1. Chapter 13
Meiosis and Sexual
Life Cycles
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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2. • Overview: Hereditary Similarity and Variation
• Living organisms
– Are distinguished by their ability to reproduce
their own kind
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3. • Heredity
– Is the transmission of traits from one generation to
the next
• Variation
– Shows that offspring differ somewhat in
appearance from parents and siblings
Figure 13.1
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4. • Genetics
– Is the scientific study of heredity and hereditary
variation
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5. • Concept 13.1: Offspring acquire genes from
parents by inheriting chromosomes
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6. Inheritance of Genes
• Genes
– Are the units of heredity
– Are segments of DNA
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7. • Each gene in an organism’s DNA
– Has a specific locus on a certain chromosome
• We inherit
– One set of chromosomes from our mother and
one set from our father
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8. Comparison of Asexual and Sexual Reproduction
• In asexual reproduction
– One parent produces genetically identical
offspring by mitosis
Parent
Bud
Figure 13.2
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0.5 mm

9. • In sexual reproduction
– Two parents give rise to offspring that have
unique combinations of genes inherited from
the two parents
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10. • 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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11. Sets of Chromosomes in Human Cells
• In humans
– Each somatic cell has 46 chromosomes, made
up of two sets
– One set of chromosomes comes from each
parent
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12. • A karyotype
– Is an ordered, visual representation of the
chromosomes in a cell
Pair of homologous
chromosomes
Centromere
Sister
chromatids
Figure 13.3
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5 µm

13. • Homologous chromosomes
– Are the two chromosomes composing a pair
– Have the same characteristics
– May also be called autosomes
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14. • Sex chromosomes
– Are distinct from each other in their
characteristics
– Are represented as X and Y
– Determine the sex of the individual, XX being
female, XY being male
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15. • A diploid cell
– Has two sets of each of its chromosomes
– In a human has 46 chromosomes (2n = 46)
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16. • In a cell in which DNA synthesis has occurred
– All the chromosomes are duplicated and thus
each consists of two identical sister chromatids
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Figure 13.4
Two nonsister
chromatids in
a homologous pair
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Pair of homologous
chromosomes
(one from each set)

17. • Unlike somatic cells
– Gametes, sperm and egg cells are haploid
cells, containing only one set of chromosomes
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18. Behavior of Chromosome Sets in the Human Life Cycle
• At sexual maturity
– The ovaries and testes produce haploid
gametes by meiosis
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19. • During fertilization
– These gametes, sperm and ovum, fuse,
forming a diploid zygote
• The zygote
– Develops into an adult organism
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20. • The human life cycle
Key
Haploid gametes (n = 23)
Haploid (n)
Diploid (2n)
Ovum (n)
Sperm
Cell (n)
FERTILIZATION
MEIOSIS
Ovary
Testis
Mitosis and
development
Figure 13.5
Multicellular diploid
adults (2n = 46)
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Diploid
zygote
(2n = 46)

21. The Variety of Sexual Life Cycles
• The three main types of sexual life cycles
– Differ in the timing of meiosis and fertilization
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22. • In animals
– Meiosis occurs during gamete formation
– Gametes are the only haploid cells
Key
Haploid
Diploid
n
n
Gametes
n
MEIOSIS
Zygote
2n
Figure 13.6 A
Diploid
multicellular
organism
FERTILIZATION
2n
Mitosis
(a) Animals
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23. • Plants and some algae
– Exhibit an alternation of generations
– The life cycle includes both diploid and haploid
multicellular stages Haploid multicellular
organism (gametophyte)
n
Mitosis
n
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
Diploid
multicellular
organism
(sporophyte)
Figure 13.6 B
2n
(b) Plants and some algae
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FERTILIZATION
2n
Mitosis
Zygote

24. • In most fungi and some protists
– Meiosis produces haploid cells that give rise to
a haploid multicellular adult organism
– The haploid adult carries out mitosis,
producing cells that will become gametes
Haploid multicellular
organism
Mitosis
n
Mitosis
n
n
n
Gametes
MEIOSIS
FERTILIZATION
2n
Figure 13.6 C
Zygote
(c) Most fungi and some protists
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n

25. • Concept 13.3: Meiosis reduces the number of
chromosome sets from diploid to haploid
• Meiosis
– Takes place in two sets of divisions, meiosis I
and meiosis II
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26. The Stages of Meiosis
• An overview of meiosis
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
1 Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Figure 13.7
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Haploid cells with unreplicated chromosomes

27. • Meiosis I
– Reduces the number of chromosomes from
diploid to haploid
• Meiosis II
– Produces four haploid daughter cells
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28. • Interphase and meiosis I
MEIOSIS I: Separates homologous chromosomes
INTERPHASE
PROPHASE I
Centrosomes
(with centriole pairs)
Sister
chromatids
Nuclear
envelope
METAPHASE I
Chiasmata
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Spindle
Metaphase
plate
Homologous
Microtubule
chromosomes
attached to
Chromatin
separate
kinetochore
Pairs of homologous
Chromosomes duplicate
Tertads line up
chromosomes split up
Homologous chromosomes
(red and blue) pair and exchange
Figure 13.8
segments; 2n = 6 in this example
Tetrad
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29. • Telophase I, cytokinesis, and meiosis II
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
Cleavage
furrow
Figure 13.8
Two haploid cells
form; chromosomes
are still double
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
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30. A Comparison of Mitosis and Meiosis
• Meiosis and mitosis can be distinguished from
mitosis
– By three events in Meiosis l
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31. • Synapsis and crossing over
– Homologous chromosomes physically connect
and exchange genetic information
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32. • Tetrads on the metaphase plate
– At metaphase I of meiosis, paired homologous
chromosomes (tetrads) are positioned on the
metaphase plates
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33. • Separation of homologues
– At anaphase I of meiosis, homologous pairs
move toward opposite poles of the cell
– In anaphase II of meiosis, the sister
chromatids separate
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34. • A comparison of mitosis and meiosis
MITOSIS
MEIOSIS
Chiasma (site of
crossing over)
Parent cell
(before chromosome replication)
MEIOSIS I
Prophase I
Prophase
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
2n = 6
Chromosomes
positioned at the
metaphase plate
Metaphase
Sister chromatids
separate during
anaphase
Anaphase
Telophase
2n
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
Figure 13.9
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Sister chromatids separate during anaphase II
n

35. • Concept 13.4: Genetic variation produced in
sexual life cycles contributes to evolution
• Reshuffling of genetic material in meiosis
– Produces genetic variation
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36. Origins of Genetic Variation Among Offspring
• In species that produce sexually
– The behavior of chromosomes during meiosis
and fertilization is responsible for most of the
variation that arises each generation
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37. Independent Assortment of Chromosomes
• Homologous pairs of chromosomes
– Orient randomly at metaphase I of meiosis
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38. • In independent assortment
–
Each pair of chromosomes sorts its maternal and paternal
homologues into daughter cells independently of the other pairs
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Figure 13.10
Combination 1
Combination 2
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Combination 3
Combination 4

39. Crossing Over
• Crossing over
–
Produces recombinant chromosomes that carry genes derived
from two different parents
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Metaphase I
Chiasma,
site of
crossing
over
Metaphase II
Daughter
cells
Figure 13.11
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Recombinant
chromosomes

40. Random Fertilization
• The fusion of gametes
– Will produce a zygote with any of about 64
trillion diploid combinations
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41. Evolutionary Significance of Genetic Variation
Within Populations
• Genetic variation
– Is the raw material for evolution by natural
selection
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42. • Mutations
– Are the original source of genetic variation
• Sexual reproduction
– Produces new combinations of variant genes,
adding more genetic diversity
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