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Chapter 8
Cellular Reproduction:
Cells from Cells
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
© 2010 Pearson Education, Inc.
WHAT CELL REPRODUCTION
ACCOMPLISHES
• Reproduction:
–
May result in the birth of new organisms
–
More commonly involves the production of new cells
• Before a parent cell splits into two, it duplicates its
chromosomes, the structures that contain most of the organism’s
DNA.
• During cell division, each daughter cell receives one set of
chromosomes.
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• Cell division:
– Replaces damaged or lost cells
– Permits growth
– Allows for reproduction
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Human kidney cell
LM
Colorized TEM
FUNCTIONS OF CELL DIVISION
Cell Replacement
Growth via Cell Division
Early human embryo
Figure 8.1a
LM
Asexual Reproduction
Figure 8.1ba
• In asexual reproduction:
– Single-celled organisms reproduce by simple cell division
– There is no fertilization of an egg by a sperm
• In asexual reproduction, the lone parent and its offspring have
identical genes.
• Mitosis is the type of cell division responsible for:
– Asexual reproduction
– Growth and maintenance of multicellular organisms
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Asexual Reproduction
Figure 8.1bb
Asexual Reproduction
Figure 8.1bc
• Sexual reproduction requires fertilization of an egg by a sperm
using a special type of cell division called meiosis.
• Thus, sexually reproducing organisms use Meiosis for
reproduction and Mitosis for growth and maintenance
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THE CELL CYCLE AND MITOSIS
• In a eukaryotic cell:
– Most genes are located on chromosomes in the cell nucleus
– A few genes are found in DNA in mitochondria and chloroplasts
• Each eukaryotic chromosome contains one very long DNA
molecule, typically bearing thousands of genes.
• The number of chromosomes in a eukaryotic cell depends on the
species.
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Species
Indian muntjac deer
Koala
Opossum
Giraffe
Mouse
Human
Duck-billed platypus
Buffalo
Dog
Red viscacha rat
Number of chromosomes
in body cells
6
16
22
30
40
46
54
60
78
102
Figure 8.2
• Chromosomes:
– Are made of chromatin, a combination of DNA and protein molecules
– Are not visible in a cell until cell division occurs
• The DNA in a cell is packed into an elaborate, multilevel system
of coiling and folding.
• Histones are proteins used to package DNA in eukaryotes.
• Nucleosomes consist of DNA wound around histone molecules.
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DNA double helix
Histones
TEM
“Beads on
a string”
Nucleosome
Tight helical fiber
Looped domains
TEM
Duplicated chromosomes
(sister chromatids)
Centromere
Figure 8.4
LM
Chromosomes
Figure 8.3
• Before a cell divides, it duplicates all of its chromosomes,
resulting in two copies called sister chromatids.
• Sister chromatids are joined together at a narrow “waist” called
the centromere.
• When the cell divides, the sister chromatids separate from each
other.
• Once separated, each chromatid is:
– Considered a full-fledged chromosome
– Identical to the original chromosome
© 2010 Pearson Education, Inc.
Chromosome
duplication
Sister
chromatids
Chromosome
distribution to
daughter cells
Figure 8.5
The Cell Cycle
• A cell cycle is the orderly sequence of events that extend from the
time a cell is first formed from a dividing parent cell to its own
division into two cells.
• The cell cycle consists of two distinct phases:
– Interphase
– The mitotic phase
• Most of a cell cycle is spent in interphase.
• During interphase, a cell:
– Performs its normal functions
– Doubles everything in its cytoplasm
– Grows in size
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INTERPHASE
Centrosomes
(with centriole pairs)
Chromatin
PROPHASE
Fragments of
Early mitotic
Centrosome nuclear envelope
spindle
Centromere
Spindle
microtubules
LM
Chromosome, consisting
Nuclear Plasma
envelope membrane of two sister chromatids
Figure 8.7.a
• The mitotic (M) phase includes two overlapping processes:
– Mitosis, in which the nucleus and its contents divide evenly into two
daughter nuclei
– Cytokinesis, in which the cytoplasm is divided in two
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S phase (DNA synthesis;
chromosome duplication)
Interphase: metabolism and
growth (90% of time)
G1
Mitotic (M) phase:
cell division
(10% of time)
Cytokinesis
(division of
cytoplasm)
G2
Mitosis
(division of nucleus)
Figure 8.6
Mitosis and Cytokinesis
• During mitosis the mitotic spindle, guides the separation of two
sets of daughter chromosomes.
• Spindle microtubules grow from two centrosomes, clouds of
cytoplasmic material that in animal cells contain centrioles.
• Mitosis consists of four distinct phases:
– (A) Prophase
– (B) Metaphase
– (C) Anaphase
– (D) Telophase
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METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Nuclear
envelope
forming
Spindle
Cleavage
furrow
Daughter
chromosomes
Figure 8.7b
• Cytokinesis typically:
– Occurs during telophase
– Divides the cytoplasm
– Is different in plant and animal cells
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SEM
Cleavage
furrow
Cleavage furrow
Contracting ring of
microfilaments
Daughter cells
Figure 8.8a
Cell plate Daughter
forming nucleus
LM
Wall of
parent cell
Cell wall
Vesicles containing
Cell plate
cell wall material
New cell wall
Daughter cells
Figure 8.8b
Cancer Cells: Growing Out of Control
• Normal plant and animal cells have a cell cycle control system
that consists of specialized proteins, which send “stop” and “goahead” signals at certain key points during the cell cycle.
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What Is Cancer?
• Cancer is a disease of the cell cycle.
• Cancer cells do not respond normally to the cell cycle control
system.
• Cancer cells can form tumors, abnormally growing masses of
body cells.
• The spread of cancer cells beyond their original site of origin is
metastasis.
• Malignant tumors can:
– Spread to other parts of the body
– Interrupt normal body functions
• A person with a malignant tumor is said to have cancer.
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Lymph
vessels
Tumor
Blood
vessel
Glandular
tissue
A tumor grows
from a single
cancer cell.
Cancer cells invade
neighboring tissue.
Metastasis: Cancer
cells spread through
lymph and blood
vessels to other parts
of the body.
Figure 8.9
Cancer Treatment
• Cancer treatment can involve:
– Radiation therapy, which damages DNA and disrupts cell division
– Chemotherapy, which uses drugs that disrupt cell division
• Certain behaviors can decrease the risk of cancer:
– Not smoking
– Exercising adequately
– Avoiding exposure to the sun
– Eating a high-fiber, low-fat diet
– Performing self-exams
– Regularly visiting a doctor to identify tumors early
© 2010 Pearson Education, Inc.
Meiosis, the Basis of Sexual Reproduction
• Sexual reproduction:
– Uses meiosis
– Uses fertilization
– Produces offspring that contain a unique combination of genes from the
parents
© 2010 Pearson Education, Inc.
Figure 8.10
Homologous Chromosomes
• Different individuals of a single species have the same number
and types of chromosomes.
• A human somatic cell:
– Is a typical body cell
– Has 46 chromosomes
• A karyotype is an image that reveals an orderly arrangement of
chromosomes.
• Homologous chromosomes are matching pairs of chromosomes
that can possess different versions of the same genes.
© 2010 Pearson Education, Inc.
LM
Pair of homologous
chromosomes
Centromere
Sister
chromatids
One duplicated
chromosome
Figure 8.11
• Humans have:
– Two different sex chromosomes, X and Y
– Twenty-two pairs of matching chromosomes, called autosomes
© 2010 Pearson Education, Inc.
Gametes and the Life Cycle of a Sexual Organism
• The life cycle of a multicellular organism is the sequence of
stages leading from the adults of one generation to the adults of
the next.
• Humans are diploid organisms in which:
– Their somatic cells contain two sets of chromosomes
– Their gametes are haploid, having only one set of chromosomes
• In humans, a haploid sperm fuses with a haploid egg during
fertilization to form a diploid zygote.
• Sexual life cycles involve an alternation of diploid and haploid
stages.
• Meiosis produces haploid gametes, which keeps the chromosome
number from doubling every generation.
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Haploid gametes (n  23)
Egg cell
n
n
Sperm cell
FERTILIZATION
MEIOSIS
Multicellular
diploid adults
(2n  46)
2n
MITOSIS
and development
Diploid
zygote
(2n  46)
Key
Haploid (n)
Diploid (2n)
Figure 8.12
Chromosomes
duplicate.
Homologous
chromosomes
separate.
Sister
chromatids
separate.
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair of
homologous
chromosomes
INTERPHASE BEFORE MEIOSIS
Sister
chromatids
MEIOSIS I
MEIOSIS II
Figure 8.13-3
The Process of Meiosis
• In meiosis:
–
Haploid daughter cells are produced in diploid organisms
–
Interphase is followed by two consecutive divisions, meiosis I and
meiosis II
–
Synapsis and Crossing over occurs
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MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
Chromatin
Chromosomes
duplicate.
PROPHASE I
Sites of
crossing over
Spindle
Sister
chromatids
Pair of
homologous
chromosomes
Homologous
chromosomes
pair up and
exchange
segments.
METAPHASE I
Microtubules
attached
to chromosome
ANAPHASE I
Sister chromatids
remain attached
Centromere
Pairs of
homologous
chromosomes
line up.
Pairs of
homologous
chromosomes
split up.
Figure 8.14a
MEIOSIS II: SISTER CHROMATIDS SEPARATE
TELOPHASE I
AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II TELOPHASE II
AND
CYTOKINESIS
Cleavage
furrow
Sister
chromatids
separate
Two haploid
cells form;
chromosomes
are still
doubled.
Haploid
daughter
cells forming
During another round of cell division, the sister
chromatids finally separate; four haploid
daughter cells result, containing single
chromosomes.
Figure 8.14b
LM
Figure 8.14bc
Review: Comparing Mitosis and Meiosis
• In mitosis and meiosis, the chromosomes duplicate only once,
during the preceding interphase.
• The number of cell divisions varies:
– Mitosis uses one division and produces two diploid cells
– Meiosis uses two divisions and produces four haploid cells
• All the events unique to meiosis occur during meiosis I.
© 2010 Pearson Education, Inc.
MEIOSIS
MITOSIS
Prophase I
Prophase
Chromosome
duplication
Duplicated chromosome
(two sister chromatids)
MEIOSIS I
Chromosome
duplication
Parent cell
(before chromosome duplication)
2n  4
Homologous
chromosomes come
together in pairs.
Site of crossing over
between homologous
(nonsister) chromatids
Metaphase I
Metaphase
Homologous pairs
align at the middle
of the cell.
Chromosomes
align at the
middle of the
cell.
Anaphase I
Telophase I
Anaphase
Telophase
2n
Sister
chromatids
separate
during
anaphase.
Daughter cells
of mitosis
2n
Homologous
chromosomes
separate during
anaphase I;
sister chromatids
remain together.
Chromosome with two
sister chromatids
Haploid
n2
Daughter
cells of meiosis I
MEIOSIS II
Sister chromatids
separate during
anaphase II.
n
n
n
Daughter cells of meiosis II
n
Figure 8.15
The Origins of Genetic Variation
• Offspring of sexual reproduction are genetically different from
their parents and one another.
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Independent Assortment of Chromosomes
• When aligned during metaphase I of meiosis, the side-by-side
orientation of each homologous pair of chromosomes is a matter
of chance.
• Every chromosome pair orients independently of the others
during meiosis.
• For any species the total number of chromosome combinations
that can appear in the gametes due to independent assortment is:
– 2n where n is the haploid number.
• For a human:
– n = 23
– 223 = 8,388,608 different chromosome combinations possible in a gamete
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POSSIBILITY 2
POSSIBILITY 1
Metaphase of
meiosis I
Metaphase
of meiosis II
Gametes
Combination a Combination b
Combination c Combination d
Figure 8.16-3
Random Fertilization
• A human egg cell is fertilized randomly by one sperm, leading to
genetic variety in the zygote.
• If each gamete represents one of 8,388,608 different chromosome
combinations, at fertilization, humans would have 8,388,608 ×
8,388,608, or more than 70 trillion, different possible
chromosome combinations.
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Figure 8.17
Crossing Over
• In crossing over:
– Homologous chromosomes exchange genetic information
– Genetic recombination, the production of gene combinations different
from those carried by parental chromosomes, occurs
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Prophase I
of meiosis
Homologous chromatids
exchange corresponding
segments.
Duplicated pair of
homologous
chromosomes
Chiasma, site of
crossing over
Metaphase I
Sister chromatids
remain joined at their
centromeres.
Spindle
microtubule
Metaphase II
Gametes
Recombinant
chromosomes
combine genetic
information from
different parents. Recombinant chromosomes
Figure 8.18-5
When Meiosis Goes Awry
• What happens when errors occur in meiosis?
• Such mistakes can result in genetic abnormalities that range from
mild to fatal.
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How Accidents during Meiosis Can Alter
Chromosome Number
• In nondisjunction, the members of a chromosome pair fail to
separate during anaphase, producing gametes with an incorrect
number of chromosomes.
• Nondisjunction can occur during meiosis I or II.
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NONDISJUNCTION IN MEIOSIS II
NONDISJUNCTION IN MEIOSIS I
Meiosis I
Nondisjunction:
Pair of homologous
chromosomes fails
to separate.
Meiosis II
Nondisjunction:
Pair of sister
chromatids
fails to separate.
Gametes
n1
n1
n–1
Abnormal gametes
Number of
n – 1 chromosomes
n1
n–1
Abnormal gametes
n
n
Normal gametes
Figure 8.20-3
Abnormal egg
cell with extra
chromosome
n1
Normal
sperm cell
n (normal)
Abnormal zygote
with extra
chromosome 2n  1
Figure 8.21
Down Syndrome: An Extra Chromosome 21
• Down Syndrome:
– Is also called trisomy 21
– Is a condition in which an individual has an extra chromosome 21
– Affects about one out of every 700 children
– The incidence of Down Syndrome increases with the age of the mother.
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LM
Chromosome 21
Figure 8.22
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
25
30
35
40
45
50
Age of mother
Figure 8.23
Abnormal Numbers of Sex Chromosomes
• Nondisjunction can also affect the sex chromosomes.
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Evolution Connection:
The Advantages of Sex
• Asexual reproduction conveys an evolutionary advantage when
plants are:
– Sparsely distributed
– Superbly suited to a stable environment
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Figure 8.24
• Sexual reproduction may convey an evolutionary advantage by:
– Speeding adaptation to a changing environment
– Allowing a population to more easily rid itself of harmful genes
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