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Chapter 8
The Cellular Basis of Reproduction
and Inheritance
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Introduction
 Cancer cells
– start out as normal body cells,
– undergo genetic mutations,
– lose the ability to control the tempo of their own division,
and
– run amok, causing disease.
© 2012 Pearson Education, Inc.
Introduction
 In a healthy body, cell division allows for
– growth,
– the replacement of damaged cells, and
– development from an embryo into an adult.
 In sexually reproducing organisms, eggs and sperm
result from
– mitosis and
– meiosis.
© 2012 Pearson Education, Inc.
Figure 8.0_2
Chapter 8: Big Ideas
Cell Division and
Reproduction
Meiosis and
Crossing Over
The Eukaryotic Cell
Cycle and Mitosis
Alterations of Chromosome
Number and Structure
Figure 8.0_3
CELL DIVISION AND
REPRODUCTION
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8.1 Cell division plays many important roles in
the lives of organisms
 Organisms reproduce their own kind, a key
characteristic of life.
 Cell division
– is reproduction at the cellular level,
– requires the duplication of chromosomes, and
– sorts new sets of chromosomes into the resulting pair
of daughter cells.
© 2012 Pearson Education, Inc.
8.1 Cell division plays many important roles in
the lives of organisms
 Cell division is used
– for reproduction of single-celled organisms,
– growth of multicellular organisms from a fertilized egg
into an adult,
– repair and replacement of cells, and
– sperm and egg production.
© 2012 Pearson Education, Inc.
8.1 Cell division plays many important roles in
the lives of organisms
 Living organisms reproduce by two methods.
– Asexual reproduction
– produces offspring that are identical to the original cell or
organism and
– involves inheritance of all genes from one parent.
– Sexual reproduction
– produces offspring that are similar to the parents, but show
variations in traits and
– involves inheritance of unique sets of genes from two parents.
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THE EUKARYOTIC CELL
CYCLE AND MITOSIS
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8.3 The large, complex chromosomes of
eukaryotes duplicate with each cell division
 Eukaryotic cells
– are more complex and larger than prokaryotic cells,
– have more genes, and
– store most of their genes on multiple chromosomes within
the nucleus.
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8.3 The large, complex chromosomes of
eukaryotes duplicate with each cell division
 Eukaryotic chromosomes are composed of
chromatin consisting of
– one long DNA molecule and
– proteins that help maintain the chromosome structure and
control the activity of its genes.
 To prepare for division, the chromatin becomes
– highly compact and
– visible with a microscope.
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Figure 8.3A
Figure 8.3B
Chromosomes
DNA molecules
Sister
chromatids
Chromosome
duplication
Centromere
Sister
chromatids
Chromosome
distribution
to the
daughter
cells
8.3 The large, complex chromosomes of
eukaryotes duplicate with each cell division
 Before a eukaryotic cell begins to divide, it
duplicates all of its chromosomes, resulting in
– two copies called sister chromatids
– joined together by a narrowed “waist” called the
centromere.
 When a cell divides, the sister chromatids
– separate from each other, now called chromosomes, and
– sort into separate daughter cells.
© 2012 Pearson Education, Inc.
Figure 8.3B_1
Chromosomes
DNA molecules
Chromosome
duplication
Centromere
Sister
chromatids
Chromosome
distribution
to the
daughter
cells
8.4 The cell cycle multiplies cells
 The cell cycle is an ordered sequence of events
that extends
– from the time a cell is first formed from a dividing
parent cell
– until its own division.
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8.4 The cell cycle multiplies cells
 The cell cycle consists of two stages,
characterized as follows:
1. Interphase: duplication of cell contents
– G1—growth, increase in cytoplasm
– S—duplication of chromosomes
– G2—growth, preparation for division
2. Mitotic phase: division
– Mitosis—division of the nucleus
– Cytokinesis—division of cytoplasm
© 2012 Pearson Education, Inc.
Figure 8.4
G1
(first gap)
S
(DNA synthesis)
M
G2
(second gap)
8.5 Cell division is a continuum of dynamic
changes
 Mitosis progresses through a series of stages:
– prophase,
– prometaphase,
– metaphase,
– anaphase, and
– telophase.
 Cytokinesis often overlaps telophase.
© 2012 Pearson Education, Inc.
Figure 8.5_1
INTERPHASE
Centrosomes
(with centriole pairs)
Centrioles
Nuclear
envelope
Chromatin
Plasma
membrane
MITOSIS
Prophase
Prometaphase
Early mitotic
spindle
Centrosome
Fragments of
the nuclear
envelope
Kinetochore
Centromere
Chromosome,
consisting of two
sister chromatids
Spindle
microtubules
Figure 8.5_left
MITOSIS
INTERPHASE
Prophase
Centrosomes
(with centriole pairs)
Centrioles
Nuclear
envelope
Early mitotic
spindle
Chromatin
Prometaphase
Centrosome
Fragments of
the nuclear envelope
Kinetochore
Plasma
membrane
Centromere
Chromosome,
consisting of two
sister chromatids
Spindle
microtubules
8.5 Cell division is a continuum of dynamic
changes
 A mitotic spindle is
– required to divide the chromosomes,
– composed of microtubules, and
– produced by centrosomes, structures in the cytoplasm
that
– organize microtubule arrangement and
– contain a pair of centrioles in animal cells.
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8.5 Cell division is a continuum of dynamic
changes
 Interphase
– The cytoplasmic contents double,
– two centrosomes form,
– chromosomes duplicate in the nucleus during the S
phase, and
– nucleoli, sites of ribosome assembly, are visible.
© 2012 Pearson Education, Inc.
Figure 8.5_2
INTERPHASE
8.5 Cell division is a continuum of dynamic
changes
 Prophase
– In the cytoplasm microtubules begin to emerge from
centrosomes, forming the spindle.
– In the nucleus
– chromosomes coil and become compact and
– nucleoli disappear.
© 2012 Pearson Education, Inc.
Figure 8.5_3
Prophase
8.5 Cell division is a continuum of dynamic
changes
 Prometaphase
– Spindle microtubules reach chromosomes, where they
– attach at kinetochores on the centromeres of sister
chromatids and
– move chromosomes to the center of the cell through
associated protein “motors.”
– Other microtubules meet those from the opposite
poles.
– The nuclear envelope disappears.
© 2012 Pearson Education, Inc.
Figure 8.5_4
Prometaphase
Figure 8.5_left
MITOSIS
INTERPHASE
Prophase
Centrosomes
(with centriole pairs)
Centrioles
Nuclear
envelope
Early mitotic
spindle
Chromatin
Prometaphase
Centrosome
Fragments of
the nuclear envelope
Kinetochore
Plasma
membrane
Centromere
Chromosome,
consisting of two
sister chromatids
Spindle
microtubules
Figure 8.5_5
MITOSIS
Anaphase
Metaphase
Metaphase
plate
Mitotic
spindle
Daughter
chromosomes
Telophase and Cytokinesis
Cleavage
furrow
Nuclear
envelope
forming
Figure 8.5_right
MITOSIS
Anaphase
Metaphase
Telophase and Cytokinesis
Metaphase
plate
Cleavage
furrow
Mitotic
spindle
Daughter
chromosomes
Nuclear
envelope
forming
8.5 Cell division is a continuum of dynamic
changes
 Metaphase
– The mitotic spindle is fully formed.
– Chromosomes align at the cell equator.
– Kinetochores of sister chromatids are facing the
opposite poles of the spindle.
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Figure 8.5_6
Metaphase
8.5 Cell division is a continuum of dynamic
changes
 Anaphase
– Sister chromatids separate at the centromeres.
– Daughter chromosomes are moved to opposite poles
of the cell as
– motor proteins move the chromosomes along the spindle
microtubules and
– kinetochore microtubules shorten.
– The cell elongates due to lengthening of
nonkinetochore microtubules.
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Figure 8.5_7
Anaphase
8.5 Cell division is a continuum of dynamic
changes
 Telophase
– The cell continues to elongate.
– The nuclear envelope forms around chromosomes at
each pole, establishing daughter nuclei.
– Chromatin uncoils and nucleoli reappear.
– The spindle disappears.
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Figure 8.5_8
Telophase and Cytokinesis
Figure 8.5_right
MITOSIS
Anaphase
Metaphase
Telophase and Cytokinesis
Metaphase
plate
Cleavage
furrow
Mitotic
spindle
Daughter
chromosomes
Nuclear
envelope
forming
8.5 Cell division is a continuum of dynamic
changes
 During cytokinesis, the cytoplasm is divided into
separate cells.
 The process of cytokinesis differs in animal and
plant cells.
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8.6 Cytokinesis differs for plant and animal cells
 In animal cells, cytokinesis occurs as
1. a cleavage furrow forms from a contracting ring of
microfilaments, interacting with myosin, and
2. the cleavage furrow deepens to separate the contents
into two cells.
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Figure 8.6A
Cytokinesis
Cleavage
furrow
Contracting ring of
microfilaments
Daughter
cells
Cleavage
furrow
8.6 Cytokinesis differs for plant and animal cells
 In plant cells, cytokinesis occurs as
1. a cell plate forms in the middle, from vesicles
containing cell wall material,
2. the cell plate grows outward to reach the edges,
dividing the contents into two cells,
3. each cell now possesses a plasma membrane and cell
wall.
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Figure 8.6B
New
cell wall
Cytokinesis
Cell wall
of the
parent cell
Cell wall
Plasma
membrane
Daughter
nucleus
Cell plate
forming
Vesicles
containing
cell wall
material
Cell plate
Daughter
cells
8.7 Anchorage, cell density, and chemical growth
factors affect cell division
 The cells within an organism’s body divide and
develop at different rates.
 Cell division is controlled by
– the presence of essential nutrients,
– growth factors, proteins that stimulate division,
– density-dependent inhibition, in which crowded cells
stop dividing, and
– anchorage dependence, the need for cells to be in
contact with a solid surface to divide.
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Figure 8.7A
Cultured cells
suspended in liquid
The addition of
growth
factor
Figure 8.7B
Anchorage
Single layer
of cells
Removal
of cells
Restoration
of single
layer by cell
division
8.8 Growth factors signal the cell cycle control
system
 The cell cycle control system is a cycling set of
molecules in the cell that
– triggers and
– coordinates key events in the cell cycle.
 Checkpoints in the cell cycle can
– stop an event or
– signal an event to proceed.
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8.8 Growth factors signal the cell cycle control
system
 There are three major checkpoints in the cell cycle.
1. G1 checkpoint
– allows entry into the S phase or
– causes the cell to leave the cycle, entering a nondividing G0
phase.
2. G2 checkpoint, and
3. M checkpoint.
 Research on the control of the cell cycle is one of
the hottest areas in biology today.
© 2012 Pearson Education, Inc.
Figure 8.8A
G1 checkpoint
G0
G1
S
Control
system
M
G2
M checkpoint
G2 checkpoint
Figure 8.8B
Growth
factor
EXTRACELLULAR FLUID
Plasma membrane
Relay proteins
Receptor
protein
Signal
transduction
pathway
G1
checkpoint
G1
S
Control
system
M
G2
CYTOPLASM
8.9 CONNECTION: Growing out of control,
cancer cells produce malignant tumors
 Cancer currently claims the lives of 20% of the
people in the United States and other
industrialized nations.
 Cancer cells escape controls on the cell cycle.
 Cancer cells
– divide rapidly, often in the absence of growth factors,
– spread to other tissues through the circulatory system,
and
– grow without being inhibited by other cells.
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8.9 CONNECTION: Growing out of control,
cancer cells produce malignant tumors
 A tumor is an abnormally growing mass of body
cells.
– Benign tumors remain at the original site.
– Malignant tumors spread to other locations, called
metastasis.
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Figure 8.9
Lymph
vessels
Blood
vessel
Tumor
Tumor in
another
part of
the body
Glandular
tissue
Growth
Invasion
Metastasis
8.9 CONNECTION: Growing out of control,
cancer cells produce malignant tumors
 Cancers are named according to the organ or
tissue in which they originate.
– Carcinomas arise in external or internal body
coverings.
– Sarcomas arise in supportive and connective tissue.
– Leukemias and lymphomas arise from blood-forming
tissues.
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8.9 CONNECTION: Growing out of control,
cancer cells produce malignant tumors
 Cancer treatments
– Localized tumors can be
– removed surgically and/or
– treated with concentrated beams of high-energy radiation.
– Chemotherapy is used for metastatic tumors.
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8.10 Review: Mitosis provides for growth, cell
replacement, and asexual reproduction
 When the cell cycle operates normally, mitosis
produces genetically identical cells for
– growth,
– replacement of damaged and lost cells, and
– asexual reproduction.
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Figure 8.10A
MEIOSIS AND
CROSSING OVER
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8.11 Chromosomes are matched in homologous
pairs
 In humans, somatic cells have
– 23 pairs of homologous chromosomes and
– one member of each pair from each parent.
 The human sex chromosomes X and Y differ in
size and genetic composition.
 The other 22 pairs of chromosomes are
autosomes with the same size and genetic
composition.
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8.11 Chromosomes are matched in homologous
pairs
 Homologous chromosomes are matched in
– length,
– centromere position, and
– gene locations.
 A locus (plural, loci) is the position of a gene.
 Different versions of a gene may be found at the
same locus on maternal and paternal
chromosomes.
© 2012 Pearson Education, Inc.
Figure 8.11
Pair of homologous
chromosomes
Locus
Centromere
Sister
chromatids
One duplicated
chromosome
8.12 Gametes have a single set of chromosomes
 An organism’s life cycle is the sequence of
stages leading
– from the adults of one generation
– to the adults of the next.
 Humans and many animals and plants are diploid,
with body cells that have
– two sets of chromosomes,
– one from each parent.
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8.12 Gametes have a single set of chromosomes
 Meiosis is a process that converts diploid nuclei to
haploid nuclei.
– Diploid cells have two homologous sets of
chromosomes.
– Haploid cells have one set of chromosomes.
– Meiosis occurs in the sex organs, producing
gametes—sperm and eggs.
 Fertilization is the union of sperm and egg.
 The zygote has a diploid chromosome number,
one set from each parent.
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Figure 8.12A
Haploid gametes (n  23)
n
Egg cell
n
Sperm cell
Meiosis
Ovary
Fertilization
Testis
Diploid
zygote
(2n  46)
2n
Key
Multicellular diploid
adults (2n  46)
Mitosis
Haploid stage (n)
Diploid stage (2n)
8.12 Gametes have a single set of chromosomes
 All sexual life cycles include an alternation
between
– a diploid stage and
– a haploid stage.
 Producing haploid gametes prevents the
chromosome number from doubling in every
generation.
© 2012 Pearson Education, Inc.
Figure 8.12B
MEIOSIS I
INTERPHASE
MEIOSIS II
Sister
chromatids
2
1
A pair of
homologous
chromosomes
in a diploid
parent cell
A pair of
duplicated
homologous
chromosomes
3
8.13 Meiosis reduces the chromosome number
from diploid to haploid
 Meiosis is a type of cell division that produces
haploid gametes in diploid organisms.
 Two haploid gametes combine in fertilization to
restore the diploid state in the zygote.
© 2012 Pearson Education, Inc.
8.13 Meiosis reduces the chromosome number
from diploid to haploid
 Meiosis and mitosis are preceded by the duplication
of chromosomes. However,
– meiosis is followed by two consecutive cell divisions and
– mitosis is followed by only one cell division.
 Because in meiosis, one duplication of
chromosomes is followed by two divisions, each of
the four daughter cells produced has a haploid set of
chromosomes.
© 2012 Pearson Education, Inc.
8.13 Meiosis reduces the chromosome number
from diploid to haploid
 Meiosis I – Prophase I – events occurring in the
nucleus.
– Chromosomes coil and become compact.
– Homologous chromosomes come together as pairs by
synapsis.
– Each pair, with four chromatids, is called a tetrad.
– Nonsister chromatids exchange genetic material by
crossing over.
© 2012 Pearson Education, Inc.
Figure 8.13_left
MEIOSIS I: Homologous chromosomes separate
INTERPHASE:
Chromosomes duplicate
Centrosomes
(with centriole
pairs)
Prophase I
Metaphase I
Sites of crossing over
Spindle microtubules
attached to a kinetochore
Centrioles
Anaphase I
Sister chromatids
remain attached
Spindle
Tetrad
Nuclear
envelope
Chromatin
Sister
chromatids
Fragments
of the
nuclear
envelope
Centromere
(with a
kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
8.13 Meiosis reduces the chromosome number
from diploid to haploid
 Meiosis I – Metaphase I – Tetrads align at the cell
equator.
 Meiosis I – Anaphase I – Homologous pairs
separate and move toward opposite poles of the cell.
© 2012 Pearson Education, Inc.
8.13 Meiosis reduces the chromosome number
from diploid to haploid
 Meiosis I – Telophase I
– Duplicated chromosomes have reached the poles.
– A nuclear envelope re-forms around chromosomes in
some species.
– Each nucleus has the haploid number of
chromosomes.
© 2012 Pearson Education, Inc.
8.13 Meiosis reduces the chromosome number
from diploid to haploid
 Meiosis II follows meiosis I without chromosome
duplication.
 Each of the two haploid products enters meiosis II.
 Meiosis II – Prophase II
– Chromosomes coil and become compact (if uncoiled
after telophase I).
– Nuclear envelope, if re-formed, breaks up again.
© 2012 Pearson Education, Inc.
Figure 8.13_right
MEIOSIS II: Sister chromatids separate
Telophase I and Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II
and Cytokinesis
Cleavage
furrow
Sister chromatids
separate
Haploid daughter
cells forming
Figure 8.13_5
Two lily cells
undergo meiosis II
8.13 Meiosis reduces the chromosome number
from diploid to haploid
 Meiosis II – Metaphase II – Duplicated
chromosomes align at the cell equator.
 Meiosis II – Anaphase II
– Sister chromatids separate and
– chromosomes move toward opposite poles.
© 2012 Pearson Education, Inc.
8.13 Meiosis reduces the chromosome number
from diploid to haploid
 Meiosis II – Telophase II
– Chromosomes have reached the poles of the cell.
– A nuclear envelope forms around each set of
chromosomes.
– With cytokinesis, four haploid cells are produced.
© 2012 Pearson Education, Inc.
8.14 Mitosis and meiosis have important
similarities and differences
 Mitosis and meiosis both
– begin with diploid parent cells that
– have chromosomes duplicated during the previous
interphase.
 However the end products differ.
– Mitosis produces two genetically identical diploid somatic
daughter cells.
– Meiosis produces four genetically unique haploid
gametes.
© 2012 Pearson Education, Inc.
Figure 8.14
MEIOSIS I
MITOSIS
Parent cell
(before chromosome duplication)
Prophase
Duplicated
chromosome
(two sister
chromatids)
Chromosome
duplication
Site of
crossing
over
Prophase I
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
duplication
2n  4
Metaphase I
Metaphase
Chromosomes
align at the
metaphase plate
Tetrads (homologous
pairs) align at the
metaphase plate
Anaphase I
Telophase I
Anaphase
Telophase
Homologous
chromosomes
separate during
anaphase I;
sister
chromatids
remain together
Sister chromatids
separate during
anaphase
Daughter
cells of
meiosis I
MEIOSIS II
2n
2n
Daughter cells of mitosis
No further
chromosomal
duplication;
sister
chromatids
separate during
anaphase II
n
n
n
n
Daughter cells of meiosis II
Haploid
n2
Figure 8.14_1
MEIOSIS I
MITOSIS
Prophase
Parent cell
(before chromosome duplication)
Chromosome
duplication
Prophase I
Site of
crossing
over
Chromosome
duplication
2n  4
Metaphase
Tetrad
Metaphase I
Chromosomes
align at the
metaphase plate
Tetrads (homologous
pairs) align at the
metaphase plate
Figure 8.14_2
MITOSIS
Metaphase
Chromosomes
align at the
metaphase plate
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
2n
Daughter cells of mitosis
Figure 8.14_3
MEIOSIS I
Metaphase I
Tetrads (homologous
pairs) align at the
metaphase plate
Anaphase I
Telophase I
Homologous
chromosomes
separate during
anaphase I;
sister
chromatids
remain together
Daughter
cells of
meiosis I
MEIOSIS II
No further
chromosomal
duplication;
sister
chromatids
separate during
anaphase II
n
n
n
n
Daughter cells of meiosis II
Haploid
n2
8.15 Independent orientation of chromosomes in
meiosis and random fertilization lead to
varied offspring
 Genetic variation in gametes results from
– independent orientation at metaphase I and
– random fertilization.
© 2012 Pearson Education, Inc.
8.15 Independent orientation of chromosomes in
meiosis and random fertilization lead to
varied offspring
 Independent orientation at metaphase I
– Each pair of chromosomes independently aligns at the
cell equator.
– There is an equal probability of the maternal or
paternal chromosome facing a given pole.
– The number of combinations for chromosomes
packaged into gametes is 2n where n = haploid number
of chromosomes.
© 2012 Pearson Education, Inc.
8.15 Independent orientation of chromosomes in
meiosis and random fertilization lead to
varied offspring
 Random fertilization – The combination of each
unique sperm with each unique egg increases
genetic variability.
© 2012 Pearson Education, Inc.
Figure 8.15_s1
Possibility A
Possibility B
Two equally probable
arrangements of
chromosomes at
metaphase I
Figure 8.15_s2
Possibility A
Possibility B
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Figure 8.15_s3
Possibility A
Possibility B
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
8.16 Homologous chromosomes may carry
different versions of genes
 Separation of homologous chromosomes during
meiosis can lead to genetic differences between
gametes.
– Homologous chromosomes may have different versions
of a gene at the same locus.
– One version was inherited from the maternal parent and
the other came from the paternal parent.
– Since homologues move to opposite poles during
anaphase I, gametes will receive either the maternal or
paternal version of the gene.
© 2012 Pearson Education, Inc.
Figure 8.16
Coat-color
genes
Eye-color
genes
Brown
C
Black
E
Meiosis
c
White
e
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
C
E
C
E
c
e
c
e
Chromosomes of
the four gametes
Brown coat (C);
black eyes (E)
White coat (c);
pink eyes (e)
8.17 Crossing over further increases genetic
variability
 Genetic recombination is the production of new
combinations of genes due to crossing over.
 Crossing over is an exchange of corresponding
segments between separate (nonsister)
chromatids on homologous chromosomes.
– Nonsister chromatids join at a chiasma (plural,
chiasmata), the site of attachment and crossing over.
– Corresponding amounts of genetic material are
exchanged between maternal and paternal (nonsister)
chromatids.
© 2012 Pearson Education, Inc.
Figure 8.17A
Chiasma
Tetrad
Figure 8.17B_1
C
E
c
e
1
Breakage of homologous chromatids
C
E
c
e
2
C
Tetrad
(pair of homologous
chromosomes in synapsis)
Joining of homologous chromatids
E
Chiasma
c
e
Figure 8.17B_2
C
E
Chiasma
c
e
3
Separation of homologous
chromosomes at anaphase I
C
E
C
e
c
E
c
e
Figure 8.17B_3
C
E
C
c
e
c
e
E
4
Separation of chromatids at
anaphase II and
completion of meiosis
C
E
C
e
c
E
c
e
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
ALTERATIONS OF
CHROMOSOME NUMBER
AND STRUCTURE
© 2012 Pearson Education, Inc.
8.18 A karyotype is a photographic inventory of
an individual’s chromosomes
 A karyotype is an ordered display of magnified
images of an individual’s chromosomes arranged
in pairs.
 Karyotypes
– are often produced from dividing cells arrested at
metaphase of mitosis and
– allow for the observation of
– homologous chromosome pairs,
– chromosome number, and
– chromosome structure.
© 2012 Pearson Education, Inc.
Figure 8.18_s5
Centromere
Sister
chromatids
Pair of
homologous
chromosomes
5
Sex chromosomes
8.19 CONNECTION: An extra copy of
chromosome 21 causes Down syndrome
 Trisomy 21
– involves the inheritance of three copies of chromosome
21 and
– is the most common human chromosome abnormality.
© 2012 Pearson Education, Inc.
8.19 CONNECTION: An extra copy of
chromosome 21 causes Down syndrome
 Trisomy 21, called Down syndrome, produces a
characteristic set of symptoms, which include:
– mental retardation,
– characteristic facial features,
– short stature,
– heart defects,
– susceptibility to respiratory infections, leukemia, and
Alzheimer’s disease, and
– shortened life span.
 The incidence increases with the age of the mother.
© 2012 Pearson Education, Inc.
Figure 8.19A
Trisomy 21
Figure 8.19B
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
25
30
35
40
Age of mother
45
50
8.20 Accidents during meiosis can alter
chromosome number
 Nondisjunction is the failure of chromosomes or
chromatids to separate normally during meiosis.
This can happen during
– meiosis I, if both members of a homologous pair go to
one pole or
– meiosis II if both sister chromatids go to one pole.
 Fertilization after nondisjunction yields zygotes with
altered numbers of chromosomes.
© 2012 Pearson Education, Inc.
Figure 8.20A_s1
MEIOSIS I
Nondisjunction
Figure 8.20A_s2
MEIOSIS I
Nondisjunction
MEIOSIS II
Normal
meiosis II
Figure 8.20A_s3
MEIOSIS I
Nondisjunction
MEIOSIS II
Normal
meiosis II
Gametes
Number of
chromosomes
n1
n1
n1
Abnormal gametes
n1
Figure 8.20B_s1
MEIOSIS I
Normal
meiosis I
Figure 8.20B_s2
MEIOSIS I
Normal
meiosis I
MEIOSIS II
Nondisjunction
Figure 8.20B_s3
MEIOSIS I
Normal
meiosis I
MEIOSIS II
Nondisjunction
n1
n1
Abnormal gametes
n
n
Normal gametes
8.21 CONNECTION: Abnormal numbers of sex
chromosomes do not usually affect survival
 Sex chromosome abnormalities tend to be less
severe, perhaps because of
– the small size of the Y chromosome or
– X-chromosome inactivation.
© 2012 Pearson Education, Inc.
8.23 CONNECTION: Alterations of chromosome
structure can cause birth defects and cancer
 Chromosome breakage can lead to
rearrangements that can produce
– genetic disorders or,
– if changes occur in somatic cells, cancer.
© 2012 Pearson Education, Inc.
8.23 CONNECTION: Alterations of chromosome
structure can cause birth defects and cancer
 These rearrangements may include
– a deletion, the loss of a chromosome segment,
– a duplication, the repeat of a chromosome segment,
– an inversion, the reversal of a chromosome segment,
or
– a translocation, the attachment of a segment to a
nonhomologous chromosome that can be reciprocal.
© 2012 Pearson Education, Inc.
8.23 CONNECTION: Alterations of chromosome
structure can cause birth defects and cancer
 Chronic myelogenous leukemia (CML)
– is one of the most common leukemias,
– affects cells that give rise to white blood cells
(leukocytes), and
– results from part of chromosome 22 switching places
with a small fragment from a tip of chromosome 9.
© 2012 Pearson Education, Inc.
Figure 8.23A
Deletion
Inversion
Duplication
Reciprocal translocation
Homologous
chromosomes
Nonhomologous
chromosomes
Figure 8.23B
Chromosome 9
Chromosome 22
Reciprocal
translocation
Activated cancer-causing gene
“Philadelphia chromosome”