Biology 201 Chapter 11

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1
Chapter 11
Lecture and
Animation Outline

2. Sexual Reproduction
and Meiosis
Chapter 11
2

3. Sexual life cycle
• Made up of meiosis and fertilization
• Diploid cells
– Somatic cells of adults have 2 sets of
chromosomes
• Haploid cells
– Gametes have only 1 set of chromosomes
• Offspring inherit genetic material from 2
parents
3

4. 4
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Haploid sperm
Haploid egg
Diploid zygote
Fertilization
Paternal
homologue
Maternal
homologue

5. 5
• Life cycles of sexually reproducing organisms
involve the alternation of haploid and diploid
stages
• Some life cycles include longer diploid phases,
some include longer haploid phases
• In most animals, diploid state dominates
– Zygote first undergoes mitosis to produce diploid cells
– Later in the life cycle, some of these diploid cells
undergo meiosis to produce haploid gametes

6. 6
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n
2n
FERTILIZAOTINMEIOSIS
MEIOSIS
Sperm
(haploid) n
Egg
(haploid) n
Zygote
(diploid) 2n
Somatic
cells
Germ-line
cells
Adult male
(diploid) 2n
Adult female
(diploid) 2n
MITOSIS
MITOSIS
Germ-line
cells

7. 7
Features of Meiosis
• Meiosis includes two rounds of division
– Meiosis I and meiosis II
– Each has prophase, metaphase, anaphase, and
telophase stages
• Synapsis
– During early prophase I
– Homologous chromosomes become closely
associated
– Includes formation of synaptonemal complexes
• Formation also called tetrad or bivalents

8. 8
• First meiotic division is termed the “reduction
division”
– Results in daughter cells that contain one homologue
from each chromosome pair
• No DNA replication between meiotic divisions
• Second meiotic division does not further reduce
the number of chromosomes
– Separates the sister chromatids for each homologue

9. The Process of Meiosis
• Meiosis I
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I
• Meiosis II
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II
• Meiotic cells have an
interphase period that
is similar to mitosis
with G1, S, and G2
phases
• After interphase,
germ-line cells enter
meiosis I
9

10. 10
Kinetochore
Sister chromatids
Homologues
a.
Centromere
Synaptonemal
complex
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11. Prophase I
• Chromosomes coil tighter and become visible,
nuclear envelope disappears, spindle forms
• Each chromosome composed of 2 sister
chromatids
• Synapsis
– Homologues become closely associated
– Crossing over occurs between nonsister chromatids
– Remain attached at chiasmata
• Chiasmata move to the end of the chromosome
arm before metaphase I
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12. 12
Crossing over
• Genetic recombination between nonsister
chromatids
• Allows the homologues to exchange
chromosomal material
• Alleles of genes that were formerly on
separate homologues can now be found
on the same homologue
• Chiasmata – site of crossing over
– Contact maintained until anaphase I

13. 13
Crossing over
Cohesin proteins
Prophase I Metaphase I Anaphase I
Site of crossover
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14. 14
Chromosome (replicated)
Spindle
Chiasmata
In prophase I, the chromosomes
begin to condense, and the
spindle of microtubules begins
to form. The DNA has been
replicated, and each
chromosome consists of two
sister chromatids attached at
the centromere. The cell
illustrated here has four
chromosomes, or two pairs of
homologues. Homologous
chromosomes pair along their
entire length during synapsis.
Crossing over occurs, forming
chiasmata, which hold
homologous chromosomes
together.
Paired homologous
chromosomes
Sister
chromatids
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15. 15
Metaphase I
• Terminal chiasmata hold homologues
together following crossing over
• Microtubules from opposite poles attach to
each homologue
– Not each sister chromatid as in mitosis
• Homologues are aligned at the metaphase
plate side-by-side
• Orientation of each pair of homologues on
the spindle is random

16. 16
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17. 17
Kinetochore microtubule
In metaphase I, the pairs of
homologous chromosomes
align at the equator of the cell.
Chiasmata keep homologous
pairs together and microtubules
from opposite poles attach to
sister kinetochores of each
homologue, producing tension.
A kinetochore microtubule from
one pole of the cell attaches to
one homologue of a
chromosome, while a
kinetochore microtubule from
the other cell pole attaches to
the other homologue of a pair.
Homologue pair
on metaphase plate
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18. 18
Anaphase I
• Microtubules of the spindle shorten
– Chiasmata break
• Homologues are separated from each other and
move to opposite poles
– Sister chromatids remain attached to each other at
their centromeres
• Each pole has a complete haploid set of
chromosomes consisting of one member of each
homologous pair
• Independent assortment of maternal and
paternal chromosomes

19. 19
Homologous chromosomes
Sister chromatids
In anaphase I, kinetochore
microtubules shorten, and
homologous pairs are pulled
apart. One duplicated homologue
goes to one pole of the cell, and
the other duplicated homologue
goes to the other pole. Sister
chromatids do not separate. This
is in contrast to mitosis, in which
duplicated homologues line up
individually on the metaphase
plate, and sister chromatids are
pulled apart in anaphase.
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20. 20
Telophase I
• Nuclear envelope re-forms around each
daughter nucleus
• Sister chromatids are no longer identical
because of crossing over (prophase I)
• Cytokinesis may or may not occur after
telophase I
• Meiosis II occurs after an interval of
variable length

21. 21
Chromosome
Nonidentical sister chromatids
In telophase I, the separated
homologues form a cluster at
each pole of the cell, and the
nuclear envelope re-forms
around each daughter cell
nucleus. Cytokinesis may occur.
The resulting two cells have
half the number of
chromosomes of the original
cell: In this example, each
nucleus contains two
chromosomes (versus four in
the original cell). Each
chromosome consists of two
sister chromatids, but sister
chromatids are not identical
because crossing over has
occurred.
Homologous
chromosomes
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22. 22
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23. 23
Meiosis II
• Resembles a mitotic division
• Prophase II: nuclear envelopes dissolve
and new spindle apparatus forms
• Metaphase II: chromosomes align on
metaphase plate
• Anaphase II: sister chromatids are
separated from each other
• Telophase II: nuclear envelope re-forms
around 4 sets of daughter chromosomes;
cytokinesis follows

24. 24
Spindle
Nuclear membrane breaking down
Following a brief interphase,
with no S phase, meiosis II
begins. During prophase II,
a new spindle apparatus forms
in each cell, and the nuclear
envelope breaks down. In some
species the nuclear envelope
does not re-form in telophase I,
obviating the need for nuclear
envelope breakdown.
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25. 25
Sister chromatids
Chromosome
In metaphase II, chromosomes
consisting of sister chromatids
joined at the centromere align
along the metaphase plate in
each cell. Now, kinetochore
microtubules from opposite
poles attach to kinetochores of
sister chromatid, as in mitosis.
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26. 26
Sister chromatids
When microtubules shorten in
anaphase II, sister chromatids
are pulled to opposite poles
of the cells, as in mitosis.
Kinetochore
microtubule
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27. 27
In telophase II, the nuclear
membranes re-form around
four different clusters of
chromosomes. After
cytokinesis, four haploid cells
result. No two cells are alike
due to the random alignment of
homologous pairs at
metaphase I and crossing over
during prophase I.
Nuclear
membrane
re-forming
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28. Final result
• Four cells containing haploid sets of
chromosomes
• In animals, develop directly into gametes
• In plants, fungi, and many protists, divide
mitotically
– Produce greater number of gametes
– Adults with varying numbers of gametes
28

29. 29
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30. Errors in Meiosis
• Nondisjunction – failure of chromosomes
to move to opposite poles during either
meiotic division
• Aneuploid gametes – gametes with
missing or extra chromosomes
• Most common cause of spontaneous
abortion in humans
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31. 31
Meiosis vs. Mitosis
Meiosis is characterized by 4 features:
1. Synapsis and crossing over
2. Sister chromatids remain joined at their
centromeres throughout meiosis I
3. Kinetochores of sister chromatids attach
to the same pole in meiosis I
4. DNA replication is suppressed between
meiosis I and meiosis II

32. 32
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Meiosis I Mitosis
Metaphase I
Anaphase I
Metaphase
Anaphase
Crossovers and
sister chromatid
cohesion lock
homologues
together.
Microtubules
connect to the
kinetochores of
sister chromatids so
that homologues are
pulled toward
opposite poles.
Microtubules pull
the homologous
chromosomes
apart, but sister
chromatids are
held together at
the centromere.
Homologues do
not pair;
kinetochores of
sister chromatids
remain separate;
microtubules
attach to both
kinetochores on
opposite sides of
the centromere.
Microtubules
pull sister
chromatids
apart.

33. 33
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Parent cell (2n)
MEIOSIS I
Prophase I Metaphase I Anaphase I Telophase I
Prophase Metaphase Anaphase Telophase
Homologous chromosomes
do not pair.
Individual homologues align
on metaphase plate.
Paternal
homologue
Homologous
chromosomes
Homologous chromosomes pair;
synapsis and crossing over occur.
Paired homologous chromosomes
align on metaphase plate.
Maternal
homologue
MITOSIS
Sister chromatids separate, cytokinesis occurs, and two cells
result, each containing the original number of homologues.
Two
daughter
cells
(each 2n)
Homologous chromosomes separate;
sister chromatids remain together.
Chromosome
replication
Chromosome
replication

34. 34
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MEIOSIS II
Prophase II Metaphase II Anaphase II Telophase II
Chromosomes align, sister chromatids separate, and four haploid cells result,
each containing half the original number of homologues.
Four
daughter
cells
(each n)