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Chapter 10
Lecture
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2. 10.1 Discovery of Meiosis
• Gametes are reproductive cells (eggs and
sperm) that contain half the complement
of chromosomes found in somatic cells
 the gametes fuse to form a new cell called a
zygote, which contains two complete copies
of each chromosome
• the fusion of gametes is called fertilization, or
syngamy

3. 10.1 Meiosis
• The formation of gametes must involve
some mechanism to halve the number of
chromosomes found in somatic cells
 if not the number of chromosomes would
double with each fertilization
 meiosis is a process of reduction division in
forming gametes
• this ensures a consistent chromosome number
across generations

4. 10.1 Discovery of Meiosis
• Meiosis and fertilization
constitute a cycle of
sexual reproduction
• Somatic cells have two
sets of chromosomes
making them diploid
• Gametes have only one
set of chromosomes,
making them haploid
Figure 10.1 Diploid cells carry
chromosomes from two parents

5. 10.1 Discovery of Meiosis
• Some organisms reproduce by mitotic
division and do not involve gametes
 this is called asexual reproduction
 an example is binary fission in prokaryotes
• Other organisms are able to reproduce
both sexually and asexually
 for example, strawberry plants flower (sexual
reproduction) and send out runners (asexual
reproduction)

6. 10.2 The Sexual Life Cycle
• In sexual reproduction, haploid cells or
organisms alternate with diploid cells or
organisms
Figure 10.3 Three types of sexual life cycles

7. 10.2 The Sexual Life Cycle
• In animals, the cells that will eventually
undergo meiosis are reserved early on for
the purpose of reproduction
 these cells are referred to as germ-line cells
and are diploid like somatic cells
 but only the germ-line cells will undergo
meiosis to produce haploid gametes

8. Figure 10.4 The sexual life cycle in animals

9. 10.3 The Stages of Meiosis
• Meiosis involves two divisions, meiosis I
and meiosis II
 meiosis I separates the homologues in a
homologous pair
 DNA is replicated only before meiosis I
 meiosis II separates the replicate sister
chromatids
 when meiosis is complete, the result is that
one diploid cell has become four haploid cells

10. Figure 10.7 How meiosis works

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12. 10.3 The Stages of Meiosis
• Meiosis I is traditionally divided into four sequential
stages
1. Prophase I
• Homologues pair up and exchange segments
1. Metaphase I
• The paired homologous chromosomes align on a central plane
1. Anaphase I
• Homologues separate from the pairing and move to opposite
poles
1. Telophase I
• Individual chromosomes gather at each of the two poles

13. 10.3 The Stages of Meiosis
• During prophase I, homologous
chromosomes line up together as a pair
 crossing over occurs between two nonsister
chromatids of homologous chromosomes
• the chromatids break in the same place, and
sections of chromosomes are swapped
• the result is a hybrid chromosome
 the pairing is held together by the cohesion
between sister chromatids and the crossovers

14. Figure 10.5 Crossing over

15. 10.3 The Stages of Meiosis
• During metaphase I, the orientation of the
homologous chromosome pairs is a
matter of chance
 each possible orientation of which homologue
in a homologous pair faces which pole results
in gametes with different combinations of
parental chromosomes
 this process is called independent
assortment

16. Figure 10.6 Independent
assortment

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18. 10.3 The Stages of Meiosis
• In anaphase I, the chromosome pairs
separate and individual homologues move
to each pole
• In telophase I, the chromosomes gather at
their respective poles to form two
chromosome clusters

19. Figure 10.8 Meiosis (just Meiosis I)

20. 10.3 The Stages of Meiosis
• After meiosis I, a brief interphase occurs
where there is no replication of DNA
• Meiosis II follows and is basically a mitotic
division of the products of meiosis I
 except that the sister chromatids are non-
identical because of crossing over in meiosis I

21. 10.3 The Stages of Meiosis
• Meiosis II is also divided into four stages
1. Prophase II
• new spindle forms to attach to chromosome clusters
1. Metaphase II
• spindle fibers bind to both sides of the centromere and individual
chromosomes align along a central plane
1. Anaphase II
• sister chromatids move to opposite poles
1. Telophase II
• the nuclear envelope is reformed around each of the four sets of
daughter chromosomes

22. Figure 10.8 Meiosis (Meiosis II only)

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24. 10.4 How Meiosis Differs from
Mitosis
• Meiosis has two unique features not found
in mitosis
 synapsis
• this is the process of drawing together homologous
chromosomes down their entire lengths so that
crossing over can occur
 reduction division
• because meiosis involves two nuclear divisions but
only one replication of DNA, the final amount of
genetic material passed to the gametes is halved

25. Figure 10.9 Unique features of meiosis

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27. Figure 10.10 A comparison of
meiosis and mitosis

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29. 10.5 Evolutionary Consequences of
Sex
• Sexual reproduction has an enormous impact
on how species evolve because it rapidly
generates new genetic combinations
• Three mechanisms help produce this variety
1. Independent assortment
2. Crossing over
3. Random fertilization

30. Inquiry & Analysis
• Is there a difference
in the rate at which
microtubules are
synthesized during
interphase and
metaphase?
• When are the
microtubules of the
spindle assembled?
Are New Microtubules Made During
Cell Division?