Principles of Life
Hillis • Sadava • Heller • Price
Instructor’s Manual
Chapter 7: The Cell Cycle and Cell Division
OVERVIEW
After a brief description of cell division in prokaryotes, Chapter 7 focuses on eukaryotic
cell division. This chapter covers the cell cycle and the principal molecules that control
cell division and then gives detailed descriptions of mitosis and meiosis. Asexual and
sexual reproduction are contrasted, and some of the consequences of meiotic errors are
examined. Chapter 7 concludes with a section on the events and regulation of
programmed cell death.
KEY CONCEPTS/CHAPTER OUTLINE
7.1 Different Life Cycles Use Different Modes of Cell ReproductionAsexual
reproduction by binary fission or mitosis results in genetic constancy
• Sexual reproduction by meiosis results in genetic diversity
Asexual reproduction (cloning) produces a new organism that is genetically identical to
the parent. Sexual reproduction forms a genetically unique organism.
7.2 Both Binary Fission and Mitosis Produce Genetically Identical
CellsProkaryotes divide by binary fission
• Eukaryotic cells divide by mitosis followed by cytokinesis
• Prophase sets the stage for DNA segregation
• Chromosome separation and movement are highly organized
• Cytokinesis is the division of the cytoplasm
Cell division consists of three steps: replication of the genetic material (DNA),
partitioning of the two DNA molecules to separate portions of the cell, and division of the
cytoplasm. In prokaryotes, cellular DNA is a single molecule, or chromosome.
Prokaryotes reproduce by binary fission, while eukaryotes divide either by mitosis or
meiosis.
7.3 Cell Reproduction Is Under Precise Control
• The eukaryotic cell division cycle is regulated internally
• The cell cycle is controlled by cyclin-dependent kinases
During most of the cell cycle, the cell is in interphase, which is divided into three
subphases: S, G1, and G2. Some cells enter a resting phase called G0. A cell can be
stimulated to begin a division cycle by its internal cyclin-Cdk complexes and by external
controls such as growth factors and hormones.
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7.4 Meiosis Halves the Nuclear Chromosome Content and Generates
DiversityMeiotic division reduces the chromosome number
• Crossing over and independent assortment generate diversity
• Meiotic errors lead to abnormal chromosome structures and numbers
Meiosis consists of two nuclear divisions, meiosis I and meiosis II. Meiosis reduces the
chromosome number from diploid to haploid and ensures that each haploid cell contains
one member of each chromosome pair. The result of meiosis is four cells, each with a
haploid chromosome content. Material may be exchanged by crossing over and
independent assortment, contributing to genetic diversity.
Meiotic errors such as nondisjunction, polyploidy, and translocation may have significant
consequences for offspring.
7.5 Programmed Cell Death Is a Necessary Process in Living Organisms
Cells may die by necrosis or may self-destruct by apoptosis, a genetically programmed
series of events that includes the detachment of the cell from its neighbors and the
fragmentation of its nuclear DNA. In apoptosis, both external and internal signals
stimulate caspases, enzymes that break down specific cell constituents.
LECTURE OUTLINE
Chapter 7 Opening Question
How does infection with HPV result in uncontrolled cell reproduction?
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.
(See Chapter 4)
FIGURE 7.1 The Importance of Cell Division
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
Sexual reproduction requires gametes—two parents each contribute one gamete to an
offspring.
Gametes form by meiosis—a process of cell division.
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Gametes—and offspring—differ genetically from each other and from the parents.
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.
(VIDEO 7.1 Cell Visualization: From DNA to chromosomes)
(LINK: The inheritance of characteristics such as seed shape is discussed in Chapter 8)
(See Chapter 4)
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.
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, produces gametes by mitosis—cells fuse to form diploid zygote
FIGURE 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)
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)
Diplontic life cycle: Animals and some plants; gametes are the only haploid stage
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)
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.
(See Part Four)
7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
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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
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
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
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.
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.
(LINK: Review the description of the cytoskeleton and its components in Concept 4.4)
FIGURE 7.4 Prokaryotic Cell Division
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.
(VIDEO 7.2 Cytokinesis in the euglenoid Phacus)
(VIDEO 7.3 Cytokinesis in a green alga, Micrasterias)
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.
(See Chapter 9)
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.
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Mitosis segregates them into two new nuclei— the cytoskeleton is involved in the
process.
Cytokinesis follows mitosis.
The process in plant cells (which have cell walls) is different than in animal cells (which
do not have cell walls).
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.
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
(ANIMATED TUTORIAL 7.1 Mitosis)
FIGURE 7.5 The Phases of the Eukaryotic Cell Cycle
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.
(VIDEO 7.4 Division of bacteria, Salmonella enteritidis)
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
(See Concept 4.3)
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, important for chromosome
movement.
(See Chapter 9)
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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.
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.
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 center
• Kinetochore microtubules—attach to kinetochores on the chromatids. Sister chromatids
attach to opposite halves of the spindle.
(VIDEO 7.5 Formation of mitotic spindle)
Chromosome separation and movement is highly organized.
During prometaphase, the nuclear envelope breaks down.
Chromosomes consisting of two chromatids attach to the kinetochore mictotubules.
(VIDEO 7.6 Cell Visualization: Mitosis and cell division)
FIGURE 7.6 The Phases of Mitosis (Part 1)
During 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 (Part 2)
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.
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
Cytokinesis:
Division of the cytoplasm differs in plant and animals
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• In animal cells, plasma membrane pinches between the nuclei because of a contractile
ring of microfilaments of actin and myosin
(VIDEO 7.7: Mitosis in a newt lung epithelial cell)
FIGURE 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 1)
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.
(VIDEO 7.8 Mitosis in a plant cell)
FIGURE 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 2)
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.
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.
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
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
Transitions also depend on activation of cyclin-dependent 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.
(See Chapter 5)
Cdk is activated by binding to cyclin (by allosteric regulation); this alters its shape and
exposes its active site.
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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.
(See Chapter 3)
FIGURE 7.10 Cyclins Are Transient in the Cell Cycle
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.
(See Chapters 3 and 5)
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
(ANIMATED TUTORIAL 7.2 Meiosis)
FIGURE 7.11 Mitosis and Meiosis: A Comparison
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.
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.
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
(VIDEO 7.9 Meiosis in a cranefly spermatocyte)
In prophase of meiosis I homologous chromosomes pair by synapsis.
The four chromatids of each pair of chromosomes form a tetrad,or bivalent.
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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, p. 138
FIGURE 7.13 Crossing Over Forms Genetically Diverse Chromosomes
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
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.
(APPLY THE CONCEPT: 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
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.
If crossing over happens between non-homologous 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
(See Chapters 10 and 11)
IN-TEXT ART, p. 140
7.5 Programmed Cell Death Is a Necessary Process in Living Organisms
Cell death occurs in two ways:
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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.
(See Concept 31.1)
(APPLY THE CONCEPT: Programmed cell death is a necessary process in living
organisms)
• Apoptosis is genetically programmed cell death. Two possible reasons:
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.
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
(VIDEO 7.10 Apoptosis)
FIGURE 7.14 Apoptosis: Programmed Cell Death (Part 1)
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
(VIDEO 7.11 Human melanoma cells dividing in culture)
FIGURE 7.15 Molecular Changes Regulate the Cell Cycle in Cancer Cells
KEY TERMS
alternation of generations
anaphase
aneuploidy
apoptosis
asexual reproduction
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binary fission
caspases
Cdk’s
cell cycle
cell cycle checkpoints
centrioles
centromere
centrosome
chromosome
clones
crossing over
cyclin
cyclin-dependent kinases
cytokinesis
daughter chromosomes
diploid
fertilization
G1
G1–S transition
G2
gametes
growth factors
haploid
homologous pairs
independent assortment
interphase
karyotype
kinetochores
meiosis
meiosis I
meiosis II
metaphase
mitosis
necrosis
nondisjunction
oncogene
polyploid
prometaphase
prophase
recombinant
replication
S phase
segregation
sexual reproduction
sister chromatids
somatic cells
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spindle
telophase
tetrad
translocation
tumor suppressors
zygote
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