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
Key Concepts
• 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—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)
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
In asexual reproduction the offspring are
clones—genetically identical to the parent.
Any genetic variations are due to mutations.
A unicellular prokaryote may reproduce itself by
binary fission.
Single-cell eukaryotes can reproduce by mitosis.
Other eukaryotes are also able to reproduce
through asexual or sexual means.
Figure 7.2 Asexual Reproduction on a Large Scale
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
Sexual reproduction requires gametes—two
parents each contribute one gamete to an
offspring.
Gametes form by meiosis—a process of cell
division.
Gametes—and offspring—differ genetically from
each other and from the parents.
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
DNA in eukaryotic cells is organized into
chromosomes.
A chromosome consists of a single molecule of
DNA and proteins.
Somatic cells—body cells not specialized for
reproduction
Each somatic cell contains two sets of
chromosomes (homologs) that occur in
homologous pairs.
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
Gametes contain only one set of
chromosomes—one homolog from each pair.
Haploid cell—Number of chromosomes = n
Fertilization—Two haploid gametes (female egg
and male sperm) fuse to form a zygote.
Chromosome number in zygote = 2n and cells
are diploid.
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
All kinds of sexual life cycles involve meiosis:
Haplontic life cycle—in protists, fungi, and some
algae—zygote is only diploid stage
After zygote forms it undergoes meiosis to form
haploid spores, which germinate to form a new
organism.
Organism is haploid, and produces gametes by
mitosis—cells fuse to form diploid 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)
Concept 7.1 Different Life Cycles Use Different Modes of Cell
Reproduction
The essence of sexual reproduction is that it
allows the random selection of half the diploid
chromosome set.
This forms a haploid gamete that fuses with
another to make a diploid cell.
Thus, no two individuals have exactly the same
genetic makeup.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Four events must occur for cell division:
• Reproductive signal—to initiate cell division
• Replication of 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
In prokaryotes, cell division results in
reproduction of the entire organism.
The cell:
• Grows in size
• Replicates its DNA
• Separates the DNA and cytoplasm into two
cells through binary fission
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Most prokaryotes have one chromosome, a
single molecule of DNA—usually circular.
Two important regions in reproduction:
• ori - where replication starts
• ter - where replication ends
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Replication occurs as the DNA is threaded
through a “replication complex” of proteins in
the center of the cell.
Replication begins at the ori site and moves
towards the ter site.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
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,
hydrolyzing ATP for energy.
An actin-like protein provides a filament along
which ori and other proteins move.
Figure 7.4 Prokaryotic Cell Division
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
In segregation of DNA after cell division, one
copy of each chromosome ends up in each of
the two new cells.
In eukaryotes, the chromosomes become highly
condensed.
Mitosis segregates them into two new nuclei—
the cytoskeleton is involved in the process.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Cytokinesis follows mitosis.
The process in plant cells (which have cell walls)
is different than in animal cells (which do not
have cell walls).
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
The cell cycle—the period between cell
divisions
In eukaryotes it is divided into mitosis and
cytokinesis—called the M phase—and a long
interphase.
During interphase, the cell nucleus is visible
and cell functions including replication occur
Interphase begins after cytokinesis and ends
when mitosis starts.
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
Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 1)
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
In mitosis, one nucleus produces two daughter
nuclei each containing the same number of
chromosomes as the parent nucleus.
Mitosis is continuous, but can be can be divided
into phases—prophase, prometaphase,
metaphase, anaphase, and telophase.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
During interphase, only the nuclear envelope
and and the nucleolus are visible.
The chromatin (DNA) is not yet condensed.
Three structures appear in prophase:
• The condensed chromosomes
• Centrosome
• Spindle
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
The karyotype of an organism reflects the
number and sizes of its condensed
chromosomes.
Karyotype analysis can be used to identify
organisms, but DNA sequence is more
commonly used.
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.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Chromosome separation and movement is
highly organized.
During prometaphase, the nuclear envelope
breaks down.
Chromosomes consisting of two chromatids
attach to the kinetochore mictotubules.
Figure 7.6 The Phases of Mitosis (1)
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Durin metaphase, chromosomes line up at the
midline of the cell.
During anaphase, the separation of sister
chromatids is controlled by M phase cyclinCdk; cohesin is hydrolyzed by separase.
After separation, they move to opposite ends of
the spindle and are referred to as daughter
chromosomes.
Figure 7.6 The Phases of Mitosis (2)
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
A protein at the kinetochores—cytoplasmic
dynein—hydrolyzes ATP for energy to move
chromosomes along the microtubules towards
the poles.
Microtubules also shorten, drawing
chromosomes toward poles.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Telophase occurs after chromosomes have
separated:
• Spindle breaks down
• Chromosomes uncoil
• Nuclear envelope and nucleoli appear
• Two daughter nuclei are formed with identical
genetic information
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Cytokinesis:
Division of the cytoplasm differs in plant and
animals
• In animal cells, plasma membrane pinches
between the nuclei because of a contractile
ring of microfilaments of actin and myosin.
Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 1)
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically
Identical Cells
Plant cells:
Vesicles from the Golgi apparatus appear along
the plane of cell division
• These fuse to form a new plasma membrane.
• Contents of vesicles form the cell plate—the
beginning of the new cell wall.
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 reproductive rates of most prokaryotes
respond to environmental conditions.
In eukaryotes, cell division is related to the
needs of the entire organism.
Cells divide in response to extracellular signals,
like growth factors.
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.
Figure 7.10 Cyclins Are Transient in the Cell Cycle
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.
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
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
Meiotic division reduces the chromosome
number. Two unique features:
• In meiosis I, homologous pairs of
chromosomes come together and line up along
their entire lengths.
• After metaphase I, the homologous
chromosome pairs separate, but individual
chromosomes made up of two sister
chromatids remain together.
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, held together by cohesin proteins.
At the end of meiosis I, two nuclei form, each
with half the original chromosomes—still
composed of sister chromatids.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Sister chromatids separate during meiosis II,
which is not proceeded by DNA replication.
The products of meiosis I and II are four cells
with a haploid number of chromosomes.
These four cells are not genetically identical.
Two processes may occur:
Crossing over and independent assortment
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.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
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
Independent assortment during anaphase I
also allows for chance combinations and
genetic diversity.
After homologous pairs of chromosomes line up
at metaphase I, it is a matter of chance which
member of a pair goes to which daughter cell.
The more chromosomes involved, the more
combinations possible.
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