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Chapter 13
Meiosis and Sexual
Life Cycles
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Hereditary Similarity and Variation
• Heredity is the transmission of traits from one
generation to the next
• Variation shows that offspring differ in appearance
from parents and siblings
• Genetics is the scientific study of heredity and
variation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Inheritance of Genes
• Genes are the units of heredity
• Genes are segments of DNA
• Locus – gene location on a chromosome
• Reproductive cells - gametes (sperm and eggs)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Comparison of Asexual and Sexual Reproduction
• Asexual reproduction, one parent produces
genetically identical offspring by mitosis
• Sexual reproduction, two parents give rise to
offspring that have unique combinations of genes
inherited from the two parents
Bud
Parent
Video: Hydra Budding
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
0.5 mm
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-3
Karyotype
Pair of homologous
Chromosomes (homologues)
Centromere
Sister
chromatids
5 µm
• The 22 pairs of chromosomes that do not
determine sex are called autosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The number of chromosomes in a single set is
represented by n
• A cell with two sets is called diploid (2n)
• For humans, the diploid number is 46 (2n = 46)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Key
After DNA Synthesis
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
• Gametes are haploid cells, containing only one
set of chromosomes
• For humans, the haploid number is 23 (n = 23)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Behavior of Chromosome Sets in the Human Life Cycle
Key
• Gametes are the
only types of human
cells produced by
meiosis, rather than
mitosis
• Fertilization, the
fusing of gametes,
restores the diploid
condition, forming a
zygote
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Haploid gametes (n = 23)
Haploid (n)
Ovum (n)
Diploid (2n)
Sperm
cell (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
LE 13-6
Key
Haploid
Diploid
n
Gametes
n
Mitosis
n
MEIOSIS
Haploid multicellular
organism (gametophyte)
n
FERTILIZATION
Diploid
multicellular
organism
Animals
Zygote 2n
Mitosis
Mitosis
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
2n
n
Haploid multicellular
organism
n
n
n
Gametes
Diploid
multicellular
organism
(sporophyte)
n
FERTILIZATION
MEIOSIS
2n
Mitosis
n
2n
Mitosis
Plants and some algae
Zygote
FERTILIZATION
2n
Zygote
Most fungi and some protists
Key
Haploid
Diploid
Haploid multicellular
organism (gametophyte)
Mitosis
n
n
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
FERTILIZATION
2n
Mitosis
Plants and some algae
Zygote
LE 13-6c
Key
Haploid
Diploid
Haploid multicellular
organism
Mitosis
Mitosis
n
n
n
n
Gametes
MEIOSIS
n
FERTILIZATION
2n
Zygote
Most fungi and some protists
Remember, only diploid
cells can undergo
meiosis.
The Stages of Meiosis
• In the first cell division (meiosis I), homologous
chromosomes separate
• Meiosis I results in two haploid daughter cells with
replicated chromosomes
• In the second cell division (meiosis II), sister
chromatids separate
• Meiosis II results in four haploid daughter cells
with unreplicated chromosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-7
Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
Sister chromatids
separate
Haploid cells with unreplicated chromosomes
LE 13-8aa
INTERPHASE
MEIOSIS I: Separates homologous chromosomes
METAPHASE I
Centrosomes
(with centriole pairs)
Chromatin
Chromosomes duplicate
Nuclear
envelope
ANAPHASE I
• Division in meiosis I occurs in four phases:
Prophase I
Metaphase I
Anaphase I
Telophase I
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Prophase I Terminology
• Synapsis, homologous chromosomes loosely pair
up, aligned gene by gene
• Crossing over, non-sister chromatids exchange
DNA segments
• Tetrad, a group of four chromatids
• Chiasmata, X-shaped regions where crossing
over occurred
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-8ab
MEIOSIS I: Separates homologous chromosomes
METAPHASE I
PROPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Sister
chromatids
Chiasmata
Metaphase
plate
Spindle
Tetrad
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
in this example
Microtubule
attached to
kinetochore
Tetrads line up
Homologous
chromosomes
separate
Pairs of homologous
chromosomes split up
Telophase I and Cytokinesis
• Cytokinesis usually occurs simultaneously,
forming two haploid daughter cells
– 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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Animation:
Telophase I and Cytokinesis
LE 13-8b
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
Two haploid cells
form; chromosomes
are still double
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
LE 13-9
MITOSIS
MEIOSIS
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Propase
Prophase I
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
2n = 6
Chromosomes
positioned at the
metaphase plate
Metaphase
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
Tetrad formed by
synapsis of homologous
chromosomes
Tetrads
positioned at the
metaphase plate
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Metaphase I
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II
n
Property
Mitosis
Meiosis
DNA
replication
Divisions
During
interphase
One
During
interphase
Two
Synapsis and
crossing over
Daughter cells,
genetic
composition
Do not occur
Role in animal
body
Produces cells
for growth and
tissue repair
Form tetrads in
prophase I
Four haploid,
different from
parent cell and
each other
Produces
gametes
Two diploid,
identical to
parent cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 13.4: Genetic variation produced in
sexual life cycles contributes to evolution
Sources of Genetic Variation/Diversity
• Mutations (changes in an organism’s DNA) are
the original source of genetic diversity
• Reshuffling of different versions of genes during
sexual reproduction produces genetic variation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Origins of Genetic Variation Among Offspring
• Three mechanisms contribute to genetic
variation in offspring:
– Independent assortment of chromosomes
– Crossing over
– Random fertilization
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-10
Independent Assortment
Key
Maternal set of
chromosomes
Possibility 2
Possibility 1
Paternal set of
chromosomes
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
LE 13-11
Nonsister
chromatids
Prophase I
of meiosis
Crossing Over
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes
Random Fertilization
• Random fertilization adds to genetic variation
because any sperm can fuse with any ovum
(unfertilized egg)
• The fusion of gametes produces a zygote with any
of about 64 trillion diploid combinations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evolutionary Significance of Genetic Variation
Within Populations
• Natural selection results in accumulation of
genetic variations favored by the environment
• Sexual reproduction contributes to the
genetic variation in a population, which
ultimately results from mutations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings