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Slide 1
Chapter 13:
Meiosis and Sexual Life
Cycles
Figure 13.1
Slide 3
The Birds and the Bees
Asexual vs. Sexual reproduction
Figure 13.2
0.5 mm
Parent
Bud
(a) Hydra
(b) Redwoods
Slide 5
Slide 6
Chromosome arrangement
• Somatic cells – typical body cells
• 46 chromosomes in 23 pairs (humans)
• The chromosomes are not normally paired up
• Each pair is called homologous chromosomes
• Gametes – sex cells
• 23 chromosomes (humans)
• One member from each pair
Slide 7
Homologous chromosomes
• 22 of the pairs (autosomes) are “true homologues”
• One of each came from mom and dad
• Identical in length and type of genes carried
• Genes on each are slightly different
• Sex chromosomes (23rd pair) don’t match up
exactly (X vs Y)
Figure 13.3a
Figure 13.3b
Pair of homologous
duplicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
5 m
Figure 13.4
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
Centromere
Pair of homologous
chromosomes
(one from each set)
Figure 13.5
Haploid gametes (n  23)
Key
Haploid (n)
Diploid (2n)
Egg (n)
Sperm (n)
The Human
Life Cycle
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n  46)
Mitosis and
development
Multicellular diploid
adults (2n  46)
Figure 13.6
Sexual Life Cycles
Key
Haploid (n)
Diploid (2n)
n
Gametes
n
Mitosis
n
n
MEIOSIS
FERTILIZATION
n
Diploid
multicellular
organism
Zygote 2n
Mitosis
(a) Animals (Including Humans)
n
Mitosis
n
Mitosis
n
Spores
Gametes
MEIOSIS
2n
Haploid unicellular or
multicellular organism
Haploid multicellular organism
(gametophyte)
n
n
n
n
Gametes
FERTILIZATION
2n Zygote
Mitosis
(b) Plants and some algae
n
FERTILIZATION
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
Mitosis
2n
Zygote
(c) Most fungi and some protists
Slide 13
Meiosis
• Reduces chromsome number
• Produces haploid cells
• Results in four daughter cells
• All are haploid
• All are genetically unique from each other and
from the parent
• DNA replicated before meiosis begins
• Two stages
• Meiosis I: separates homologous chromosomes
• Meiosis II: separates sister chromatids
Slide 14
How does meiosis reduce chromosome
number?
2 divisions
Figure 13.7-1
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Figure 13.7-2
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Meiosis I
1
Homologous
chromosomes separate
Haploid cells with
duplicated chromosomes
Figure 13.7-3
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Meiosis I
1
Homologous
chromosomes separate
Haploid cells with
duplicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unduplicated chromosomes
Figure 13.8
MEIOSIS I: Separates sister chromatids
MEIOSIS I: Separates homologous chromosomes
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Telophase I and
Cytokinesis
Anaphase I
Duplicated homologous
chromosomes (red and blue)
pair and exchange segments;
2n  6 in this example.
Anaphase II
Telophase II and
Cytokinesis
Centromere
(with kinetochore)
Metaphase
plate
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Cleavage
furrow
Fragments
of nuclear
envelope
Metaphase II
Sister chromatids
remain attached
Spindle
Homologous
chromosomes
Prophase II
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Chromosomes line up
by homologous pairs.
Each pair of homologous
chromosomes separates.
Sister chromatids
separate
Two haploid cells
form; each chromosome
still consists of two
sister chromatids.
Haploid daughter
cells forming
Figure 13.8a
Prophase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Telophase I and
Cytokinesis
Anaphase I
Metaphase I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Cleavage
furrow
Each pair of homologous
chromosomes separates.
Chromosomes line up
Duplicated homologous
chromosomes (red and blue) by homologous pairs.
pair and exchange segments;
2n  6 in this example.
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.
Figure 13.8b
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
Haploid daughter
cells forming
Figure 13.9a
MEIOSIS
MITOSIS
Parent cell
MEIOSIS I
Chiasma
Prophase
Prophase I
Duplicated
chromosome
Chromosome
duplication
2n  6
Chromosome
duplication
Homologous
chromosome pair
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Daughter
cells of
meiosis I
2n
Daughter cells
of mitosis
Haploid
n3
MEIOSIS II
2n
n
n
n
n
Daughter cells of meiosis II
Figure 13.9b
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, 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 chromosomes
by half and introduces genetic variability among the
gametes
Slide 23
Sexual Reproduction leads to genetic
variation!
• 3 processes that contribute to variation
1. Independent assortment of chromosomes
2. Crossing over
3. Random fertilization
Figure 13.10-1
1. Independent Assortment
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Each homologous pair lines up independently of the other
pairs at the Metaphase Plate.
Figure 13.10-2
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Figure 13.10-3
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1 Combination 2
Combination 3 Combination 4
Slide 27
2. Crossing Over
Figure 13.11-1
Prophase I
of meiosis
Pair of homologs
Nonsister chromatids
held together
during synapsis
Figure 13.11-2
Prophase I
of meiosis
Pair of homologs
Chiasma
Centromere
TEM
Nonsister chromatids
held together
during synapsis
Figure 13.11-3
Prophase I
of meiosis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Nonsister chromatids
held together
during synapsis
Figure 13.11-4
Prophase I
of meiosis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Nonsister chromatids
held together
during synapsis
Figure 13.11-5
Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
Slide 33
3. Random fertilization
Which sperm ends up fertilizing which egg is “random”
• In humans:
• There are 23 pairs of chromosomes, so there are
223 combinations during independent assortment
• 223 =
Slide 34
Variation in Humans
• Independent Assortment
• There are 23 pairs of chromosomes, so there are 223
possible combinations during independent assortment
• 223 = 8,388,608 combinations of chromosomes from
one meiotic event
• Random fertilization
• 8,388,608 x 8,388,608 = 70,368,744,177,664
• That’s 70.3 TRILLION possible combinations of sperm
and eggs
• Crossing over
• Makes the variation essentially limitless
Slide 35
Darwin vs. Mendel