Warm up
1. Compare sexual to asexual reproduction.
2. What are homologous chromosomes?
© 2014 Pearson Education, Inc.
Warm up
1. Describe what occurs during crossing over.
2. What are 3 sources of genetic variation?
3. Mitosis, Meiosis, or Both?
A. Chromosomes line up at metaphase plate
B. Crossing over
C. Cytokinesis
D. Chromosomes are replicated
E. Four haploid daughter cells result
F. Two diploid daughter cells result
© 2014 Pearson Education, Inc.
Chapter 10 Meiosis
 I can Explain:
 The differences between asexual and sexual
reproduction.
 The role of meiosis and fertilization in sexually
reproducing organisms.
 The importance of homologous chromosomes to meiosis.
 How the chromosome number is reduced from diploid to
haploid through the stages of meiosis.
 Three important differences between mitosis and meiosis.
 The importance of crossing over, independent
assortment, and random fertilization to increasing genetic
variability.
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What is it?
© 2014 Pearson Education, Inc.
Sperm and egg.
Overview: 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
© 2014 Pearson Education, Inc.
How chromosomes pass from parents to offspring
in sexual reproduction
 Meiosis- a type of cell division that produces
gametes
 Gametes- sperm and eggs
 Fertilization- union of gametes to produce a zygote
 Both meiosis and fertilization contribute go genetic
diversity
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Concept 10.1: Offspring acquire genes from
parents by inheriting chromosomes
 In a literal sense, children do not inherit particular
physical traits from their parents
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Figure 10.1
Genes: segments of DNA that
code for basic units of heredity
Offspring acquire genes from
parents by inheriting
chromosomes
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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)
© 2014 Pearson Education, Inc.
 Most DNA is packaged into chromosomes
 For example, humans have 46 chromosomes in their
somatic cells, the cells of the body except for
gametes and their precursors
 Each gene has a specific position, or locus, on a
certain chromosome
© 2014 Pearson Education, Inc.
Comparison of Asexual and Sexual Reproduction
 In asexual reproduction, a single individual passes
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 Major advantage of
sexual reproduction over asexual
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What if?
 A horticulturist breeds orchids, trying to obtain a
plant with a specific combination of traits. After may
tries she succeeds. To produce more of these plants
like this one, should she cross-breed with another
plan or make more through asexual reproduction?
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Figure 10.2
0.5 mm
Parent
Bud
(a) Hydra
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(b) Redwoods
Concept 10.2: 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
© 2014 Pearson Education, Inc.
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
 Human gametes have 23 chromosomes no pairs
© 2014 Pearson Education, Inc.
Chromosome Structure
Chromosomes also have banding patterns unique to each one.
These bands are caused by certain dyes. Chromosome
banding can help to determine homologs on a karyotype.
Each chromosome has mane alleles, or alternate forms of
genes
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Homologous Chromosomes
- each chromosome has a match,
called a homolog.
This is why normal organisms
always have an even number
of chromosomes.
One homolog you received from
your mother, the other you
received from your father. They
are not exactly alike, but they
are the same size, shape, and
have the same banding
pattern.
Chromosomes are numbered
according to their size.
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Karyotype showing
homologous pairs.
Figure 10.1b
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Sex Chromosomes
The last set of
chromosomes are the
sex chromosomes. In
humans...
XX = female
XY = male= boy
© 2014 Pearson Education, Inc.
Figure 10.3
Application
Technique
Pair of homologous
duplicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
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5 m
Figure 10.3a
Application
© 2014 Pearson Education, Inc.
Figure 10.3b
Technique
Pair of homologous
duplicated chromosomes
5 m
Centromere
Sister
chromatids
Metaphase
chromosome
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Figure 10.3c
What gender is this person?
male
© 2014 Pearson Education, Inc.
5 m
Boy or Girl?
The Y Chromosome Decides
Y - Chromosome
X - Chromosome
© 2014 Pearson Education, Inc.
24
 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)
 Diploid-full set of chromosomes (2n)
 Haploid- no pairs 1 chromosome from each set (n)
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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
© 2014 Pearson Education, Inc.
Figure 10.4
Key
2n  6
Maternal set of
chromosomes (n  3)
Paternal set of
chromosomes (n  3)
Sister chromatids
of one duplicated
chromosome
Two nonsister
chromatids in
a homologous pair
© 2014 Pearson Education, Inc.
Centromere
Pair of homologous
chromosomes
(one from each set)
 A gamete (sperm or egg) contains a single set of
chromosomes and is haploid (n)
 For humans, the haploid number is 23 (n = 23)
 Haploid-half
 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
© 2014 Pearson Education, Inc.
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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Meiosis
------ the production of haploid cells with
unpaired chromosomes - word means "to
diminish".
© 2014 Pearson Education, Inc.
Figure 10.5
Haploid gametes (n  23)
Key
Haploid (n)
Diploid (2n)
Egg (n)
Sperm (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n  46)
Mitosis and
development
Multicellular diploid
adults (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
© 2014 Pearson Education, Inc.
 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
© 2014 Pearson Education, Inc.
Figure 10.6
Key
Haploid (n)
Diploid (2n)
n
Gametes
n
Mitosis
n
n
MEIOSIS
Diploid
multicellular
organism
(a) Animals
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2n
Mitosis
Mitosis
Mitosis
n
n
Spores
FERTILIZATION
Zygote
n
n
MEIOSIS
2n
Haploid unicellular or
multicellular organism
Haploid multicellular organism
(gametophyte)
Gametes
n
n
n
Gametes
Diploid
multicellular
organism
(sporophyte)
n
FERTILIZATION
FERTILIZATION
MEIOSIS
2n
Mitosis
n
2n
Mitosis
(b) Plants and some algae
Zygote
2n
Zygote
(c) Most fungi and some protists
 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
© 2014 Pearson Education, Inc.
Figure 10.6b
Haploid multicellular organism
(gametophyte)
Mitosis
n
Key
Haploid (n)
Diploid (2n)
Mitosis
n
n
Spores
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
Gametes
FERTILIZATION
2n
Mitosis
(b) Plants and some algae
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n
n
Zygote
 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
© 2014 Pearson Education, Inc.
Figure 10.6c
Haploid unicellular or
multicellular organism
Mitosis
Key
Haploid (n)
Diploid (2n)
Mitosis
n
n
n
n
Gametes
FERTILIZATION
MEIOSIS
2n
Zygote
(c) Most fungi and some protists
© 2014 Pearson Education, Inc.
n
 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 contribute to genetic variation in
offspring
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 meiosis video
 phases of meiosis video
© 2014 Pearson Education, Inc.
Concept 10.3: 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 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
© 2014 Pearson Education, Inc.
Figure 10.7
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Chromosomes
duplicate
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Diploid cell with
duplicated
chromosomes
Meiosis I
1 Homologous
chromosomes
separate
Meiosis II
Haploid cells with
duplicated chromosomes
2 Sister chromatids
separate
Haploid cells with unduplicated chromosomes
© 2014 Pearson Education, Inc.
Interphase
Prophase I
Metaphase I
Anaphase I
Telophase (cytokinesis) I
Prophase II
Metaphase II
Anaphase II
Telophase (cytokinesis) II
This is a good time to
watch the MEIOSIS
SQUARE DANCE.
© 2014 Pearson Education, Inc.
Figure 10.7a
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
© 2014 Pearson Education, Inc.
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Figure 10.7b
Meiosis I
1 Homologous
chromosomes
separate
Meiosis II
Haploid cells with
duplicated chromosomes
2 Sister chromatids
separate
Haploid cells with unduplicated chromosomes
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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
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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
 Synapsis is the formation of a tetrad
 Tetrad-pair of homologous chromosomes joined
together in meiosis
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 In crossing over, nonsister chromatids exchange
DNA segments- contributes to genetic diversity
 Each homologous pair has one or more X-shaped
regions called chiasmata
 Chiasmata exist at points where crossing over has
occurred.
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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
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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
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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
 Cytokinesis- division of cytoplasm to separate
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
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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 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
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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
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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.
 Sister chromatid cohesion keeps sister chromatids
of a single chromosome attached during meiosis
and mitosis
 Protein complexes called cohesins are responsible
for this cohesion
 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)
© 2014 Pearson Education, Inc.
 Meiosis I is called the reductional division because it
halves the number of chromosome sets per cell from
diploid (2n) to haploid (n)
 Meiosis II is called the equational division because
the haploid cells divide to produce haploid daughter
cells
 The mechanism of sister chromatid separation in
meiosis II is identical to that in mitosis
© 2014 Pearson Education, Inc.
Concept 10.4: 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
© 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
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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 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
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 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
Animation: Genetic Variation
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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
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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
© 2014 Pearson Education, Inc.
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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 Asexual reproduction is less expensive than sexual
reproduction
 Nonetheless, sexual reproduction is nearly universal
among animals
 Overall, genetic variation is evolutionarily
advantageous
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Figure 10.12
200 m
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Figure 10.UN01
© 2014 Pearson Education, Inc.
Figure 10.UN02
Prophase I: Each homologous pair undergoes
synapsis and crossing over between nonsister
chromatids with the subsequent appearance
of chiasmata.
Metaphase I: Chromosomes line up as
homologous pairs on the metaphase plate.
Anaphase I: Homologs separate from each
other; sister chromatids remain joined at
the centromere.
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Figure 10.UN03
F
H
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© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Check for understanding
1. What phase directly follows metaphase I?
anaphase I
2. How many cells are present at the end of meiosis I ? 2
3. A cell has a diploid number of 60, what is the organism's
haploid number? 30
4. Meiosis occurs in what type of cells? gametes
5. In what phase do homologous chromosomes pair up and
crossing-over can occur?
prophase 1
6. In what phase do the CHROMATIDS separate?
anaphase 2
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7. Identify the phase:
anaphase 1
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8. Identify the phase:
anaphase 2
See also: Meiosis animation at http://www.johnkyrk.com/meiosis.html
Videos: http://www.youtube.com/watch?v=D1_-mQS_FZ0
Meiosis Square Dance at http://www.youtube.com/watch?v=eaf4j19_3Zg
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Karyotype review
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Making a karyotype – unsorted chromosomes
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22 pairs of autosomes + 1 pair of sex chromosomes
Male or female?
© 2014 Pearson Education, Inc.
Male or female?
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Cancer cells
• Some have abnormal #’s of chromosomes
Karyotype of
Metastatic
Melanoma
© 2014 Pearson Education, Inc.
Breast Cancer Cell Karyotype
© 2014 Pearson Education, Inc.
• meiosis and mitosis interactive review
© 2014 Pearson Education, Inc.
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