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
Chapter 7 The Cell Cycle and Cell Division
7.4 Meiosis Halves the Nuclear Chromosome
Content and Generates Diversity
7.5 Programmed Cell Death Is a Necessary
Process in Living Organisms
Chapter 7 Opening Question
How does infection with HPV result in
uncontrolled cell reproduction?
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
The lifespan of an organism is linked to cell
reproduction, or cell division: a parent cell
duplicates its genetic material and then divides
into two similar cells.
Cell division is important in growth and repair of
multicellular organisms and the reproduction
of all organisms.
Figure 7.1 The Importance of Cell Division
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
Organisms have two basic strategies for
reproducing themselves:
•  Asexual reproduction
•  Sexual reproduction
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
Asexual reproduction
•  The offspring are clones—genetically
identical to the parent
•  Any genetic variations are due to mutations
(changes in DNA sequences due to
environmental factors or copying errors)
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
Single-celled prokaryotes usually reproduce by
binary fission
Single-celled eukaryotes can reproduce by
mitosis and cytokinesis
Many multicellular eukaryotes can also
reproduce by asexual means
Figure 7.2 Asexual Reproduction on a Large Scale
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
Sexual reproduction
•  Involves fusion of gametes
•  Results in offspring with genetic variation
•  Gametes form by meiosis—a process of
cell division that reduces genetic material
by half
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
DNA in eukaryotic cells is organized into
chromosomes.
Somatic cells: body cells not specialized for
reproduction
Each somatic cell contains two sets of
chromosomes that occur in homologous
pairs.
•  One homolog came from the female parent
and one from the male parent and have
corresponding genetic information.
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
Gametes have only one set of chromosomes—
one homolog from each pair.
•  They are haploid; number of
chromosomes = n
Fertilization: two haploid gametes fuse to form
a zygote
•  They are diploid; number of chromosome
in zygote = 2n
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
All sexual life cycles involve meiosis:
•  Gametes may develop immediately after
meiosis
•  Or each haploid cell may develop into a
haploid organism (haploid stage of the life
cycle) that eventually produces gametes by
mitosis
Fertilization results in a zygote and begins the
diploid stage of the life cycle.
Figure 7.3 Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)
Figure 7.3 Sexual Life Cycles Involve Fertilization and Meiosis (Part 2)
Figure 7.3 Sexual Life Cycles Involve Fertilization and Meiosis (Part 3)
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
The essence of sexual reproduction is:
•  Random selection of half the diploid
chromosome set to form a haploid gamete
•  Followed by fusion of haploid gametes from
separate parents to make a diploid cell
This results in shuffling of genetic information in
a population, and no two individuals have
exactly the same genetic makeup.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Four events in cell division:
•  Reproductive signals initiate cell division
•  DNA replication
•  DNA segregation—distribution of the DNA
into the two new cells
•  Cytokinesis—division of the cytoplasm and
separation of the two new cells
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Prokaryotes divide by binary fission: results in
reproduction of the entire organism.
Reproductive signals may be environmental
factors such as nutrient availability.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Replication:
Most prokaryotes have one circular
chromosome with two important regions:
•  ori—where replication starts
•  ter—where replication ends
Replication occurs as the DNA is threaded
through a “replication complex” of proteins at
the center of the cell.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Segregation:
As replication proceeds, the ori complexes
move to opposite ends of the cell.
DNA sequences adjacent to the ori region
actively bind proteins for the segregation,
using ATP.
An actin-like protein provides a filament along
which ori and other proteins move.
Figure 7.4 Prokaryotic Cell Division: Binary Fission
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Cytokinesis:
After chromosome segregation, the cell
membrane pinches in by contraction of a ring
of protein fibers under the surface.
As the membrane pinches in, new cell wall
materials are deposited, 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.
•  Reproductive signals are usually related to
functions of the entire organism, not the
environment of a single cell.
•  Most cells in a multicellular organism are
specialized and do not divide.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
•  Replication of each chromosome occurs as
they 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
•  In segregation, one copy of each
chromosome ends up in each of the two
new cells.
§ 
More complex than in prokaryotes:
eukaryotes have a nuclear envelope,
and there are multiple chromosomes.
•  Cytokinesis in plant cells (which have cell
walls) is different than in animal cells (no
cell walls).
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
In mitosis, one nucleus produces two daughter
nuclei, each containing the same number of
chromosomes as the parent nucleus.
Mitosis is continuous, but it is convenient to
subdivide it into phases.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
The cell cycle is the period from one cell
division to the next, divided into stages in
eukaryotes.
M phase: Mitosis (segregation of
chromosomes into two new nuclei), followed
by cytokinesis.
Interphase: cell nucleus is visible and cell
functions occur, including DNA replication.
Figure 7.5 The Phases of the Eukaryotic Cell Cycle
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Interphase has three subphases:
§ 
§ 
§ 
G1 (Gap 1)—variable, may last a long
time
S phase (synthesis)—DNA is replicated
G2 (Gap 2)—the cell prepares for
mitosis; synthesizes microtubules for
segregating chromosomes
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Prophase: three structures appear
•  Condensed chromosomes
•  Reoriented centrosomes
•  Spindle
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Even during interphase, DNA is packaged by
winding around specific proteins, and other
proteins coat the DNA coils.
In prophase, the chromosomes become much
more tightly coiled and condensed.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
After replication, each chromosome has two
DNA molecules called sister chromatids,
joined at a region called the centromere.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Karyotype: the condensed chromosomes for a
given organism can be distinguished by their
sizes and centromere positions
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Karyotype analysis was used to identify and
classify organisms, but DNA sequencing is
more commonly used today.
Karotype analysis is still used to identify
chromosome abnormalities.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
The centrosome determines orientation of the
spindle.
•  Consists of two centrioles—hollow tubes
formed by microtubules.
The centrosome is duplicated during S phase;
centrosomes move towards opposite sides of
the nucleus at the G2–M transition.
Centrosome position determines the plane of
cell division—important in the development of
multicellular organisms.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Centrosomes serve as poles toward which the
chromosomes move.
The spindle forms between the poles from
microtubules:
•  Polar microtubules overlap in the middle
region of the cell and keep the poles apart.
•  Astral microtubules interact with proteins
attached to the cell membrane; also help
keep the poles apart.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
•  Kinetochore microtubules attach to
kinetochores on the chromatid
centromeres.
§  Sister chromatids attach to kinetochore
microtubules from opposite sides so that
the two chromatids will move to opposite
poles.
§ 
Sister chromatids become daughter
chromatids after separation.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Prometaphase: the nuclear envelope breaks
down and chromatids attach to the
kinetochore microtubules.
Metaphase: the chromosomes line up at the
midline of the cell.
Anaphase: the chromatids separate, and
daughter chromosomes move toward the
poles.
Figure 7.6 The Phases of Mitosis (1)
Figure 7.6 The Phases of Mitosis (2)
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Two mechanisms move the chromosomes to
opposite poles:
•  Kinetochores have molecular motor
proteins (kinesin and dynein), which move
the chromosomes along the microtubules.
•  The kinetochore microtubules shorten from
the poles, drawing the chromosomes
toward the poles.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Telophase: nuclear envelopes form around
each set of chromosomes and nucleoli
appear, and the spindle breaks down and
chromosomes become less compact.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Cytokinesis:
In animal cells, the cell membrane pinches in
between the nuclei.
A contractile ring of actin and myosin
microfilaments forms on the inner surface of
the cell membrane; the two proteins produce a
contraction to pinch the cell in two.
Figure 7.7 Cytokinesis Differs in Animal and Plant Cells
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
In plant cells, vesicles from the Golgi apparatus
appear along the plane of cell division.
The vesicles fuse to form a new cell membrane.
Contents of vesicles also contribute to forming
the cell plate—the beginning of the new cell
wall.
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 there does not appear to be
a precise mechanism for distribution of the
cytoplasmic contents.
Table 7.1
Concept 7.3 Cell Reproduction Is Under Precise Control
Cell reproduction must be under precise control.
If single-celled organisms had no control over
reproduction, they would soon overrun the
environment and starve to death.
In multicellular organisms, cell reproduction
must be controlled to maintain body form and
function.
Concept 7.3 Cell Reproduction Is Under Precise Control
Prokaryotic cells divide in response to
environmental conditions.
In eukaryotes, cell division is related to the
needs of the entire organism.
Mammals produce growth factors that
stimulate cell division and differentiation.
•  Example: platelets in the blood secrete
growth factors that stimulate cells to divide
to heal wounds.
Concept 7.3 Cell Reproduction Is Under Precise Control
Progression through the eukaryotic cell cycle 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 substances trigger the transition from
one phase to another.
The first evidence for these substances came
from cell fusion experiments.
Fusion of mammalian cells at G1 and S phases
showed that a cell in S phase produces a
substance that activates DNA replication.
Figure 7.9 Regulation of the Cell Cycle (Part 1)
Figure 7.9 Regulation of the Cell Cycle (Part 2)
Figure 7.9 Regulation of the Cell Cycle (Part 3)
Concept 7.3 Cell Reproduction Is Under Precise Control
The trigger substances turned out to be protein
kinases: cyclin-dependent kinases (CDKs).
They catalyze phosphorylation of proteins that
regulate the cell cycle and are activated by
binding to cyclin, which exposes the active site
(allosteric regulation).
Concept 7.3 Cell Reproduction Is Under Precise Control
CDKs function at cell cycle checkpoints:
•  G1 checkpoint is triggered by DNA
damage.
•  S checkpoint is triggered by incomplete
replication or DNA damage.
•  G2 checkpoint is triggered by DNA
damage.
•  M checkpoint is triggered by a chromosome
that fails to attach to the spindle.
Concept 7.3 Cell Reproduction Is Under Precise Control
Each CDK has a cyclin to activate it, which is
made only at the right time.
After the CDK acts, the cyclin is broken down by
a protease.
Synthesis and breakdown of cyclins is important
in controlling the cell cycle.
Cyclins are synthesized in response to various
signals, such as growth factors.
Figure 7.10 Cyclins Are Transient in the Cell Cycle
Concept 7.3 Cell Reproduction Is Under Precise Control
•  Example: control of the restriction point (R)
§ 
§ 
G1–S cyclin–CDK catalyzes
phosphorylation of retinoblastoma
protein (RB).
RB normally inhibits the cell cycle at R,
but when phosphorylated, it becomes
inactive and no longer blocks the cell
cycle.
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 haploid cells produced by meiosis are
genetically different from one another and
from the parent cell.
Figure 7.11 Mitosis and Meiosis: A Comparison (Part 1)
Figure 7.11 Mitosis and Meiosis: A Comparison (Part 2)
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
The function of meiosis is to:
•  Reduce the chromosome number from
diploid to haploid
•  Ensure that each haploid cell has a
complete set of chromosomes
•  Generate diversity among the products
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Meiosis I
•  Homologous chromosomes come together
and line up along their entire lengths.
•  The homologous chromosome pairs
separate, but individual chromosomes
made up of two sister chromatids remain
together.
Figure 7.12 Meiosis: Generating Haploid Cells (1)
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Meiosis I is preceded by an S phase during
which DNA is replicated.
Each chromosome then consists of two sister
chromatids.
At the end of meiosis I, two nuclei form, each
with half the original chromosomes (one
member of each homologous pair).
The centromeres did not separate, so each
chromosome is still two sister chromatids.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Meiosis II
•  Not preceded by DNA replication
•  Sister chromatids separate
•  End result: four haploid cells that are not
genetically identical
Figure 7.12 Meiosis: Generating Haploid Cells (2)
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Shuffling of genetic material during meiosis
occurs by two processes:
•  Crossing over
§ 
In prophase I homologous chromosomes
(synapsis) and the four chromatids form
a tetrad, or bivalent.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
§ 
The homologs seem to repel each other
at the centromeres but remain attached
at chiasmata.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
§ 
§ 
§ 
Genetic material is exchanged between
nonsister chromatids at the chiasmata.
Any of the four chromatids in the tetrad
can participate, and a single chromatid
can exchange material at more than one
point.
Crossing over results in recombinant
chromatids and increases genetic
variability of the products.
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.
o 
o 
Human males: prophase I lasts about
1 week, and 1 month for entire
meiotic cycle
Human females: prophase I begins
before birth, meiosis continues up to
decades later during the monthly
ovarian cycle and is completed only
after fertilization.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
•  Independent assortment
§ 
§ 
§ 
At anaphase I, it is a matter of chance
which member of a homologous pair
goes to which daughter cell.
The greater the number of
chromosomes, the greater the potential
for genetic diversity.
In humans, 223 (8,388,608) different
combinations of maternal and paternal
chromosomes can be produced.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Meiosis is complex, and errors can occur.
Nondisjunction
•  Homologous pair fails to separate at
anaphase I
•  Sister chromatids fail to separate at
anaphase II
Both result in aneuploidy—an abnormal
number of chromosomes.
Figure 7.14 Nondisjunction Leads to Aneuploidy
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Most human embryos from aneuploid zygotes
do not survive. Many miscarriages are due to
this.
The most common human aneuploidy is trisomy
16.
Trisomy 21 (Down syndrome) is one of the few
aneuploidies that allow survival.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Polyploidy
Sometimes, organisms with triploid (3n),
tetraploid (4n), and even higher numbers can
form.
This can occur through an extra round of DNA
replication before meiosis, or 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
Translocation
Crossing over between non-homologous
chromosomes in meiosis I
Location of genes relative to other DNA
sequences is important, and translocations
can have profound effects on gene
expression.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
A translocation that occurs in humans between
chromosomes 9 and 22 can result in a form of
leukemia.
Concept 7.5 Programmed Cell Death Is a Necessary Process in
Living Organisms
Cells can die in one of 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.
Concept 7.5 Programmed 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 are prone to genetic damage
that can lead to cancer—especially true
of epithelial cells that 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
•  DNA is cut into small fragments
•  Membranous lobes (“blebs”) form and
break into fragments
•  Surrounding living cells usually ingest
remains of the dead cell by phagocytosis
Figure 7.15 Apoptosis: Programmed Cell Death
Concept 7.5 Programmed Cell Death Is a Necessary Process in
Living Organisms
Plants use apoptosis in the hypersensitive
response.
They protect themselves from disease by
undergoing apoptosis at the site of infection by
a fungus or bacterium, preventing spread to
other parts of the plant.
Concept 7.5 Programmed Cell Death Is a Necessary Process in
Living Organisms
Programmed cell death is controlled by signals:
•  Internal signals may be linked to cell age or
damaged DNA.
•  Both internal and external signals lead to
activation of caspases, which hydrolyze
target proteins in a cascade of events.
The cell dies as caspases hydrolyze proteins of
the nuclear envelope, nucleosomes, and cell
membrane.
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 mutated positive
regulators of the cell cycle—in cancer cells
they are overactive or present in excess.
Answer to Opening Question
•  Tumor suppressors are negative
regulators of the cell cycle, but are inactive
in cancer cells.
§ 
Example: RB blocks the cell cycle at R.
HPV causes synthesis of E7 protein,
which fits into the protein-binding site of
RB, thereby inactivating it.
Figure 7.16 Molecular Changes Regulate the Cell Cycle in Cancer Cells
Answer to Opening Question
Chemotherapy drugs stop cell division by
targeting cell cycle events.
Some drugs block DNA replication; others
damage DNA, stopping cells at G2; and still
others prevent normal functioning of the
mitotic spindle.
Unfortunately, these drugs also act on normal
cells and are toxic to rapidly dividing cells in
the intestines, skin, and bone marrow.
Answer to Opening Question
Research into more specific chemotherapy
drugs is ongoing.
•  Example: a drug has been identified that
affects the protein produced as a result of
the translocation between chromosomes 9
and 22.
•  It has been successful at treating leukemia
caused by this translocation.