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Meiosis and Sexual Reproduction
Chapter 10
Impacts, Issues:
Why Sex?
 An adaptive trait tends to spread more quickly
through a sexually reproducing population than
through an asexually reproducing one
Fig. 10-1b, p. 154
10.1 Introducing Alleles
 Asexual reproduction produces genetically
identical copies of a parent (clones)
 Sexual reproduction introduces variation in the
combinations of traits among offspring
Genes and Alleles
 Genes are regions in an organism’s DNA that
encode information about heritable traits
• In sexual reproduction, pairs of genes are
inherited on pairs of chromosomes, one maternal
and one paternal
 Alleles are different forms of the same gene
• Offspring of sexual reproducers inherit new
combinations of alleles, the basis of traits
Chromosome Pair: Maternal and Paternal
10.1 Key Concepts
Sexual vs Asexual Reproduction
 In asexual reproduction, one parent transmits its
genetic information to offspring
 In sexual reproduction, offspring typically inherit
information from two parents who differ in their
alleles
 Alleles are different forms of the same gene;
they specify different versions of a trait
10.2 What Meiosis Does
 Meiosis
• A nuclear division mechanism that precedes
cytoplasmic division of immature reproductive
cells in sexually-reproducing eukaryotic species
Halving the Diploid Number
 A diploid cell has two nonidentical copies of every
chromosome (except XY sex chromosomes)
• Humans have 23 pairs of homologous
chromosomes
 Meiosis in germ cells halves the diploid number
of chromosomes (2n) to the haploid number (n),
producing haploid gametes
• Eggs and sperm have 23 unpaired chromosomes
Gamete Production
 Gametes are produced in specialized
reproductive structures or organs
Fig. 10-3a, p. 156
anther (where
sexual spores
that give rise to
sperm form)
ovules inside an
ovary (where sexual
spores that give rise
to eggs form)
a Flowering plant
Fig. 10-3a, p. 156
Fig. 10-3b, p. 156
testis
(where sperm
originate)
b Human male
Fig. 10-3b, p. 156
Fig. 10-3c, p. 156
ovary
(where eggs
develop)
c Human female
Fig. 10-3c, p. 156
Restoring the Diploid Number
 Human gametes (eggs and sperm) have 23
chromosomes – one of each homologous pair
 The diploid number (23 pairs) is restored at
fertilization, when two haploid gametes fuse and
form a diploid zygote, the first cell of a new
individual
Human Chromosomes:
Homologous Pairs
Two Divisions, Not One
 In meiosis, DNA is replicated once and divided
twice (meiosis I and meiosis II), forming four
haploid nuclei
 In meiosis I, each duplicated homologous
chromosome is separated from its partner
 In meiosis II, sister chromatids are separated
Two Divisions, Not One
 Meiosis I
 Meiosis II
each chromosome in the cell
pairs with its homologous
partner
then the partners separate
p. 157
two chromosomes
(unduplicated)
one chromosome
(duplicated)
p. 157
10.3 Visual Tour of Meiosis
10.3 Visual Tour of Meiosis
Meiosis I
newly forming
microtubules
plasma
membrane of the spindle
breakup
of nuclear
envelope
A Prophase I
one pair of
homologous
chromosomes
centrosome with
a pair of centrioles,
moving to opposite
sides of nucleus
B Metaphase I
C Anaphase I
D Telophase I
Fig. 10-5a, p. 158
Fig. 10-5a (1), p. 158
Fig. 10-5a (1), p. 158
Fig. 10-5a (2), p. 158
Fig. 10-5a (2), p. 158
Fig. 10-5a (3), p. 158
Fig. 10-5a (3), p. 158
Fig. 10-5a (4), p. 158
Fig. 10-5a (4), p. 158
Fig. 10-5b (1), p. 159
Fig. 10-5b (2), p. 159
Fig. 10-5b (3), p. 159
Fig. 10-5b (4), p. 159
Meiosis I
newly forming
microtubules
plasma
membrane of the spindle
breakup
of nuclear
envelope
A Prophase I
one pair of
homologous
chromosomes
centrosome with
a pair of centrioles, moving to
opposite sides of nucleus
B Metaphase I
C Anaphase I
D Telophase I
Stepped Art
Fig. 10-5a, p. 158
Meiosis II
There is
no DNA
replication
between the
two nuclear
divisions.
E Prophase II
F Metaphase II
G Anaphase II
H Telophase II
Stepped Art
Fig. 10-5b, p. 159
Animation: Meiosis step-by-step
10.2-10.3 Key Concepts
Stages of Meiosis
 Meiosis reduces the chromosome number
 Meiosis occurs only in cells set aside for sexual
reproduction
 Meiosis sorts a reproductive cell’s chromosomes
into four haploid nuclei, which are distributed to
descendent cells by cytoplasmic division
10.4 How Meiosis
Introduces Variation in Traits
 Crossovers and the random sorting of
chromosomes in meiosis introduce novel
combinations of alleles into gametes, resulting in
new combinations of traits among offspring
Crossing Over in Prophase I
 Crossing over
• The process by which a chromosome and its
homologous partner exchange heritable
information in corresponding segments
• Occurs during condensation in prophase I
Crossing Over
Between Homologous Chromosomes
Fig. 10-6a, p. 160
Fig. 10-6b, p. 160
A
A a
a
B
B b
b
B Here, we focus on only two
genes. One gene has alleles A and
a; the other has alleles B and b.
Fig. 10-6b, p. 160
Fig. 10-6c, p. 160
crossover
C Close contact between the
homologous chromosomes promotes
crossing over between nonsister
chromatids, so paternal and maternal
chromatids exchange segments.
Fig. 10-6c, p. 160
Fig. 10-6d, p. 160
D Crossing over mixes up
paternal and maternal
alleles on homologous
chromosomes.
A
A a
a
B
B b
b
Fig. 10-6d, p. 160
Animation: Crossing over
Segregation of
Chromosomes into Gametes
 Homologous chromosomes can be attached to
either spindle pole in prophase I, so each
homologue can be packaged into either one of
the two new nuclei
 Random assortment produces 1023 (8,388,608)
possible combinations of homologous
chromosomes
Random Assortment
A Alignment
in nucleus at
metaphase I
B Alignments C Nuclei of the
in two nuclei
four resulting
at metaphase II gametes
Possible
lineup #1
Possible
lineup #2
Possible
lineup #3
Possible
lineup #4
Fig. 10-7, p. 161
A Alignment
in nucleus at
metaphase I
B Alignments C Nuclei of the
in two nuclei
four resulting
at metaphase II gametes
Possible
lineup #1
Possible
lineup #2
Possible
lineup #3
Possible
lineup #4
Stepped Art
Fig. 10-7, p. 161
Animation: Random alignment
10.4 Key Concepts: Chromosome
Recombinations and Shufflings
 During meiosis, each pair of maternal and
paternal chromosomes swaps segments
 Then, each chromosome is randomly
segregated into one of the new nuclei
 Both processes lead to novel combinations of
alleles – and traits – among offspring
10.5 From Gametes to Offspring
 Aside from meiosis, the details of gamete
formation and fertilization differ among plants
and animals
Gamete Formation in Plants
 Sporophytes
• Diploid bodies with specialized structures that
form spores (haploid cells) that give rise to
gametophytes through mitosis
 Gametophytes
• A multicelled haploid body inside which one or
more gametes form
Life Cycles of Plants and Animals
Fig. 10-8a, p. 162
mitosis
multicelled
sporophyte
(2n)
zygote
(2n)
fertilization
DIPLOID
meiosis
HAPLOID
spores
(n)
gametes
(n)
mitosis
multicelled
gametophyte
(n)
mitosis
a Plant life cycle
Fig. 10-8a, p. 162
Fig. 10-8b, p. 162
mitosis
multicelled body
(2n)
zygote
(2n)
fertilization
DIPLOID
meiosis
HAPLOID
gametes
(n)
b Animal life cycle
Fig. 10-8b, p. 162
Gamete Formation in Animals
 Males
• Meiosis of primary spermatocytes produces four
haploid spermatids, which mature into sperm
 Females
• Meiosis of a primary oocyte forms cells of
different sizes; the secondary oocyte gets most of
the cytoplasm and matures into an ovum (egg);
other cells (polar bodies) get little cytoplasm and
degenerate
Sperm Formation in Animals
sperm (mature,
haploid male
gametes)
secondary
spermatocytes
(haploid)
diploid
male
germ cell
primary
spermatocyte
(diploid)
spermatids (haploid)
A Growth
B Meiosis I
and cytoplasmic
division
C Meiosis II and
cytoplasmic division
Fig. 10-9, p. 163
sperm
(mature,
haploid male
gametes)
secondary
spermatocytes
(haploid)
diploid male
germ cell
A Growth
primary
spermatocyte
(diploid)
B Meiosis I
and cytoplasmic
division
spermatids (haploid)
C Meiosis II and
cytoplasmic division
Stepped Art
Fig. 10-9, p. 163
Animation: Sperm formation
Egg Formation in Animals
Fig. 10-10 (left), p. 163
first polar
body
(haploid)
oogonium
(diploid
female
germ cell)
primary oocyte
(diploid)
secondary
oocyte
(haploid)
A Growth
three polar
bodies
(haploid)
B Meiosis I and
cytoplasmic division
ovum
(haploid)
C Meiosis II and
cytoplasmic division
Fig. 10-10 (left), p. 163
three
polar
bodies
first polar
(haploid)
body
(haploid)
oogonium primary oocyte
(diploid)
(diploid
female
germ cell)
A Growth B Meiosis I and
cytoplasmic
division
secondary
oocyte
(haploid)
ovum
(haploid)
C Meiosis II and
cytoplasmic division
Stepped Art
Fig. 10-10, p. 163
Animation: Egg formation
More Shufflings at Fertilization
 Chance combinations of maternal and paternal
chromosomes through fertilization produce a
unique combination of genetic information
 Fertilization
• The fusion of two haploid gametes (sperm and
egg) resulting in a diploid zygote
10.5 Key Concepts: Sexual
Reproduction in Context of Life Cycles
 Gametes form by different mechanisms in males
and females
 In most plants, spore formation and other events
intervene between meiosis and gamete
formation
10.6 Mitosis and Meiosis –
An Ancestral Connection?
 Though they have different results, mitosis and
meiosis are fundamentally similar processes
 Meiosis may have evolved by the remodeling of
existing mechanisms of mitosis
Comparing Mitosis and Meiosis
Fig. 10-11a (1), p. 165
Fig. 10-11a (2), p. 165
Fig. 10-11a (3), p. 165
Fig. 10-11b, p. 165
one diploid nucleus
Prophase
• Chromosomes
condense.
• Nuclear envelope
breaks up.
• Bipolar spindle
forms; it attaches
chromosomes to
spindle poles.
two diploid nuclei
Metaphase
• Chromosomes
align midway
between spindle
poles.
Anaphase
• Sister chromatids
separate as they
are pulled toward
spindle poles.
Telophase
• Chromosome
clusters arrive at
spindle poles.
• Chromosomes
decondense.
• New nuclear
envelopes form.
Fig. 10-11b, p. 165
Fig. 10-11c, p. 164
one diploid nucleus
Prophase I
• Chromosomes
condense.
• Homologous
chromosomes pair.
• Bipolar spindle
forms; it attaches
chromosomes to
spindle poles.
• Crossovers occur.
• Nuclear envelope
breaks up.
two haploid nuclei
Metaphase I
• Chromosomes
align midway
between spindle
poles.
Anaphase I
• Homologous
chromosomes
separate as they
are pulled toward
spindle poles.
Telophase I
• Chromosome
clusters arrive
at spindle poles.
• New nuclear
envelopes form.
• Chromosomes
decondense.
Fig. 10-11c, p. 164
Fig. 10-11d, p. 165
two haploid nuclei
Prophase II
• Chromosomes
condense.
• Bipolar spindle
forms; it attaches
chromosomes to
spindle poles.
• Nuclear envelope
breaks up.
four haploid nuclei
Metaphase II
• Chromosomes
align midway
between spindle
poles.
Anaphase II
• Sister chromatids
separate as they
are pulled toward
spindle poles.
Telophase II
• Chromosome
clusters arrive at
spindle poles.
• New nuclear
envelopes form.
• Chromosomes
decondense.
Fig. 10-11d, p. 165
10.6 Key Concepts
Mitosis and Meiosis Compared
 Meiosis may have originated by evolutionary
remodeling of mechanisms that already existed
for mitosis, and before that, for repairing
damaged DNA
Animation: Comparing mitosis and
meiosis
Animation: Generalized life cycles
Animation: Meiosis
Animation: Meiosis I and II
Animation: Reproductive organs
Video: Why sex?