Download CELL DIVISION AND REPRODUCTION

Chapter 8
The Cellular Basis of Reproduction
and Inheritance
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.
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
© 2012 Pearson Education, Inc.
Introduction
Figure 8.0_2
Chapter 8: Big Ideas
  In a healthy body, cell division allows for
–  growth,
–  the replacement of damaged cells, and
–  development from an embryo into an adult.
Cell Division and
Reproduction
The Eukaryotic Cell
Cycle and Mitosis
  In sexually reproducing organisms, eggs and sperm
result from
–  mitosis and
–  meiosis.
Meiosis and
Crossing Over
Alterations of Chromosome
Number and Structure
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Figure 8.0_3
CELL DIVISION AND
REPRODUCTION
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1
8.1 Cell division plays many important roles in
the lives of organisms
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 used
  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.
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–  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.
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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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8.3 The large, complex chromosomes of
eukaryotes duplicate with each cell division
THE EUKARYOTIC CELL
CYCLE AND MITOSIS
  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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2
8.3 The large, complex chromosomes of
eukaryotes duplicate with each cell division
Figure 8.3A
  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 8.3B
Chromosomes
DNA molecules
Sister
chromatids
Chromosome
duplication
Centromere
Sister
chromatids
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
Chromosome
distribution
to the
daughter
cells
–  separate from each other, now called chromosomes, and
–  sort into separate daughter cells.
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Figure 8.3B_1
Chromosomes
DNA molecules
Chromosome
duplication
Centromere
Sister
chromatids
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.
Chromosome
distribution
to the
daughter
cells
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3
Figure 8.4
8.4 The cell cycle multiplies cells
INT
E RPHASE
  The cell cycle consists of two stages,
characterized as follows:
1.  Interphase: duplication of cell contents
G1
(first gap)
–  G1—growth, increase in cytoplasm
–  S—duplication of chromosomes
–  Mitosis—division of the nucleus
s
ito
G2
(second gap)
M
IC
OT
MIT
2.  Mitotic phase: division
M
si
esis
okin
Cyt
–  G2—growth, preparation for division
S
(DNA synthesis)
A
PH
–  Cytokinesis—division of cytoplasm
SE
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Figure 8.5_1
8.5 Cell division is a continuum of dynamic
changes
MITOSIS
INTERPHASE
  Mitosis progresses through a series of stages:
Centrosomes
(with centriole pairs)
–  prophase,
Centrioles
–  prometaphase,
Chromatin
Prophase
Prometaphase
Early mitotic
spindle
Centrosome
Fragments of
the nuclear
envelope
Kinetochore
–  metaphase,
–  anaphase, and
–  telophase.
  Cytokinesis often overlaps telophase.
Nuclear
envelope
Plasma
membrane
Centromere
Chromosome,
consisting of two
sister chromatids
Spindle
microtubules
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Figure 8.5_left
8.5 Cell division is a continuum of dynamic
changes
MITOSIS
INTERPHASE
Prophase
Prometaphase
  A mitotic spindle is
–  required to divide the chromosomes,
–  composed of microtubules, and
Centrosomes
(with centriole pairs)
Centrioles
Chromatin
Early mitotic
spindle
Centrosome
Fragments of
the nuclear envelope
Kinetochore
–  produced by centrosomes, structures in the cytoplasm
that
–  organize microtubule arrangement and
–  contain a pair of centrioles in animal cells.
Nuclear
envelope
Plasma
membrane
Centromere
Chromosome,
consisting of two
sister chromatids
Spindle
microtubules
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4
8.5 Cell division is a continuum of dynamic
changes
Figure 8.5_2
  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.
INTERPHASE
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8.5 Cell division is a continuum of dynamic
changes
Figure 8.5_3
  Prophase
–  In the cytoplasm microtubules begin to emerge from
centrosomes, forming the spindle.
–  In the nucleus
–  chromosomes coil and become compact and
–  nucleoli disappear.
Prophase
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8.5 Cell division is a continuum of dynamic
changes
Figure 8.5_4
  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.
Prometaphase
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5
Figure 8.5_left
Figure 8.5_5
MITOSIS
INTERPHASE
Prophase
Prometaphase
MITOSIS
Anaphase
Metaphase
Metaphase
plate
Centrosomes
(with centriole pairs)
Centrioles
Chromatin
Early mitotic
spindle
Cleavage
furrow
Fragments of
the nuclear envelope
Centrosome
Kinetochore
Daughter
chromosomes
Mitotic
spindle
Nuclear
envelope
Telophase and Cytokinesis
Plasma
membrane
Centromere
Chromosome,
consisting of two
sister chromatids
Spindle
microtubules
Figure 8.5_right
MITOSIS
Anaphase
Metaphase
Nuclear
envelope
forming
Telophase and Cytokinesis
8.5 Cell division is a continuum of dynamic
changes
  Metaphase
–  The mitotic spindle is fully formed.
–  Chromosomes align at the cell equator.
Metaphase
plate
Mitotic
spindle
Cleavage
furrow
Daughter
chromosomes
–  Kinetochores of sister chromatids are facing the
opposite poles of the spindle.
Nuclear
envelope
forming
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Figure 8.5_6
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.
Metaphase
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Figure 8.5_7
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.
Anaphase
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Figure 8.5_8
Figure 8.5_right
MITOSIS
Anaphase
Metaphase
Metaphase
plate
Telophase and Cytokinesis
Cleavage
furrow
Telophase and Cytokinesis
Mitotic
spindle
Daughter
chromosomes
Nuclear
envelope
forming
8.5 Cell division is a continuum of dynamic
changes
8.6 Cytokinesis differs for plant and animal cells
  During cytokinesis, the cytoplasm is divided into
separate cells.
  In animal cells, cytokinesis occurs as
  The process of cytokinesis differs in animal and
plant cells.
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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
8.6 Cytokinesis differs for plant and animal cells
Cytokinesis
Cleavage
furrow
  In plant cells, cytokinesis occurs as
Contracting ring of
microfilaments
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,
Daughter
cells
3.  each cell now possesses a plasma membrane and cell
wall.
Cleavage
furrow
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Figure 8.6B
8.7 Anchorage, cell density, and chemical growth
factors affect cell division
New
cell wall
Cytokinesis
Cell wall
of the
parent cell
Cell wall
  Cell division is controlled by
Plasma
membrane
–  the presence of essential nutrients,
Daughter
nucleus
Cell plate
forming
  The cells within an organism’s body divide and
develop at different rates.
Vesicles
containing
cell wall
material
Cell plate
Daughter
cells
–  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
Figure 8.7B
Anchorage
Cultured cells
suspended in liquid
The addition of
growth
factor
Single layer
of cells
Removal
of cells
Restoration
of single
layer by cell
division
8
8.8 Growth factors signal the cell cycle control
system
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
  There are three major checkpoints in the cell cycle.
1.  G1 checkpoint
–  triggers and
–  allows entry into the S phase or
–  coordinates key events in the cell cycle.
  Checkpoints in the cell cycle can
–  causes the cell to leave the cycle, entering a nondividing G0
phase.
2.  G2 checkpoint, and
–  stop an event or
3.  M checkpoint.
–  signal an event to proceed.
  Research on the control of the cell cycle is one of
the hottest areas in biology today.
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Figure 8.8A
Figure 8.8B
G1 checkpoint
Growth
factor
G0
EXTRACELLULAR FLUID
Plasma membrane
Relay proteins
G1
S
Control
system
M
Receptor
protein
Signal
transduction
pathway
G2
G1
checkpoint
G1
S
Control
system
M
G2
M checkpoint
CYTOPLASM
G2 checkpoint
8.9 CONNECTION: Growing out of control,
cancer cells produce malignant tumors
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.
  A tumor is an abnormally growing mass of body
cells.
  Cancer cells escape controls on the cell cycle.
  Cancer cells
–  Benign tumors remain at the original site.
–  Malignant tumors spread to other locations, called
metastasis.
–  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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9
Figure 8.9
8.9 CONNECTION: Growing out of control,
cancer cells produce malignant tumors
Lymph
vessels
Blood
vessel
Tumor
Tumor in
another
part of
the body
Glandular
tissue
Growth
Invasion
  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.
Metastasis
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8.9 CONNECTION: Growing out of control,
cancer cells produce malignant tumors
8.10 Review: Mitosis provides for growth, cell
replacement, and asexual reproduction
  Cancer treatments
  When the cell cycle operates normally, mitosis
produces genetically identical cells for
–  Localized tumors can be
–  removed surgically and/or
–  growth,
–  treated with concentrated beams of high-energy radiation.
–  replacement of damaged and lost cells, and
–  Chemotherapy is used for metastatic tumors.
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–  asexual reproduction.
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Figure 8.10A
MEIOSIS AND
CROSSING OVER
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10
8.11 Chromosomes are matched in homologous
pairs
8.11 Chromosomes are matched in homologous
pairs
  In humans, somatic cells have
  Homologous chromosomes are matched in
–  23 pairs of homologous chromosomes and
–  length,
–  one member of each pair from each parent.
–  centromere position, and
  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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–  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.
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Figure 8.11
8.12 Gametes have a single set of chromosomes
Pair of homologous
chromosomes
  An organism’s life cycle is the sequence of stages
leading
–  from the adults of one generation
Locus
–  to the adults of the next.
Centromere
Sister
chromatids
One duplicated
chromosome
  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
Figure 8.12A
Haploid gametes (n = 23)
n
Egg cell
n
Sperm cell
  Meiosis is a process that converts diploid nuclei to
haploid nuclei.
–  Diploid cells have two homologous sets of
chromosomes.
Meiosis
Fertilization
–  Haploid cells have one set of chromosomes.
–  Meiosis occurs in the sex organs, producing gametes
—sperm and eggs.
Ovary
Testis
Diploid
zygote
(2n = 46)
  Fertilization is the union of sperm and egg.
  The zygote has a diploid chromosome number,
one set from each parent.
2n
Key
Multicellular diploid
adults (2n = 46)
Mitosis
Haploid stage (n)
Diploid stage (2n)
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11
8.12 Gametes have a single set of chromosomes
Figure 8.12B
INTERPHASE
  All sexual life cycles include an alternation
between
MEIOSIS II
Sister
chromatids
–  a diploid stage and
–  a haploid stage.
  Producing haploid gametes prevents the
chromosome number from doubling in every
generation.
MEIOSIS I
2
1
A pair of
homologous
chromosomes
in a diploid
parent cell
3
A pair of
duplicated
homologous
chromosomes
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8.13 Meiosis reduces the chromosome number
from diploid to haploid
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.
  Meiosis and mitosis are preceded by the duplication
of chromosomes. However,
  Two haploid gametes combine in fertilization to
restore the diploid state in the zygote.
–  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.
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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.
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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
–  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.
Tetrad
Nuclear
envelope
Chromatin
Sister
chromatids
Fragments
of the
nuclear
envelope
Centromere
(with a
kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
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12
8.13 Meiosis reduces the chromosome number
from diploid to haploid
8.13 Meiosis reduces the chromosome number
from diploid to haploid
  Meiosis I – Metaphase I – Tetrads align at the cell
equator.
  Meiosis I – Telophase I
  Meiosis I – Anaphase I – Homologous pairs
separate and move toward opposite poles of the cell.
–  Duplicated chromosomes have reached the poles.
–  A nuclear envelope re-forms around chromosomes in
some species.
–  Each nucleus has the haploid number of
chromosomes.
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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.
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
  Meiosis II – Prophase II
Sister chromatids
separate
–  Chromosomes coil and become compact (if uncoiled
after telophase I).
Haploid daughter
cells forming
–  Nuclear envelope, if re-formed, breaks up again.
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Figure 8.13_5
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.
Two lily cells
undergo meiosis II
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13
8.13 Meiosis reduces the chromosome number
from diploid to haploid
8.14 Mitosis and meiosis have important
similarities and differences
  Meiosis II – Telophase II
  Mitosis and meiosis both
–  Chromosomes have reached the poles of the cell.
–  begin with diploid parent cells that
–  A nuclear envelope forms around each set of
chromosomes.
–  have chromosomes duplicated during the previous
interphase.
  However the end products differ.
–  With cytokinesis, four haploid cells are produced.
–  Mitosis produces two genetically identical diploid somatic
daughter cells.
–  Meiosis produces four genetically unique haploid
gametes.
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© 2012 Pearson Education, Inc.
Figure 8.14
Figure 8.14_1
MEIOSIS I
MITOSIS
Parent cell
(before chromosome duplication)
Prophase
Duplicated
chromosome
(two sister
chromatids)
Chromosome
duplication
Site of
crossing
over
Prophase I
MEIOSIS I
MITOSIS
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
duplication
2n = 4
Prophase
Parent cell
(before chromosome duplication)
Prophase I
Site of
crossing
over
Metaphase I
Metaphase
Chromosomes
align at the
metaphase plate
Chromosome
duplication
Tetrads (homologous
pairs) align at the
metaphase plate
Chromosome
duplication
Tetrad
2n = 4
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
Metaphase
Metaphase I
Chromosomes
align at the
metaphase plate
Haploid
n=2
Tetrads (homologous
pairs) align at the
metaphase plate
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
Figure 8.14_2
Figure 8.14_3
MEIOSIS I
MITOSIS
Metaphase I
Metaphase
Chromosomes
align at the
metaphase plate
Tetrads (homologous
pairs) align at the
metaphase plate
Anaphase I
Telophase I
Homologous
chromosomes
separate during
anaphase I;
sister
chromatids
remain together
Anaphase
Telophase
2n
Daughter cells of mitosis
Haploid
n=2
MEIOSIS II
Sister chromatids
separate during
anaphase
2n
Daughter
cells of
meiosis I
No further
chromosomal
duplication;
sister
chromatids
separate during
anaphase II
n
n
n
n
Daughter cells of meiosis II
14
8.15 Independent orientation of chromosomes in
meiosis and random fertilization lead to
varied offspring
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
–  independent orientation at metaphase I and
–  random fertilization.
–  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.
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© 2012 Pearson Education, Inc.
8.15 Independent orientation of chromosomes in
meiosis and random fertilization lead to
varied offspring
Figure 8.15_s1
Possibility A
Possibility B
Two equally probable
arrangements of
chromosomes at
metaphase I
  Random fertilization – The combination of each
unique sperm with each unique egg increases
genetic variability.
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Figure 8.15_s2
Figure 8.15_s3
Possibility A
Possibility B
Possibility A
Possibility B
Two equally probable
arrangements of
chromosomes at
metaphase I
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
15
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.
Figure 8.16
Coat-color
genes
Eye-color
genes
Brown
C
Black
E
c
White
e
Pink
Meiosis
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)
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8.17 Crossing over further increases genetic
variability
Figure 8.17A
  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.
Chiasma
–  Nonsister chromatids join at a chiasma (plural,
chiasmata), the site of attachment and crossing over.
Tetrad
–  Corresponding amounts of genetic material are
exchanged between maternal and paternal (nonsister)
chromatids.
© 2012 Pearson Education, Inc.
Figure 8.17B_1
C
E
c
e
1
E
c
e
C
E
c
e
3
Joining of homologous chromatids
E
Chiasma
c
C
Chiasma
Breakage of homologous chromatids
C
2
Figure 8.17B_2
Tetrad
(pair of homologous
chromosomes in synapsis)
Separation of homologous
chromosomes at anaphase I
C
E
C
e
c
E
c
e
e
16
Figure 8.17B_3
C
E
C
c
e
E
c
e
4
Separation of chromatids at
anaphase II and
completion of meiosis
C
E
C
e
c
E
c
e
ALTERATIONS OF
CHROMOSOME NUMBER
AND STRUCTURE
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
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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
Figure 8.18_s5
Centromere
Sister
chromatids
Pair of
homologous
chromosomes
–  homologous chromosome pairs,
5
–  chromosome number, and
Sex chromosomes
–  chromosome structure.
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8.19 CONNECTION: An extra copy of
chromosome 21 causes Down syndrome
8.19 CONNECTION: An extra copy of
chromosome 21 causes Down syndrome
  Trisomy 21
  Trisomy 21, called Down syndrome, produces a
characteristic set of symptoms, which include:
–  involves the inheritance of three copies of chromosome
21 and
–  is the most common human chromosome abnormality.
–  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.
© 2012 Pearson Education, Inc.
17
Figure 8.19A
Figure 8.19B
Trisomy 21
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
8.20 Accidents during meiosis can alter
chromosome number
25
30
35
40
Age of mother
45
50
Figure 8.20A_s1
MEIOSIS I
  Nondisjunction is the failure of chromosomes or
chromatids to separate normally during meiosis. This
can happen during
Nondisjunction
–  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_s2
Figure 8.20A_s3
MEIOSIS I
MEIOSIS I
Nondisjunction
MEIOSIS II
Nondisjunction
MEIOSIS II
Normal
meiosis II
Normal
meiosis II
Gametes
Number of
chromosomes
n+1
n+1
n-1
n-1
Abnormal gametes
18
Figure 8.20B_s1
Figure 8.20B_s2
MEIOSIS I
MEIOSIS I
Normal
meiosis I
Normal
meiosis I
MEIOSIS II
Nondisjunction
Figure 8.20B_s3
8.21 CONNECTION: Abnormal numbers of sex
chromosomes do not usually affect survival
MEIOSIS I
Normal
meiosis I
  Sex chromosome abnormalities tend to be less
severe, perhaps because of
MEIOSIS II
–  the small size of the Y chromosome or
–  X-chromosome inactivation.
Nondisjunction
n+1
n-1
Abnormal gametes
n
n
Normal gametes
© 2012 Pearson Education, Inc.
8.23 CONNECTION: Alterations of chromosome
structure can cause birth defects and cancer
8.23 CONNECTION: Alterations of chromosome
structure can cause birth defects and cancer
  Chromosome breakage can lead to
rearrangements that can produce
  These rearrangements may include
–  a deletion, the loss of a chromosome segment,
–  genetic disorders or,
–  a duplication, the repeat of a chromosome segment,
–  if changes occur in somatic cells, cancer.
–  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.
© 2012 Pearson Education, Inc.
19
8.23 CONNECTION: Alterations of chromosome
structure can cause birth defects and cancer
Figure 8.23A
  Chronic myelogenous leukemia (CML)
–  is one of the most common leukemias,
Deletion
Inversion
Duplication
Reciprocal translocation
–  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.
Homologous
chromosomes
Nonhomologous
chromosomes
© 2012 Pearson Education, Inc.
Figure 8.23B
Chromosome 9
Chromosome 22
Reciprocal
translocation
Activated cancer-causing gene
Philadelphia chromosome
20