
threaded through replication complexes.
DNA replication only occurs during a specific
stage of the cell cycle.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Interphase has three subphases: G1, S, and G2.
G1 (Gap 1)—variable, a cell may spend a long
time in this phase carrying out its functions
S phase (Synthesis)—DNA is replicated
G2 (Gap 2)—the cell prepares for mitosis,
synthesizes microtubules for segregating
chromosomes
Anatomy of a Chromosome
Anatomy of a Chromosome
Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 2)
Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 3)
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Condensed chromosomes appear during
prophase.
Sister chromatids—two DNA molecules on
each chromosome after replication
Centromere—region where chromatids are
joined
Kinetochores are protein structures on the
centromeres, and are important for
chromosome movement.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Segregation is aided by other structures:
The centrosome determines the orientation of
the spindle apparatus.
Each centrosome can consist of two
centrioles—hollow tubes formed by
microtubules.
Centrosome is duplicated during S phase and
each moves towards opposite sides of the
nucleus.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Centrosomes serve as mitotic centers or poles;
the spindle forms between the poles from two
types of microtubules:
• Polar microtubules form a spindle and overlap
in the center.
• Kinetochore microtubules—attach to
kinetochores on the chromatids. Sister
chromatids attach to opposite halves of the
spindle.
Figure 7.6 The Phases of Mitosis (1)
Figure 7.6 The Phases of Mitosis (2)
Kinetochore
Chromosome Walking
Experiments: Spindle
fibers shorten during
anaphase from
the end attached to
the chromosome, not
the centrosome.
Fig. 12.7b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 1)
Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 2)
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
After cytokinesis:
Each daughter cell contains all of the
components of a complete cell.
Chromosomes are precisely distributed.
The orientation of cell division is important to
development, but organelles are not always
evenly distributed.
Concept 7.3 Cell Reproduction Is Under Precise Control
The eukaryotic cell cycle has four stages: G1, S,
G2, and M.
Progression is tightly regulated—the G1-S
transition is called R, the restriction point.
Passing this point usually means the cell will
proceed with the cell cycle and divide.
Figure 7.8 The Eukaryotic Cell Cycle
Concept 7.3 Cell Reproduction Is Under Precise Control
Specific signals trigger the transition from one
phase to another.
Evidence for substances as triggers came from
cell fusion experiments.
Nuclei in cells at different stages, fused by
polyethylene glycol, both entered the phase of
DNA replication (S).
Figure 7.9 Regulation of the Cell Cycle (Part 1)
Concept 7.3 Cell Reproduction Is Under Precise Control
Transitions also depend on activation of cyclindependent kinases (Cdk’s).
A protein kinase is an enzyme that catalyzes
phosphorylation from ATP to a protein.
Phosphorylation changes the shape and function
of a protein by changing its charges.
Concept 7.3 Cell Reproduction Is Under Precise Control
Cdk is activated by binding to cyclin (by
allosteric regulation); this alters its shape and
exposes its active site.
The G1-S cyclin-Cdk complex acts as a protein
kinase and triggers transition from G1 to S.
Other cyclin-Cdk’s act at different stages of the
cell cycle, called cell cycle checkpoints.
Cyclins
Concept 7.3 Cell Reproduction Is Under Precise Control
Example of G1-S cyclin-Cdk regulation:
Progress past the restriction point in G1 depends
on retinoblastoma protein (RB).
RB normally inhibits the cell cycle, but when
phosphorylated by G1-S cyclin-Cdk, RB
becomes inactive and no longer blocks the cell
cycle.
Growth Factors
Density normally inhibits growth
•Anchorage dependence
•Senescence and Immortals
•(HeLa Cell Line)
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Meiosis consists of two nuclear divisions but
DNA is replicated only once. The function of
meiosis is to:
• Reduce the chromosome number from diploid
to haploid
• Ensure that each haploid has a complete set of
chromosomes
• Generate diversity among the products
Sexual Reproduction
Figure 7.11 Mitosis and Meiosis: A Comparison
Figure 7.12 Meiosis: Generating Haploid Cells (Part 1)
Figure 7.12 Meiosis: Generating Haploid Cells (Part 2)
Figure 7.12 Meiosis: Generating Haploid Cells (Part 3)
Figure 7.12 Meiosis: Generating Haploid Cells (Part 4)
Figure 7.12 Meiosis: Generating Haploid Cells (Part 5)
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
In prophase of meiosis I homologous chromosomes
pair by synapsis.
The four chromatids of each pair of chromosomes
form a tetrad,or bivalent.
The homologs seem to repel each other but are held
together at chiasmata.
Crossing over is an exchange of genetic material
that occurs at the chiasma.
Crossing over results in recombinant chromatids
and increases genetic variability of the products
In-Text Art, Ch. 7, p. 138
Figure 7.13 Crossing Over Forms Genetically Diverse Chromosomes
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Prophase I may last a long time.
• Human males: Prophase I lasts about 1 week,
and 1 month for entire meiotic cycle
• Human females: Prophase I begins before
birth, and ends up to decades later during the
monthly ovarian cycle
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Meiotic errors:
Nondisjunction—homologous pairs fail to
separate at anaphase I—sister chromatids fail
to separate, or homologous chromosomes may
not remain together
Either results in aneuploidy—chromosomes
lacking or present in excess
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Organisms with triploid (3n), tetraploid (4n), and
even higher levels are called polyploid.
This can occur through an extra round of DNA
duplication before meiosis, or the lack of
spindle formation in meiosis II.
• Polyploidy occurs naturally in some species,
and can be desirable in plants.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
If crossing over happens between nonhomologous chromosomes, the result is a
translocation.
A piece of chromosome may rejoin another
chromosome, and its location can have
profound effects on the expression of other
genes.
Example: Leukemia
In-Text Art, Ch. 7, p. 140
Concept 7.5 Programmed Cell Death Is a Necessary Process in
Living Organisms
Cell death occurs in two ways:
• In necrosis, the cell is damaged or starved for
oxygen or nutrients. The cell swells and bursts.
Cell contents are released to the extracellular
environment and can cause inflammation.
7.5 ProConcept 7.ammed Cell Death Is a Necessary Process in
Living Organisms
• Apoptosis is genetically programmed cell
death. Two possible reasons:
The cell is no longer needed, e.g., the
connective tissue between the fingers of a
fetus.
Old cells may be prone to genetic damage that
can lead to cancer—blood cells and epithelial
cells die after days or weeks.
Concept 7.5 Programmed Cell Death Is a Necessary Process in
Living Organisms
Events of apoptosis:
• Cell detaches from its neighbors
• Cuts up its chromatin into nucleosome-sized
pieces
• Forms membranous lobes called “blebs” that
break into fragments
• Surrounding living cells ingest the remains of
the dead cell
Figure 7.14 Apoptosis: Programmed Cell Death (Part 1)
Concept 7.5 Programmed Cell Death Is a Necessary Process in
Living Organisms
Cell death cycle is controlled by signals:
• Lack of a mitotic signal (growth factor)
• Recognition of damaged DNA
External signals cause membrane proteins to
change shape and activate enzymes called
caspases—hydrolyze proteins of membranes.
Figure 7.14 Apoptosis: Programmed Cell Death (Part 2)
Answer to Opening Question
Human papilloma virus (HPV) stimulates the
cell cycle when it infects the cervix.
Two proteins regulate the cell cycle:
Oncogene proteins are positive regulators
of the cell cycle—in cancer cells they are
overactive or present in excess
Tumor suppressors are negative
regulators of the cell cycle, but in cancer
cells they are inactive—can be blocked by
a virus such as HPV
Figure 7.15 Molecular Changes Regulate the Cell Cycle in Cancer Cells