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Anu Singh-Cundy • Gary Shin
Discover Biology
SIXTH EDITION
CHAPTER 7
Cell Division
© 2015 W. W. Norton & Company, Inc.
CHAPTER 7
Cell Division, Part 1
OLYMPIC-CLASS ALGAL BLOOM
7.1 Why Cells Divide
Many bacteria use binary fission for asexual reproduction
Eukaryotes use mitosis to generate identical daughter cells
Meiosis is necessary for sexual reproduction
Cell divisions grow, maintain, and reproduce the human body
7.2 The Cell Cycle
DNA is replicated in the S phase
Most cell types in the adult body do not divide
The cell cycle is tightly regulated
7.3 The Chromosomal Organization of Genetic Material
A karyotype displays all the chromosomes in a nucleus
Most human cells have two copies of each type of chromosome
CHAPTER 7
Cell Division, Part 2
7.4 Mitosis and Cytokinesis: From One Cell to Two Identical Cells
Chromosomes are compacted during early prophase
Chromosomes are attached to the spindle in late prophase
Chromosomes line up in the middle of the cell during metaphase
Chromatids separate during anaphase
New nuclei form during telophase
The cytoplasm is divided during cytokinesis
7.5 Meiosis: Halving the Chromosome Set to Make Gametes
Gametes contain half the chromosomes found in somatic cells
Meiosis I is the reduction division
Meiosis II segregates sister chromatids into separate daughter cells
Meiosis and fertilization contribute to genetic variation in a population
Crossing-over shuffles alleles
The independent assortment of homologous pairs generates diverse gametes
BIOLOGY MATTERS: PROGRAMMED CELL DEATH
APPLYING WHAT WE LEARNED: THE GREAT DIVIDE
Olympic-Class Algal Bloom
•
•
Algae are photosynthetic eukaryotes that divide rapidly when nutrients are abundant.
Large mats of floating algae are capable of stopping boats and entangling wildlife.
The Capacity to Reproduce through
Cell Division Is a Defining Feature of Life
• The continuity of life depends
on cell division.
• It takes trillions of cell
divisions to turn a fertilized
egg into an adult human.
• Millions of cell divisions take
place in our bodies every day
to replace the cells that have
died.
• Cell division is the generation
of daughter cells from a
parent cell.
Why Cells Divide
•
•
•
•
Cell division is necessary for reproduction in all life forms, and for growth and repair in a
multicellular body.
Asexual reproduction results in offspring that are genetically identical to the parent.
Sexual reproduction combines genetic information from two individuals of opposite
mating types to produce offspring.
Offspring resulting from sexual reproduction are similar, but not identical, to the parents.
Many Bacteria Use Binary Fission for
Asexual Reproduction
• Binary fission begins with replication of
the genetic material.
• Each of the two copies of DNA are
segregated to opposite sides of the
parent cell.
• A partition develops to separate the
cytoplasm, with each side containing a
single copy of the DNA.
• The resulting two daughter cells replace
the single parent cell and are
genetically identical to each other and
to the original parent cell.
Eukaryotes Use Mitosis to
Produce Identical
Daughter Cells
• Mitotic division is the process in
eukaryotes that generates two
genetically identical daughter cells
from a single parent cell.
• Mitotic cell division involves the
even distribution of replicated DNA
into new daughter cells.
• During a mitotic division, the
nucleus divides in a process called
mitosis, followed by a division of
the cytoplasm called cytokinesis.
Meiosis Is Necessary for Sexual Reproduction
•
Meiosis is a specialized form of cell
division that is needed to make
gametes (sex cells).
•
Meiosis in female animals results in
gametes that will mature into eggs.
•
Meiosis in male animals produces
gametes called sperm.
•
Meiosis reduces the genetic
information passed to the daughter
cells by half (n).
•
The fusion of information from the
egg (n) and information from the
sperm (n) restores a full set of genetic
information (2n) in the zygote
(fertilized egg).
Fertilization Is the Merging of a Male and
Female Gamete to Produce a Zygote
•
A gamete has half the amount of genetic information as a somatic cell (nonsex cell in the
body).
•
A somatic cell contains a diploid set of genetic information (2n), whereas a gamete
contains a haploid set (n).
•
The zygote receives one haploid set from each gamete (n + n).
•
The resulting embryo divides mitotically as it develops.
Cell Divisions Grow, Maintain, and
Reproduce the Human Body
•
Cell differentiation allows the daughter
cells to become specialized for a specific
set of functions.
•
All of the genetic material present in the
zygote remains present in the somatic
cells of the adult body, regardless of
differentiation.
•
A small group of gamete-producing cells,
called germ line cells, are reserved early
in embryonic development for eventually
producing gametes in the individual.
•
Adult stem cells are unspecialized cells
that can divide to grow, regenerate, and
repair tissues throughout our life span.
In Humans, Meiosis Resumes (Females) or
Begins (Males) at Puberty
• In a female, meiosis begins while she’s still a fetus: about a million
gametocytes (diploid precursor cells) begin meiosis, but then the
process is put on hold until puberty! So, baby girls are born with
about a million gametocytes that are arrested in an early stage of
meiosis.
• Typically, one arrested gametocyte resumes meiosis each month in
one of the ovaries of a fertile female once she reaches puberty; one
resulting daughter cell differentiates into an egg cell (ova).
• In males, meiosis begins at puberty, when hormones signal millions
of gametocytes, in both testes, to begin producing haploid cells that
differentiate into sperm.
• Male meiosis occurs daily and continues well into old age, but in
women the supply of functioning gametocytes dwindles and usually
disappears by age 50.
The Cell Cycle Is Divided into Two Main
Stages: Interphase and Cell Division
The cell cycle can be
completed in as little as
90 minutes in yeast
cells. It takes about 24
hours in most human
cells.
• The cell cycle refers to the life cycle of a eukaryotic cell from its origin to its
division by mitosis or meiosis.
• Cell division includes:
– Nuclear division, involving separation of chromosomes (mitosis)
– Cytoplasmic division (cytokinesis), creating two daughter cells
During Interphase, the Cell Grows
and May Prepare to Divide
Interphase is divided
into three phases:
G1, S, and G2
•
During interphase, the cell:
– Takes in nutrients and manufactures proteins and other substances
– Grows
– Conducts its specialized functions
– Prepares to divide, including replicating the genetic material
DNA Is Replicated in S Phase
• The cell’s size and protein content increase during G1 and G2 phases, and checks are
made to assess whether it is safe to progress to the next stage of the cell cycle.
• The replication of DNA occurs in the S phase (“S” for synthesis of DNA).
The G0 State Represents a “Safe Haven”
Against Progression to Cancer
• Many terminally differentiated cells
enter a nondividing phase called G0.
• The G0 phase can last from a few days to
the lifetime of the organism.
• Liver cells stay in G0 phase much of the
time but divide once a year on average.
• Neurons stay in G0 phase for their entire
life span.
• Many of the genes needed for DNA
replication and cell division are absent in
cells in the G0 phase, so these cells are
resistant to entering and advancing
through the cell cycle.
The Cell Cycle Is Tightly Highly Regulated
• Cell cycle checkpoints ensure
that the cell cycle does not
progress if conditions are not
suitable for cell division.
• The checkpoints are controlled
by cell cycle regulatory
proteins.
• Internal and external signals
can prompt the cell cycle
regulatory proteins to advance
the cell cycle.
• However, cell cycle regulatory
proteins can stop or pause the
cell cycle if conditions are not
favorable for cell division.
Genetic Material Is Organized into Chromosomes,
Each Chromosome Containing a Single DNA Molecule
• Each DNA double helix is
packaged with special
proteins to form long strands
of chromatin.
• Chromatin is further
compacted to form
chromosomes.
At the End of S Phase, the Replicated DNA
Helices Remain Attached as Sister Chromatids
• DNA is replicated during S phase, resulting in two identical double helices
known as sister chromatids.
• The identical sister chromatids are held together at a region called the
centromere.
The Karyotype Describes All
the Chromosomes in a Nucleus
• Every species has its own
characteristic number of
chromosomes.
• The display of all the chromosomes
in a somatic cell is called the
karyotype.
• Karyotypes can be seen more easily
during mitosis, when chromosomes
are highly compacted.
• The number of chromosomes found
in any given organism does not
indicate the total number of genes,
nor does it reflect the complexity of
the organism.
Somatic Cells Have Two Copies of Each
Type of Chromosome
• Eukaryotes carry two copies of each type
of chromosome; together the two copies
make up a pair of homologous
chromosomes.
• Humans have 46 chromosomes divided
into 23 homologous pairs.
• One set of chromosomes, called sex
chromosomes, determines the sex of an
individual among mammals.
Mitosis and Cytokinesis: From One Cell to
Two Identical Cells
• Mitosis can be divided into four main phases:
– Prophase
– Metaphase
– Anaphase
– Telophase
• The main role of mitosis is to separate sister chromatids and distribute
one of each chromosome into each of the daughter cells.
Chromosomes Condense and the Mitotic
Spindle Forms in Early Prophase
The Nuclear Envelope
Disassembles
and the Mitotic
Spindle Captures
the Replicated
Chromosomes
by the End of Late
Prophase
During Prophase, the Cell Prepares for the
Segregation of Chromosomes
•
•
•
•
•
The chromatin becomes highly compacted,
which reduces the risk of tangling and breakage
at later steps.
Two cytoskeletal structures called centrosomes
begin to move toward opposite ends of the
cell.
Special proteins form long cylinders, called
microtubules, that assemble into the mitotic
spindle, which is anchored to a centrosome at
its two ends.
The nuclear envelope breaks down, enabling
spindle microtubules to capture all the
replicated chromosomes: the spindle
microtubules attach to patches of protein
(kinetochores) on each side of the centromere.
The spindle will guide sister chromatids to the
metaphase plate, and later to the opposite
ends of the cell, as the later phases of mitosis
unfold.
Chromosomes Are Positioned at the
Middle of the Cell during Metaphase
• During metaphase, all of the
replicated chromosomes are
arranged in a plane that
typically lies at the center of
the cell.
• The function of metaphase is
to align the chromosomes
precisely in order to facilitate
equal distribution of
chromosomes to each end of
the cell.
Chromatids Separate during Anaphase
• During anaphase, sister
chromatids are separated
and pulled to opposite ends
of the cell by the progressive
shortening of the spindle
microtubules.
• Once separated, each
chromatid is considered a
new chromosome.
New Nuclei Form during Telophase
• During telophase, the mitotic
spindle breaks down, and
nuclear envelopes begin to
form around the two new
sets of chromosomes at each
end of the cell.
• Within each nucleus, the
chromosomes decondense,
as each new daughter cell
begins its G1 phase.
The Cytoplasm Is Divided during Cytokinesis
• Cytokinesis is the process of
dividing the parent cell’s
cytoplasm to produce two
daughter cells.
• In animal cells, actin
microfilaments form at the
metaphase plate; as the actin
filaments contract, they draw
the plasma membrane in until
the original cytoplasm is
pinched into two new cells,
the daughter cells.
• Cytokinesis is the last step in
the cell cycle.
Cell Division in Plants
Meiosis Is Necessary for
Gamete Formation
• In sexual reproduction, gametes of the
opposite type (egg and sperm) fuse during
fertilization to produce a zygote.
• Each gamete is haploid (n): it contains half the
diploid set of chromosomes (2n).
• The zygote formed by the two haploid
gametes contains one paternal homologue
and one maternal homologue for each pair of
homologous chromosomes.
• Meiosis is a special type of cell division: its
function is to generate haploid cells from a
diploid parent cell by splitting up every
homologous pair and sorting its members into
different gametes.
The Chromosome Set Is Halved During Meiosis
• Meiosis occurs in two stages:
--Meiosis I sorts each member
of a homologous pair into two
different daughter cells,
reducing the chromosome sets
from 2n to n.
--Meiosis II separates sister
chromatids in each cell
produced by meiosis I into two
different daughter cells.
•
•
Meiosis produces four daughter cells from the division of each diploid precursor cell
(gametocyte).
In animals, the daughter cells differentiate into gametes (egg or sperm).
Meiosis I is a Reduction Division: It Sorts the
Two Members of Each Pair of Homologous
Chromosome into Different Daughter Cells
•
Paternal and maternal partners of each homologous chromosome pair align themselves next
to each other during prophase of meiosis I (prophase I).
•
A tetrad consists of one replicated maternal chromosome aligned parallel to its matching
replicated paternal chromosomes, for a total of four chromatids.
Crossing-Over in Prophase of Meiosis I Shuffles
the Genes on the Homologous Pairs Before They
Are Inherited by the Daughter Cells
• During prophase I, DNA
segments are swapped
between nonsister
chromatids in a process
called crossing-over.
• Crossing-over is a random
process: the number and the
location of a crossover can
vary from one meiosis to
another.
• Meiosis I continues through
prophase I and into
metaphase I, anaphase I, and
telophase I.
Meiosis II Segregates Sister Chromatids
into Separate Daughter Cells
• Meiosis II cell division is almost exactly like mitosis.
• The two haploid cells produced by meiosis I give rise to a total of four haploid
cells.
• The reduction in chromosome numbers achieved through meiosis I offsets
the combining of chromosomes when gametes fuse during fertilization.
Meiosis and Fertilization Contribute
to Genetic Variation in a Population
• Genetic variation is the raw material for evolution.
• Mutations are the ultimate source of genetic
variation in all types of organisms.
• Different variations of a particular gene created
through mutations are called alleles.
• Through meiosis and fertilization, sexual
reproduction produces offspring with new
combinations of alleles, adding to the genetic
diversity in a population.
Multiple embryos inside a
shark “egg case.” Just one fetal
shark will survive after
cannibalizing its siblings.
Crossing-Over during Meiosis I Generates
Gametes with Recombinant Chromosomes
•
Meiosis magnifies diversity by
shuffling alleles between
homologous pairs and then sorting
these scrambled homologues
randomly into gametes.
•
Crossing-over is the physical
exchange of chromosomal
segments between nonsister
chromatids in paired-off paternal
and maternal homologues.
•
Crossing-over exchanges alleles
between the paternal and
maternal chromatids, creating new
groupings of alleles through the
exchange of DNA segments in a
process known as genetic
recombination.
The Independent Assortment of Homologous Pairs
Generates Diverse Gametes
• Independent
assortment is the
random distribution
of the different
homologous
chromosome pairs
into daughter cells
during meiosis I.
• Independent assortment of chromosomes produces gametes that are different
from the parent and from each other.
• The random events in meiosis, and the random fertilization of a certain egg by a
particular sperm, contribute to the genetic uniqueness of offspring.
BIOLOGY MATTERS
PROGRAMMED CELL DEATH: GOING OUT IN STYLE
• Some cells are killed off neatly
and systematically when they
are no longer needed or to help
sculpt tissues during
development.
Characteristics of death by apoptosis (main type of programmed cell
death in human body):
• No spillage of cytoplasm, no inflammation
• Mitochondria degrade
• Proteins degraded
• Fragmentation of DNA
• Cell shrinks; eventually engulfed and digested inside immune cells
APPLYING WHAT WE LEARNED:
THE GREAT DIVIDE
• Nutrients help cells divide rapidly, as seen in the bloom of “sea
lettuce” that threatened to halt the 2008 Olympics.
• Eutrophication refers to the overfertilization of an area,
typically due to agricultural runoff.
• Eutrophication can lead to large algal blooms.
List of Key Terms: Chapter 7
anaphase (p. 160)
asexual reproduction (p. 150)
binary fission (p. 151)
cancer cell (p. 156)
cell cycle (p. 153)
cell differentiation (p. 153)
cell division (p. 153)
cell plate (p. 161)
centromere (p. 157)
centrosome (p. 160)
chromatin (p. 156)
chromosome (p. 156)
crossing-over (p. 166)
cytokinesis (p. 152)
diploid (p. 162)
embryo (p. 152)
fertilization (p. 152)
G0 phase (p. 154)
G1 phase (p. 154)
G2 phase (p. 154)
gamete (p. 152)
genetic recombination (p. 166)
germ line cell (p. 153)
haploid (p. 162)
homologous chromosome (p. 158)
independent assortment of
chromosomes (p. 167)
interphase (p. 153)
karyotype (p. 157)
maternal homologue (p. 163)
meiosis (p. 152)
meiosis I (p. 163)
meiosis II (p. 163)
metaphase (p. 160)
mitosis (p. 152)
mitotic division (p. 152)
mitotic spindle (p. 160)
paternal homologue (p. 163)
prophase (p. 160)
S phase (p. 154)
sex chromosome (p. 158)
sexual reproduction (p. 150)
sister chromatids (p. 157)
somatic cell (p. 152)
telophase (p. 161)
tetrad (p. 164)
tumor (p. 156)
zygote (p. 152)
Class Quiz, Part 1
Which of the following is not true about
interphase?
A. The cell grows larger during interphase.
B. Chromosomes are replicated during
interphase.
C. A cell can go from G1 to S but not from S to G1.
D. Chromosomes are segregated to daughter
cells during interphase.
Class Quiz, Part 2
The HeLa cells in the photo are undergoing mitosis in a lab dish. DNA is stained pink, and the
mitotic spindle is yellow. (The multiple pink dots within the nuclei are regions called nucleoli
where ribosomal RNA is made.)
The cell labeled A is at which stage
of mitosis?
A. prophase
B. metaphase
C. anaphase
D. telophase
Class Quiz, Part 3
Which of the following is true about
homologous chromosomes?
A. You received both homologues in each pair of
homologous chromosomes from your mother.
B. The two homologues in each pair are sorted into
different daughter cells at the end of meiosis I.
C. The two daughter cells produced by mitosis have
only one member of each homologous pair.
D. Human skin cells have 46 pairs of homologous
chromosomes.
Class Quiz, Part 4
Meiosis ensures that
A. all gamete receives the same alleles.
B. chromosome number is doubled in the
gametes.
C. gametes have a haploid number of
chromosomes.
D. all paternal chromosomes end up in the same
gamete.
Class Quiz, Part 5
Compare mitotic cell divisions with meiosis by filling out the table below. The
response to the first item is already filled in, by way of example.
1. IN HUMANS, THE CELL UNDERGOING THIS TYPE OF DIVISION IS DIPLOID.
2. A TOTAL OF FOUR DAUGHTER CELLS ARE PRODUCED WHEN ONE PARENT CELL
UNDERGOES THIS TYPE OF DIVISION.
3. OUR SKIN MAKES MORE SKIN CELLS USING THIS TYPE OF CELL DIVISION.
4. THE DAUGHTER CELLS ARE GENETICALLY IDENTICAL TO THE PARENT CELL IN THIS
TYPE OF DIVISION.
5. THIS TYPE OF DIVISION INVOLVES TWO NUCLEAR DIVISIONS.
6. THIS TYPE OF DIVISION INVOLVES TWO ROUNDS OF CYTOKINESIS.
7. MATERNAL AND PATERNAL HOMOLOGUES PAIR UP TO FORM TETRADS AT SOME
POINT DURING THIS TYPE OF DIVISION.
8. SISTER CHROMATIDS SEPARATE FROM EACH OTHER AT SOME POINT DURING THIS
TYPE OF CELL DIVISION.
MITOTIC
CELL DIVISION
true
MEIOSIS
true
Relevant Art from Other
Chapters
All art files from the book are available in
JPEG and PPT formats online
Cell Division in Prokaryotes
Structure of a Prokaryote
Asexual and Sexual
Reproduction in
Eukaryotes
Crossing-Over Causes Offspring to Have a Genotype
That Differs from the Genotype of Either Parent
• Crossing-over is a reciprocal exchange of segments of nonsister chromatids in
prophase I of meiosis, which produces chromosomes with new combinations of
alleles.
The Sorting of One
Pair of Homologous
Chromosomes Is
Independent of the
Sorting of Any Other
Pair of Homologous
Chromosomes
• Which homologue of each pair
goes to which pole is essentially
random.
7.1 Concept Check, Part 1
1. In what way is binary fission similar to
mitotic cell division? Give one difference.
ANSWER: Both produce genetically identical daughter
cells, but only prokaryotes undergo binary fission.
7.1 Concept Check, Part 2
2. What is the function of mitosis?
ANSWER: Mitosis is necessary for asexual
reproduction in both single-celled and multicellular
eukaryotes, and for growth and repair of the
multicellular body.
7.2 Concept Check, Part 1
1. When in the cell cycle does copying
(replication) of DNA take place?
ANSWER: In the S (synthesis) phase of interphase.
7.2 Concept Check, Part 2
2. What is the significance of the G₀ phase?
ANSWER: A cell that leaves the cell cycle and stops
dividing is in the G₀ phase, either temporarily or
permanently.
7.3 Concept Check, Part 1
1. The house cat karyotype displays a total of 38 chromosomes.
How many distinct DNA molecules are present in a cat skin cell
toward the end of (a) G₀ phase; (b) G₁ phase; (c) S phase; (d) G₂
phase?
ANSWER: (a) 38
(b) 38
(c) 76
(d) 76
7.3 Concept Check, Part 2
2. How is the karyotype of men different from
that of women?
ANSWER: All humans have 22 pairs of autosomes and
a pair of sex chromosomes. Women have a pair of
identical sex chromosomes called X chromosomes.
Men have two dissimilar sex chromosomes: one X and
one Y.
7.4 Concept Check, Part 1
1. What are sister chromatids? When during mitosis do
they separate to become chromosomes in their own
right?
ANSWER: Sister chromatids are two identical DNA
molecules, replicated from a single parental
chromosome, that remain attached during mitosis until
anaphase, when they separate to become two
independent chromosomes, each headed to a different
daughter cell.
7.4 Concept Check, Part 2
2. What is the difference between mitosis and
cytokinesis?
ANSWER: Mitosis is nuclear division; cytokinesis is the
partitioning of the cytoplasm, and ultimately the whole
parent cell, into two daughter cells.
7.5 Concept Check, Part 1
1. Meiosis I reduces the diploid chromosome set.
What role does meiosis II play in gamete
formation?
ANSWER: Meiosis II separates the two sister
chromatids of each replicated chromosome, to form
gametes with 23 chromosomes each.
7.5 Concept Check, Part 2
2. How does meiosis contribute to genetic
variation in a population?
ANSWER: Meiosis generates genetically diverse
gametes through two different mechanisms:
recombination of alleles via crossing-over, and random
assortment of homologous chromosomes.