Download PPT 11-12

Chapter 11 & 12:
How Cells Divide
Learning Outcomes
I can explain why cells
divide
Figure 9.2
100 m
200 m
(a) Reproduction
(b) Growth and
development
20 m
(c) Tissue renewal
© 2014 Pearson Education, Inc.
Learning Outcomes
I can explain the steps
of cell division
 Somatic cells (nonreproductive cells) have two sets
of chromosomes
 Gametes (reproductive cells: sperm and eggs) have
one set of chromosomes
Figure 9.6
G1
S
(DNA synthesis)
G2
© 2014 Pearson Education, Inc.
 Interphase (about 90% of the cell cycle) can be
divided into subphases
 G1 phase (“first gap”)
 S phase (“synthesis”)
 G2 phase (“second gap”)
 The cell grows during all three phases, but
chromosomes are duplicated only during the
S phase
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 Mitosis- division of the genetic material
 Prophase
 Metaphase
 Anaphase
 Telophase
 Cytokinesis- division of cytoplasm
© 2014 Pearson Education, Inc.
Cellular Organization of the Genetic Material
 All the DNA in a cell constitutes the cell’s genome
 A genome can consist of a single DNA molecule
(common in prokaryotic cells) or a number of DNA
molecules (common in eukaryotic cells)
 DNA molecules in a cell are packaged into
chromosomes
© 2014 Pearson Education, Inc.
Learning Outcomes
I can explain the
cellular organization of
genetic material
 Eukaryotic chromosomes consist of chromatin, a
complex of DNA and protein
 Every eukaryotic species has a characteristic
number of chromosomes in each cell nucleus
© 2014 Pearson Education, Inc.
Distribution of Chromosomes During Eukaryotic
Cell Division
 In preparation for cell division, DNA is replicated and
the chromosomes condense
 Each duplicated chromosome has two sister
chromatids, joined identical copies of the original
chromosome
 The centromere is where the two chromatids are
most closely attached
© 2014 Pearson Education, Inc.
Figure 9.4
Sister
chromatids
Centromere
0.5 m
 During cell division, the two sister chromatids of each
duplicated chromosome separate and move into two
nuclei
 Once separate, the chromatids are called
chromosomes
© 2014 Pearson Education, Inc.
Figure 9.5-1
Chromosomes
1
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Figure 9.5-2
Chromosomes
1
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Chromosome duplication
2
Sister
chromatids
Figure 9.5-3
Chromosomes
1
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Chromosome duplication
2
Sister
chromatids
Separation of sister
chromatids
3
Concept 1: Most cell division results in genetically
identical daughter cells
 Most cell division results in the distribution of
identical genetic material—DNA—to two daughter
cells
 DNA is passed from one generation of cells to the
next with remarkable fidelity
© 2014 Pearson Education, Inc.
Figure 9.11
Nucleus
Chromosomes
Nucleolus condensing
Chromosomes
10 m
1 Prophase
2 Prometaphase
Cell plate
3 Metaphase
4 Anaphase
5 Telophase
Figure 9.11a
Chromosomes
Nucleus
Nucleolus condensing
10 m
1 Prophase
Figure 9.11c
3 Metaphase
10 m
Figure 9.11d
4 Anaphase
10 m
Figure 9.11e
Cell plate
5 Telophase
10 m
The Mitotic Spindle: A Closer Look
 The mitotic spindle is a structure made of
microtubules and associated proteins
 It controls chromosome movement during mitosis
 In animal cells, assembly of spindle microtubules
begins in the centrosome, the microtubule
organizing center
© 2014 Pearson Education, Inc.
 The centrosome replicates during interphase,
forming two centrosomes that migrate to opposite
ends of the cell during prophase and prometaphase
 An aster (radial array of short microtubules) extends
from each centrosome
 The spindle includes the centrosomes, the spindle
microtubules, and the asters
© 2014 Pearson Education, Inc.
 During prometaphase, some spindle microtubules
attach to the kinetochores of chromosomes and
begin to move the chromosomes
 Kinetochores are protein complexes that assemble
on sections of DNA at centromeres
 At metaphase, the centromeres of all the
chromosomes are at the metaphase plate, an
imaginary structure at the midway point between
the spindle’s two poles
© 2014 Pearson Education, Inc.
Figure 9.8
Aster
Sister
chromatids
Centrosome
Metaphase plate
(imaginary)
Kinetochores
Microtubules
Chromosomes
Overlapping
nonkinetochore
microtubules Kinetochore
microtubules
1 m
0.5 m
© 2014 Pearson Education, Inc.
Centrosome
Figure 9.8a
Microtubules
Chromosomes
1 m
Centrosome
© 2014 Pearson Education, Inc.
Figure 9.8b
Kinetochores
Kinetochore
microtubules
0.5 m
© 2014 Pearson Education, Inc.
 Nonkinetochore microtubules from opposite poles
overlap and push against each other, elongating
the cell
 At the end of anaphase, duplicate groups of
chromosomes have arrived at opposite ends of the
elongated parent cell
 Cytokinesis begins during anaphase or telophase
and the spindle eventually disassembles
© 2014 Pearson Education, Inc.
Learning Outcomes
I can explain the
difference with
cytokinesis in plant vs
animal cells
Cytokinesis: A Closer Look
 In animal cells, cytokinesis occurs by a process
known as cleavage, forming a cleavage furrow
 In plant cells, a cell plate forms during cytokinesis
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Animation: Cytokinesis
Figure 9.10
(a) Cleavage of an animal cell (SEM)
Cleavage furrow
Contractile ring of
microfilaments
100 m
(b) Cell plate formation in a plant cell (TEM)
Vesicles
forming
cell plate
Wall of parent
1 m
cell
Cell plate New cell wall
Daughter cells
Daughter cells
© 2014 Pearson Education, Inc.
Figure 9.10a
(a) Cleavage of an animal cell (SEM)
Cleavage furrow
Contractile ring of
microfilaments
© 2014 Pearson Education, Inc.
100 m
Daughter cells
Figure 9.10b
(b) Cell plate formation in a plant cell (TEM)
Vesicles
forming
cell plate
Wall of parent
1 m
cell
Cell plate New cell wall
Daughter cells
© 2014 Pearson Education, Inc.
Figure 9.10ba
Vesicles
forming
cell plate
© 2014 Pearson Education, Inc.
Wall of parent
cell
1 m
Figure 9.UN02
G1
S
Cytokinesis
Mitosis
G2
MITOTIC (M) PHASE
Prophase
Telophase and
Cytokinesis
Prometaphase
Anaphase
Metaphase
© 2014 Pearson Education, Inc.
Figure 9.UN03
© 2014 Pearson Education, Inc.
Learning Outcomes
I can explain how cell
division is controlled
Concept 2: The eukaryotic cell cycle is
regulated by a molecular control system
 The frequency of cell division varies with the type
of cell
 These differences result from regulation at the
molecular level
 Cancer cells manage to escape the usual controls
on the cell cycle
© 2014 Pearson Education, Inc.
Checkpoints of the Cell Cycle Control System
 The sequential events of the cell cycle are directed
by a distinct cell cycle control system, which is
similar to a timing device of a washing machine
 The cell cycle control system is regulated by both
internal and external controls
 The clock has specific checkpoints where the cell
cycle stops until a go-ahead signal is received
© 2014 Pearson Education, Inc.
Figure 9.15
G1 checkpoint
Control
system
G1
M
G2
M checkpoint
G2 checkpoint
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S
 For many cells, the G1 checkpoint seems to be the
most important
 If a cell receives a go-ahead signal at the G1
checkpoint, it will usually complete the S, G2, and
M phases and divide
 If the cell does not receive the go-ahead signal, it
will exit the cycle, switching into a nondividing state
called the G0 phase
© 2014 Pearson Education, Inc.
Figure 9.16
G1 checkpoint
G0
G1
G1
Without go-ahead signal,
cell enters G0.
(a) G1 checkpoint
S
M
G1
With go-ahead signal,
cell continues cell cycle.
G2
G1
G1
M G2
M
G2
M checkpoint
Prometaphase
Without full chromosome
attachment, stop signal is
received.
(b) M checkpoint
© 2014 Pearson Education, Inc.
Anaphase
G2
checkpoint
Metaphase
With full chromosome
attachment, go-ahead signal
is received.
Figure 9.16a
G1 checkpoint
G0
G1
Without go-ahead signal,
cell enters G0.
(a) G1 checkpoint
© 2014 Pearson Education, Inc.
G1
With go-ahead signal,
cell continues cell cycle.
Figure 9.16b
G1
G1
M G2
M
G2
M checkpoint
Prometaphase
Without full chromosome
attachment, stop signal is
received.
(b) M checkpoint
© 2014 Pearson Education, Inc.
Anaphase
G2
checkpoint
Metaphase
With full chromosome
attachment, go-ahead signal
is received.
 The cell cycle is regulated by a set of regulatory
proteins and protein complexes including kinases
and proteins called cyclins
© 2014 Pearson Education, Inc.
 An example of an internal signal occurs at the M
phase checkpoint
 In this case, anaphase does not begin if any
kinetochores remain unattached to spindle
microtubules
 Attachment of all of the kinetochores activates a
regulatory complex, which then activates the enzyme
separase
 Separase allows sister chromatids to separate,
triggering the onset of anaphase
© 2014 Pearson Education, Inc.
 Another example of external signals is densitydependent inhibition, in which crowded cells stop
dividing
 Most animal cells also exhibit anchorage
dependence, in which they must be attached to a
substratum in order to divide
 Cancer cells exhibit neither density-dependent
inhibition nor anchorage dependence
© 2014 Pearson Education, Inc.
Figure 9.18
Anchorage dependence: cells
require a surface for division
Density-dependent inhibition:
cells form a single layer
Density-dependent inhibition:
cells divide to fill a gap and
then stop
20 m
(a) Normal mammalian cells
© 2014 Pearson Education, Inc.
20 m
(b) Cancer cells
Loss of Cell Cycle Controls in Cancer Cells
 Cancer cells do not respond to signals that normally
regulate the cell cycle
 Cancer cells may not need growth factors to grow
and divide
 They may make their own growth factor
 They may convey a growth factor’s signal without the
presence of the growth factor
 They may have an abnormal cell cycle control system
© 2014 Pearson Education, Inc.
 A normal cell is converted to a cancerous cell by a
process called transformation
 Cancer cells that are not eliminated by the immune
system form tumors, masses of abnormal cells within
otherwise normal tissue
 If abnormal cells remain only at the original site, the
lump is called a benign tumor
 Malignant tumors invade surrounding tissues and
can metastasize, exporting cancer cells to other
parts of the body, where they may form additional
tumors
© 2014 Pearson Education, Inc.
 Recent advances in understanding the cell cycle
and cell cycle signaling have led to advances in
cancer treatment
 Medical treatments for cancer are becoming more
“personalized” to an individual patient’s tumor
 One of the big lessons in cancer research is how
complex cancer is
© 2014 Pearson Education, Inc.
Learning Outcomes
I can explain the
stages of meiosis
Concept 1: Meiosis reduces the number of chromosome
sets from diploid to haploid
• Like mitosis, meiosis is preceded by the replication
of chromosomes
• Meiosis takes place in two sets of cell divisions,
called meiosis I and meiosis II
• The two cell divisions result in four daughter cells,
rather than the two daughter cells in mitosis
• Each daughter cell has only half as many
chromosomes as the parent cell
© 2014 Pearson Education, Inc.
The Stages of Meiosis
• For a single pair of homologous chromosomes in a
diploid cell, both members of the pair are
duplicated
• The resulting sister chromatids are closely
associated all along their lengths
• Homologs may have different versions of genes,
each called an allele
• Homologs are not associated in any obvious way
except during meiosis
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• Meiosis halves the total number of chromosomes
very specifically
• It reduces the number of sets from two to one,
with each daughter cell receiving one set of
chromosomes
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• In the first meiotic division, homologous pairs of
chromosomes pair and separate
• In the second meiotic division, sister chromatids
of each chromosome separate
• Four new haploid cells are produced as a result
© 2014 Pearson Education, Inc.
Figure 10.8
MEIOSIS I: Separates homologous chromosomes
Prophase I
Metaphase I
Anaphase I
Telophase I and
Cytokinesis
MEIOSIS II: Separates sister chromatids
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
Sister
chromatids
Centromere
(with kinetochore) Sister chromatids
remain attached
Centrosome
(with centriole
Cleavage
pair)
furrow
Chiasmata
Metaphase
Spindle
plate
Sister chromatids
separate
Homologous
chromosomes
separate
Fragments
of nuclear
envelope
Homologous
chromosomes
Microtubule
attached to
kinetochore
© 2014 Pearson Education, Inc.
Haploid
daughter
cells forming
Figure 10.8a
MEIOSIS I: Separates homologous chromosomes
Prophase I
Metaphase I
Anaphase I
Telophase I and
Cytokinesis
Sister
chromatids
Centromere
(with kinetochore) Sister chromatids
remain attached
Centrosome
(with centriole
Cleavage
pair)
furrow
Chiasmata Metaphase
Spindle
plate
Fragments
of nuclear
envelope
Homologous
chromosomes
© 2014 Pearson Education, Inc.
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Figure 10.8b
MEIOSIS II: Separates sister chromatids
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
Sister chromatids
separate
Haploid
daughter
cells forming
© 2014 Pearson Education, Inc.
Prophase I
• Prophase I typically occupies more than 90% of the
time required for meiosis
• Chromosomes begin to condense
• In synapsis, homologous chromosomes loosely
pair up, aligned gene by gene
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• In crossing over, nonsister chromatids exchange
DNA segments
• Each homologous pair has one or more X-shaped
regions called chiasmata
• Chiasmata exist at points where crossing over has
occurred.
© 2014 Pearson Education, Inc.
Metaphase I
 In metaphase I, tetrads line up at the metaphase
plate, with one chromosome facing each pole
 Microtubules from one pole are attached to the
kinetochore of one chromosome of each tetrad
 Microtubules from the other pole are attached to the
kinetochore of the other chromosome
© 2014 Pearson Education, Inc.
Anaphase I
• In anaphase I, pairs of homologous chromosomes
separate
• One chromosome moves toward each pole, guided
by the spindle apparatus
• Sister chromatids remain attached at the
centromere and move as one unit toward the pole
© 2014 Pearson Education, Inc.
Telophase I and Cytokinesis
• In the beginning of telophase I, each half of the
cell has a haploid set of chromosomes; each
chromosome still consists of two sister chromatids
• Cytokinesis usually occurs simultaneously,
forming two haploid daughter cells
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• In animal cells, a cleavage furrow forms; in plant
cells, a cell plate forms
• No chromosome replication occurs between the
end of meiosis I and the beginning of meiosis II
because the chromosomes are already replicated
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• Division in meiosis II also occurs in four phases
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II and cytokinesis
• Meiosis II is very similar to mitosis
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Prophase II
• In prophase II, a spindle apparatus forms
• In late prophase II, chromosomes (each still
composed of two chromatids) move toward the
metaphase plate
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Metaphase II
• In metaphase II, the sister chromatids are
arranged at the metaphase plate
• Because of crossing over in meiosis I, the two
sister chromatids of each chromosome are no
longer genetically identical
• The kinetochores of sister chromatids attach to
microtubules extending from opposite poles
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Anaphase II
• In anaphase II, the sister chromatids separate
• The sister chromatids of each chromosome now
move as two newly individual chromosomes
toward opposite poles
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Telophase II and Cytokinesis
• In telophase II, the chromosomes arrive at
opposite poles
• Nuclei form, and the chromosomes begin
decondensing
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• At the end of meiosis, there are four daughter
cells, each with a haploid set of unduplicated
chromosomes
• Each daughter cell is genetically distinct from the
others and from the parent cell
© 2014 Pearson Education, Inc.
Learning Outcomes
 I can compare mitosis
and meiosis
© 2014 Pearson Education, Inc.
A Comparison of Mitosis and Meiosis
• Mitosis conserves the number of chromosome sets,
producing cells that are genetically identical to the
parent cell
• Meiosis reduces the number of chromosome sets
from two (diploid) to one (haploid), producing
cells that differ genetically from each other and
from the parent cell
• Meiosis includes two divisions after replication,
each with specific stages
© 2014 Pearson Education, Inc.
• Three events are unique to meiosis, and all three
occur in meiosis l
– Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and
exchange genetic information
– Homologous pairs at the metaphase plate:
Homologous pairs of chromosomes are positioned
there in metaphase I
– Separation of homologs during anaphase I
© 2014 Pearson Education, Inc.
Figure 10.9
MITOSIS
MEIOSIS
Parent cell
MEIOSIS I
Chiasma
Prophase I
Prophase
Duplicated
chromosome
Chromosome
duplication
2n = 6
Chromosome
duplication
Metaphase
Individual
chromosomes
line up.
Pairs of
chromosomes
line up.
Anaphase
Telophase
Sister chromatids
separate.
Homologs
separate.
2n
Sister
chromatids
separate.
2n
Mitosis
Metaphase I
Anaphase I
Telophase I
Daughter
cells of
meiosis I
MEIOSIS II
n
n
n
n
Daughter cells of meiosis II
Daughter cells
of mitosis
Property
Homologous
chromosome
pair
SUMMARY
Meiosis
DNA replication
Occurs during interphase before mitosis begins
Occurs during interphase before meiosis I begins
Number of divisions
One, including prophase, prometaphase,
metaphase, anaphase, and telophase
Two, each including prophase, metaphase, anaphase, and telophase
Synapsis of homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing over between nonsister chromatids;
resulting chiasmata hold pairs together due to sister chromatid cohesion
Number of daughter cells
and genetic composition
Two, each diploid (2n) and genetically
identical to the parent cell
Four, each haploid (n), containing half as many chromosomes as the parent cell;
genetically different from the parent cell and from each other
Role in the animal body
Enables multicellular adult to arise from zygote;
produces cells for growth, repair, and, in some
species, asexual reproduction
Produces gametes; reduces number of chromosome sets by half and introduces
genetic variability among the gametes
© 2014 Pearson Education, Inc.
Figure 10.9a
MITOSIS
MEIOSIS
Parent cell
Chiasma
MEIOSIS I
Prophase I
Prophase
Duplicated
chromosome
Metaphase
Anaphase
Telophase
2n
Daughter cells
of mitosis
© 2014 Pearson Education, Inc.
Chromosome
duplication
2n = 6
Chromosome
duplication
Individual
chromosomes
line up.
Pairs of
chromosomes
line up.
Sister chromatids
separate.
Homologs
separate.
2n
Sister
chromatids
separate.
Homologous
chromosome
pair
Metaphase I
Anaphase I
Telophase I
Daughter
cells of
meiosis I
MEIOSIS II
n
n
n
n
Daughter cells of meiosis II
Figure 10.9aa
MITOSIS
Prophase
Duplicated
chromosome
MEIOSIS
Parent cell
Chromosome
Chromosome
duplication 2n = 6 duplication
Individual
chromosomes
line up.
Metaphase
© 2014 Pearson Education, Inc.
Chiasma
Pairs of
chromosomes
line up.
MEIOSIS I
Prophase I
Homologous
chromosome
pair
Metaphase I
Figure 10.9ab
MEIOSIS
MITOSIS
Anaphase
Telophase
Sister chromatids
separate.
2n
Daughter cells
of mitosis
© 2014 Pearson Education, Inc.
2n
Anaphase I
Telophase I
Homologs
separate.
Sister
chromatids
separate.
Daughter
cells of
meiosis I
MEIOSIS II
n
n
n
n
Daughter cells of meiosis II
Figure 10.9b
SUMMARY
Property
Mitosis
Meiosis
DNA
replication
Occurs during interphase
before mitosis begins
Occurs during interphase before meiosis I
begins
Number of
divisions
One, including prophase,
prometaphase, metaphase,
anaphase, and telophase
Two, each including prophase, metaphase,
anaphase, and telophase
Synapsis of
homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing
over between nonsister chromatids; resulting
chiasmata hold pairs together due to sister
chromatid cohesion
Number of
daughter cells
and genetic
composition
Two, each diploid (2n) and
genetically identical to the
parent cell
Four, each haploid (n), containing half as
many chromosomes as the parent cell;
genetically different from the parent cell and
from each other
Role in the
animal body
Enables multicellular adult to
arise from zygote; produces
cells for growth, repair, and,
in some species, asexual
reproduction
Produces gametes; reduces number of
chromosome sets by half and introduces
genetic variability among the gametes
© 2014 Pearson Education, Inc.
Figure 10.9ba
Property
© 2014 Pearson Education, Inc.
Mitosis
DNA
replication
Occurs during interphase
before mitosis begins
Number of
divisions
One, including prophase,
prometaphase, metaphase,
anaphase, and telophase
Synapsis of
homologous
chromosomes
Does not occur
Number of
daughter cells
and genetic
composition
Two, each diploid (2n) and
genetically identical to the
parent cell
Role in the
animal body
Enables multicellular adult to arise
from zygote; produces cells for
growth, repair, and, in some
species, asexual reproduction
Figure 10.9bb
Property
Meiosis
DNA
replication
Occurs during interphase before meiosis I
begins
Number of
divisions
Two, each including prophase, metaphase,
anaphase, and telophase
Synapsis of
homologous
chromosomes
Occurs during prophase I along with crossing
over between nonsister chromatids; resulting
chiasmata hold pairs together due to sister
chromatid cohesion
Number of
daughter cells
and genetic
composition
Four, each haploid (n), containing half as
many chromosomes as the parent cell;
genetically different from the parent cell and
from each other
Role in the
animal body
Produces gametes; reduces number of
chromosome sets by half and introduces
genetic variability among the gametes
© 2014 Pearson Education, Inc.
Learning Outcomes
 I can explain the reasons
for genetic
recombination in
meiosis
© 2014 Pearson Education, Inc.
Origins of Genetic Variation Among Offspring
• The behavior of chromosomes during meiosis and
fertilization is responsible for most of the variation
that arises in each generation
• Three mechanisms contribute to genetic variation
– Independent assortment of chromosomes
– Crossing over
– Random fertilization
© 2014 Pearson Education, Inc.
Independent Assortment of Chromosomes
• Homologous pairs of chromosomes orient
randomly at metaphase I of meiosis
• In independent assortment, each pair of
chromosomes sorts maternal and paternal
homologs into daughter cells independently of the
other pairs
© 2014 Pearson Education, Inc.
• The number of combinations possible when
chromosomes assort independently into gametes is
2n, where n is the haploid number
• For humans (n = 23), there are more than 8 million
(223) possible combinations of chromosomes
© 2014 Pearson Education, Inc.
Figure 10.10-1
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
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Figure 10.10-2
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
© 2014 Pearson Education, Inc.
Figure 10.10-3
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1 Combination 2
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Combination 3 Combination 4
Crossing Over
• Crossing over produces recombinant
chromosomes, which combine DNA inherited
from each parent
• Crossing over begins very early in prophase I, as
homologous chromosomes pair up gene by gene
© 2014 Pearson Education, Inc.
• In crossing over, homologous portions of two
nonsister chromatids trade places
• Crossing over contributes to genetic variation by
combining DNA, producing chromosomes with
new combinations of maternal and paternal alleles
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Animation: Genetic Variation
Right click slide / Select play
Figure 10.11-1
Prophase I
of meiosis
Pair of
homologs
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Nonsister chromatids
held together
during synapsis
Figure 10.11-2
Prophase I
of meiosis
Pair of
homologs
Chiasma
Centromere
TEM
© 2014 Pearson Education, Inc.
Nonsister chromatids
held together
during synapsis
Synapsis and
crossing over
Figure 10.11-3
Prophase I
of meiosis
Pair of
homologs
Chiasma
Nonsister chromatids
held together
during synapsis
Synapsis and
crossing over
Centromere
TEM
Anaphase I
© 2014 Pearson Education, Inc.
Breakdown of
proteins holding sister
chromatid arms together
Figure 10.11-4
Prophase I
of meiosis
Pair of
homologs
Chiasma
Nonsister chromatids
held together
during synapsis
Synapsis and
crossing over
Centromere
TEM
Anaphase I
Anaphase II
© 2014 Pearson Education, Inc.
Breakdown of
proteins holding sister
chromatid arms together
Figure 10.11-5
Prophase I
of meiosis
Pair of
homologs
Chiasma
Nonsister chromatids
held together
during synapsis
Synapsis and
crossing over
Centromere
TEM
Anaphase I
Breakdown of
proteins holding sister
chromatid arms together
Anaphase II
Daughter
cells
Recombinant chromosomes
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Figure 10.11a
Chiasma
Centromere
TEM
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Random Fertilization
• Random fertilization adds to genetic variation
because any sperm can fuse with any ovum
(unfertilized egg)
• The fusion of two gametes (each with 8.4 million
possible chromosome combinations from
independent assortment) produces a zygote with
any of about 70 trillion diploid combinations
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• Crossing over adds even more variation
• Each zygote has a unique genetic identity
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The Evolutionary Significance of Genetic
Variation Within Populations
• Natural selection results in the accumulation of
genetic variations favored by the environment
• Sexual reproduction contributes to the genetic
variation in a population, which originates from
mutations
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Learning Outcomes
 I can explain how cells
program their death
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But sometimes cells need to die…
• Lysosomes can be used to kill cells when they are supposed to
be destroyed
– some cells have to die for proper development in an
organism
• apoptosis
– “auto-destruct” process
– lysosomes break open & kill cell
• ex: tadpole tail gets re-absorbed
when it turns into a frog
• ex: loss of webbing between your
fingers during fetal development
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Fetal development
syndactyly
6 weeks
15 weeks
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Apoptosis
• programmed destruction of cells in multicellular organisms
– programmed development
– control of cell growth
• example:
if cell grows uncontrollably this self-destruct mechanism
is triggered to remove damaged cell
• cancer must over-ride this to enable tumor growth
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