13 lecture biol 1030-30 gillette college
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Meiosis & Sexual Life CyclesTRANSCRIPT
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CAMPBELL
BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson
© 2014 Pearson Education, Inc.
TENTH
EDITION
CAMPBELL
BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson
TENTH
EDITION
13Meiosis and
Sexual Life
Cycles
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
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Variations on a Theme
Living organisms are distinguished by their ability
to reproduce their own kind
Heredity is the transmission of traits from one
generation to the next
Variation is demonstrated by the differences in
appearance that offspring show from parents and
siblings
Genetics is the scientific study of heredity and
variation
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Figure 13.1
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Figure 13.1a
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Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes
In a literal sense, children do not inherit particular
physical traits from their parents
It is genes that are actually inherited
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Inheritance of Genes
Genes are the units of heredity and are made up of
segments of DNA
Genes are passed to the next generation via
reproductive cells called gametes (sperm and eggs)
Most DNA is packaged into chromosomes
Humans have 46 chromosomes in their somatic cells,
all cells of the body except gametes and their
precursors
A gene’s specific position along a chromosome is
called the locus
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Comparison of Asexual and Sexual Reproduction
In asexual reproduction, a single individual
passes all of its genes to its offspring without the
fusion of gametes
A clone is a group of genetically identical
individuals from the same parent
In sexual reproduction, two parents give rise to
offspring that have unique combinations of genes
inherited from the two parents
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Figure 13.2
0.5 mm
Bud
Parent
(a) (b)Hydra Redwoods
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Video: Hydra Budding
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Concept 13.2: Fertilization and meiosis alternate in sexual life cycles
A life cycle is the generation-to-generation
sequence of stages in the reproductive history of
an organism
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Sets of Chromosomes in Human Cells
Human somatic cells have 23 pairs of
chromosomes
A karyotype is an ordered display of the pairs of
chromosomes from a cell
The two chromosomes in each pair are called
homologous chromosomes, or homologs
Chromosomes in a homologous pair are the same
length and shape and carry genes controlling the
same inherited characters
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Figure 13.3
TechniqueApplicationPair of homologous
duplicated chromosomes
Centromere
Metaphase
chromosome
Sister
chromatids
5 µm
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The sex chromosomes, which determine the sex
of the individual, are called X and Y
Human females have a homologous pair of X
chromosomes (XX)
Human males have one X and one Y chromosome
The remaining 22 pairs of chromosomes are
called autosomes
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Each pair of homologous chromosomes includes
one chromosome from each parent
The 46 chromosomes in a human somatic cell are
two sets of 23: one from the mother and one from
the father
A diploid cell (2n) has two sets of chromosomes
For humans, the diploid number is 46 (2n = 46)
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In a cell in which DNA synthesis has occurred,
each chromosome is replicated
Each replicated chromosome consists of two
identical sister chromatids
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Figure 13.4
Key
Maternal set ofchromosomes (n = 3)
Paternal set ofchromosomes (n = 3)
2n = 6
Sister chromatidsof one duplicatedchromosome
Two nonsisterchromatids ina homologous pair
Centromere
Pair of homologouschromosomes(one from each set)
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A gamete (sperm or egg) contains a single set of
chromosomes and is haploid (n)
For humans, the haploid number is 23 (n = 23)
Each set of 23 consists of 22 autosomes and a
single sex chromosome
In an unfertilized egg (ovum), the sex
chromosome is X
In a sperm cell, the sex chromosome may be
either X or Y
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Behavior of Chromosome Sets in the Human Life Cycle
Fertilization is the union of gametes (the sperm
and the egg)
The fertilized egg is called a zygote and has one
set of chromosomes from each parent
The zygote produces somatic cells by mitosis and
develops into an adult
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At sexual maturity, the ovaries and testes produce
haploid gametes
Gametes are the only types of human cells
produced by meiosis, rather than mitosis
Meiosis results in one set of chromosomes in each
gamete
Fertilization and meiosis alternate in sexual life
cycles to maintain chromosome number
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Figure 13.5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Egg (n)
Sperm (n)
MEIOSIS FERTILIZATION
Testis
Diploidzygote(2n = 46)
Ovary
Mitosis anddevelopment
Multicellular diploidadults (2n = 46)
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The Variety of Sexual Life Cycles
The alternation of meiosis and fertilization is
common to all organisms that reproduce sexually
The three main types of sexual life cycles differ in
the timing of meiosis and fertilization
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Figure 13.6
Key
Haploid (n)Diploid (2n)
Gametes
Haploid multi-cellular organism(gametophyte)
MitosisMitosis Mitosis Mitosis
Haploid unicellular ormulticellular organism
Gametes
FERTILIZATIONMEIOSIS
n
2n
n
nn
n
nn
n
nn
2n2n
Zygote
MEIOSIS FERTILIZATION
SporesGametes
MitosisDiploidmulticellularorganism(sporophyte)
Mitosis
Diploidmulticellularorganism
(a) Animals (b) Plants and some algae (c) Most fungi andsome protists
2n 2n
MEIOSIS FERTILIZATION
Zygote
n
Zygote
nn
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Gametes are the only haploid cells in animals
They are produced by meiosis and undergo no
further cell division before fertilization
Gametes fuse to form a diploid zygote that divides
by mitosis to develop into a multicellular organism
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Figure 13.6a
Key
Haploid (n)Diploid (2n)
Gametes
Mitosis
Diploidmulticellularorganism
(a) Animals
2n 2n
MEIOSIS FERTILIZATION
Zygote
n
nn
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Plants and some algae exhibit an alternation of
generations
This life cycle includes both a diploid and haploid
multicellular stage
The diploid organism, called the sporophyte,
makes haploid spores by meiosis
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Each spore grows by mitosis into a haploid
organism called a gametophyte
A gametophyte makes haploid gametes by mitosis
Fertilization of gametes results in a diploid
sporophyte
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Figure 13.6b
Haploid multi-cellular organism(gametophyte)
Mitosis
n n
n
2n2n
Zygote
MEIOSIS FERTILIZATION
SporesGametes
Diploidmulticellularorganism(sporophyte)
(b) Plants and some algae
n n
Key
Haploid (n)Diploid (2n)
Mitosis
Mitosis
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In most fungi and some protists, the only diploid
stage is the single-celled zygote; there is no
multicellular diploid stage
The zygote produces haploid cells by meiosis
Each haploid cell grows by mitosis into a haploid
multicellular organism
The haploid adult produces gametes by mitosis
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Figure 13.6c
Mitosis Mitosis
Haploid unicellular ormulticellular organism
Gametes
FERTILIZATIONMEIOSIS
n
2n
n
nn
n
(c) Most fungi andsome protists
Zygote
Key
Haploid (n)Diploid (2n)
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Depending on the type of life cycle, either haploid
or diploid cells can divide by mitosis
However, only diploid cells can undergo meiosis
In all three life cycles, the halving and doubling
of chromosomes contributes to genetic variation
in offspring
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Concept 13.3: 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 consecutive 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
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The Stages of Meiosis
Chromosomes duplicate during interphase
The resulting sister chromatids are closely
associated along their lengths
This is called sister chromatid cohesion
The chromatids are sorted into four haploid
daughter cells
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Figure 13.7
Interphase
Meiosis I
Meiosis II
Pair ofhomologouschromosomesin diploidparent cell
Pair of duplicatedhomologouschromosomes
Chromosomesduplicate
Diploid cell withduplicatedchromosomes
Sisterchromatids
Homologouschromosomesseparate
Haploid cells withduplicated chromosomes
Sister chromatidsseparate
Haploid cells with unduplicated chromosomes
1
2
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Figure 13.7a
Interphase
Pair ofhomologouschromosomesin diploidparent cell
Pair of duplicatedhomologouschromosomes
Chromosomesduplicate
Diploid cell withduplicatedchromosomes
Sisterchromatids
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Figure 13.7b
Meiosis I
Meiosis II
Homologouschromosomesseparate
Haploid cells withduplicated chromosomes
Sister chromatidsseparate
Haploid cells with unduplicated chromosomes
1
2
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Figure 13.8
MEIOSIS I: Separateshomologous chromosomes
Prophase I Metaphase I Anaphase ITelophase I
andCytokinesis
Prophase II Metaphase II Anaphase IITelophase II
andCytokinesis
Centrosome
(with
centriole
pair)
Centromere
(with
kineto-
chore)
Sister
chromatids
remain
attachedMeta-
phase
plate
Sister
chroma-
tidsChiasmata
Spindle
Homo-
logous
chromo-
somes Fragments
of nuclear
envelopeMicrotubules
attached to kinetochore
Homologous
chromo-
somes
separate
Cleavage
furrow
Sister
chromatids
separate
Haploid
daughter
cells
forming
MEIOSIS II: Separatessister chromatids
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Division in meiosis I occurs in four phases
Prophase I
Metaphase I
Anaphase I
Telophase I and cytokinesis
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Prophase I
In early prophase I each chromosome pairs
with its homolog and crossing over occurs
X-shaped regions called chiasmata are sites
of crossover
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Metaphase I
In metaphase I, pairs of homologs 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
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Anaphase I
In anaphase I, pairs of homologous
chromosomes separate
One chromosome of each pair moves toward
opposite poles, guided by the spindle apparatus
Sister chromatids remain attached at the
centromere and move as one unit toward the
pole
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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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Figure 13.8a
MEIOSIS I: Separateshomologous chromosomes
Prophase I Metaphase I Anaphase ITelophase I
andCytokinesis
Centrosome
(with
centriole
pair)Sister
chroma-
tidsChiasmata
Spindle
Centromere
(with
kineto-
chore)
Sister
chromatids
remain
attachedMeta-
phase
plate
Homo-
logous
chromo-
somes Fragments
of nuclear
envelopeMicrotubules
attached to kinetochore
Homologous
chromo-
somes
separate
Cleavage
furrow
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BioFlix: Meiosis
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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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Cytokinesis separates the cytoplasm
At the end of meiosis, there are four daughter
cells, each with a haploid set of unreplicated
chromosomes
Each daughter cell is genetically distinct from the
others and from the parent cell
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Figure 13.8b
MEIOSIS II: Separatessister chromatids
Prophase II Metaphase II Anaphase IITelophase II
andCytokinesis
Sister
chromatids
separate
Haploid
daughter
cells
forming
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Crossing Over and Synapsis During Prophase I
After interphase the sister chromatids are held
together by proteins called cohesins
The nonsister chromatids are broken at precisely
corresponding positions
A zipper-like structure called the synaptonemal
complex holds the homologs together tightly
DNA breaks are repaired, joining DNA from one
nonsister chromatid to the corresponding segment
of another
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Figure 13.9
DNA
breaks
CohesinsCentromere
DNA
breaks
Pair of
homologous
chromosomes:
Paternal
sister
chromatids
Maternal
sister
chromatids
Synaptonemal
complex forming
Chiasmata
Crossover Crossover
1
2 4
3
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Figure 13.9a
DNA
breaks
CohesinsCentromere
DNA
breaks
Pair of
homologous
chromosomes:
Paternal
sister
chromatids
Maternal
sister
chromatids
Synaptonemal
complex forming
1
2
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Figure 13.9b
Crossover
3
4
Crossover
Chiasmata
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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 chromosomes sets
from two (diploid) to one (haploid), producing cells
that differ genetically from each other and from the
parent cell
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Figure 13.10a
MITOSIS MEIOSIS
Prophase
Duplicated
chromosome
Metaphase
Anaphase
Telophase
2n 2nDaughter cells
of mitosis
Sister
chromatids
separate.
Individual
chromosomes
line up.
Chromosome
duplication
Chromosome
duplication2n = 6
Parent cell Chiasma MEIOSIS I
Prophase I
Homologous
chromosome
pair
Metaphase I
Anaphase ITelophase I
MEIOSIS
II
Pairs of
homologous
chromosomes
line up.
Homologs
separate.
Sister
chroma-
tids
separate.
Daughter cells of meiosis II
Daughter
cells of
meiosis I
n n n n
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Figure 13.10aa
MITOSIS MEIOSIS
MEIOSIS I
Prophase I
Metaphase I
Homo-
logous
chromosome
pair
ChiasmaParent cellProphase
Duplicated
chromosome
Metaphase
Chromosome
duplication
Chromosome
duplication
Individual
chromosomes
line up.
Pairs of
homologous
chromosomes
line up.
2n = 6
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Figure 13.10ab
MITOSIS MEIOSIS
Anaphase IAnaphase
Telophase
2n 2n
n n n nDaughter cells
of mitosis
Sister
chromatids
separate.
Homologs
separate.
Sister
chroma-
tids
separate.
Daughter cells of meiosis II
Daughter
cells of
meiosis I
MEIOSIS
II
Telophase I
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Figure 13.10b
SUMMARY
PropertyMitosis (occurs in both diploid
and haploid cells)Meiosis (can only occur in diploid cells)
DNA
replication
Number of
divisions
Synapsis of
homologous
chromosomes
Number of
daughter cells
and genetic
composition
Role in the
animal or
plant body
Occurs during interphase
before mitosis begins
One, including prophase,
prometaphase, metaphase,
anaphase, and telophase
Does not occur
Two, each genetically
identical to the parent
cell, with the same number
of chromosomes
Enables multicellular animal
or plant (gametophyte or
sporophyte) to arise from a
single cell; produces cells
for growth, repair, and, in
some species, asexual
reproduction; produces
gametes in the gametophyte
plant
Occurs during interphase before
meiosis I begins
Two, each including prophase, metaphase,
anaphase, and telophase
Occurs during prophase I along with
crossing over between nonsister
chromatids; resulting chiasmata hold
pairs together due to sister chromatid
cohesion
Four, each haploid (n); genetically different
from the parent cell and from each other
Produces gametes (in animals) or spores
(in the sporophyte plant); reduces number
of chromosomes sets by half and introduces
genetic variability among the gametes or
spores
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Figure 13.10ba
PropertyMitosis (occurs in both diploid
and haploid cells)
DNA
replication
Number of
divisions
Synapsis of
homologous
chromosomes
Number of
daughter cells
and genetic
composition
Role in the
animal or
plant body
Occurs during interphase
before mitosis begins
One, including prophase,
prometaphase, metaphase,
anaphase, and telophase
Does not occur
Two, each genetically
identical to the parent
cell, with the same number
of chromosomes
Enables multicellular animal
or plant (gametophyte or
sporophyte) to arise from a
single cell; produces cells
for growth, repair, and, in
some species, asexual
reproduction; produces
gametes in the gametophyte
plant
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Figure 13.10bb
Property
DNA
replication
Number of
divisions
Synapsis of
homologous
chromosomes
Number of
daughter cells
and genetic
composition
Role in the
animal or
plant body
Meiosis (can only occur in diploid cells)
Occurs during interphase before
meiosis I begins
Two, each including prophase, metaphase,
anaphase, and telophase
Occurs during prophase I along with
crossing over between nonsister
chromatids; resulting chiasmata hold
pairs together due to sister chromatid
cohesion
Four, each haploid (n); genetically different
from the parent cell and from each other
Produces gametes (in animals) or spores
(in the sporophyte plant); reduces number
of chromosomes sets by half and introduces
genetic variability among the gametes or
spores
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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
Separation of homologs during anaphase I
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Sister chromatid cohesion allows sister chromatids
to stay together through meiosis I
In mitosis, cohesins are cleaved at the end of
metaphase
In meiosis, cohesins are cleaved along the
chromosome arms in anaphase I (separation of
homologs) and at the centromeres in anaphase II
(separation of sister chromatids)
Meiosis I is called the reductional division because
it reduces the number of chromosomes per cell
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Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution
Mutations (changes in an organism’s DNA) are the
original source of genetic diversity
Mutations create different versions of genes called
alleles
Reshuffling of alleles during sexual reproduction
produces genetic variation
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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
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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
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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
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Figure 13.11-1
Possibility 1 Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
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Figure 13.11-2
Possibility 1 Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
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Figure 13.11-3
Possibility 1 Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1 Combination 2 Combination 3 Combination 4
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Crossing Over
Crossing over produces recombinant
chromosomes, which combine DNA inherited
from each parent
Crossing over contributes to genetic variation by
combining DNA from two parents into a single
chromosome
In humans an average of one to three crossover
events occurs per chromosome
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Figure 13.12-1
Prophase I
of meiosis
Pair of
homologs
Nonsister chromatids
held together
during synapsis
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Figure 13.12-2
Prophase I
of meiosis
Pair of
homologs
Chiasma
Centromere
TEM
Nonsister chromatids
held together
during synapsis
Synapsis and
crossing over
1
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Figure 13.12-3
Prophase I
of meiosis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Nonsister chromatids
held together
during synapsis
Synapsis and
crossing over
Movement to
the metaphase
I plate
1
2
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Figure 13.12-4
Prophase I
of meiosis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Nonsister chromatids
held together
during synapsis
Synapsis and
crossing over
Movement to
the metaphase
I plate
Breakdown of
proteins holding
sister chromatid
arms together
1
2
3
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Figure 13.12-5
Prophase I
of meiosis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Nonsister chromatids
held together
during synapsis
Synapsis and
crossing over
Movement to
the metaphase
I plate
Breakdown of
proteins holding
sister chromatid
arms together
Recombinant chromosomes
1
2
3
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Animation: Genetic Variation
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Figure 13.12a
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
Animals that always reproduce asexually are quite
rare
Organisms like the bdelloid rotifer increase their
genetic diversity through horizontal gene transfer
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Figure 13.13
200 µm
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Figure 13.UN01a
Time After Induction (hours)Average Amount of DNA per
Cell (fg)
0.0
2.0
1.0
3.0
4.0
5.0
6.0
7.0
7.5
8.0
9.0
9.5
10.0
11.0
12.0
13.0
14.0
14.0
13.0
12.5
12.0
12.5
12.0
23.5
24.0
25.0
47.5
48.0
48.0
47.5
47.0
40.0
24.0
24.0
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Figure 13.UN01b
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Figure 13.UN02
Prophase I: Each homologous pairundergoes synapsis and crossing overbetween nonsister chromatids with thesubsequent appearance of chiasmata.
Metaphase I: Chromosomes line upas homologous pairs on the metaphaseplate.
Anaphase I: Homologs separate fromeach other; sister chromatids remainjoined at the centromere.
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Figure 13.UN03
F
H
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Figure 13.UN04