1. Chapter 11 Lecture Outline
See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes and animations. 1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

4. Haploid sperm Paternal homologue Fertilization Maternal homologue
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Haploid sperm
Paternal
homologue
Fertilization
Maternal
homologue
Diploid zygote
Haploid egg

5. In most animals, diploid state dominates
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. Sperm (haploid) n Egg (haploid) n n 2n Zygote (diploid) 2n MITOSIS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sperm
(haploid) n
Egg
(haploid) n
MEIOSIS
FERTILIZAOTIN
MEIOSIS n 2n
Zygote
(diploid) 2n
MITOSIS
Somatic
cells
Germ-line
cells
Germ-line
cells
MITOSIS
Adult male
(diploid) 2n
Adult female
(diploid) 2n

7. Features of Meiosis Meiosis includes two rounds of division Synapsis
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. 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

10. a. Kinetochore Sister chromatids Synaptonemal complex Homologues
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Kinetochore
Sister chromatids
Synaptonemal
complex
Homologues
Centromere a.

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

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. Crossing over Cohesin proteins Site of crossover Prophase I
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cohesin proteins
Site of crossover
Prophase I
Metaphase I
Anaphase I

14. Sister chromatids Paired homologous chromosomes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chromosome (replicated)
Spindle
Sister
chromatids
Paired homologous
chromosomes
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.

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. Copyright © The McGraw-Hill Companies, Inc
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

17. Homologue pair on metaphase plate
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Kinetochore microtubule
Homologue pair
on metaphase plate
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.

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. Homologous chromosomes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sister chromatids
Homologous chromosomes
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.

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. Homologous chromosomes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nonidentical sister chromatids
Chromosome
Homologous
chromosomes
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.

22. 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

23. Nuclear membrane breaking down
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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.

24. In metaphase II, chromosomes consisting of sister chromatids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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.

25. Kinetochore microtubule
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Kinetochore
microtubule
Sister chromatids
When microtubules shorten in
anaphase II, sister chromatids
are pulled to opposite poles
of the cells, as in mitosis.

26. Nuclear membrane re-forming
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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

27. 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. 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

29. Meiosis vs. Mitosis Meiosis is characterized by 4 features:
Synapsis and crossing over
Sister chromatids remain joined at their centromeres throughout meiosis I
Kinetochores of sister chromatids attach to the same pole in meiosis I
DNA replication is suppressed between meiosis I and meiosis II

30. Meiosis I Mitosis Metaphase I Crossovers and sister chromatid
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Meiosis I
Mitosis
Metaphase I
Crossovers and
sister chromatid
cohesion lock
homologues
together.
Microtubules
connect to the
kinetochores of
sister chromatids so
that homologues are
pulled toward
opposite poles.
Metaphase
Homologues do
not pair;
kinetochores of
sister chromatids
remain separate;
microtubules
attach to both
kinetochores on
opposite sides of
the centromere.
Anaphase I
Anaphase
Microtubules pull
the homologous
chromosomes
apart, but sister
chromatids are
held together at
the centromere.
Microtubules
pull sister
chromatids
apart.

31. Homologous chromosomes pair; synapsis and crossing over occur.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
MEIOSIS I
Prophase I
Metaphase I
Anaphase I
Telophase I
Parent cell (2n)
Paternal
homologue
Chromosome
replication
Homologous chromosomes pair;
synapsis and crossing over occur.
Paired homologous chromosomes
align on metaphase plate.
Homologous chromosomes separate;
sister chromatids remain together.
MITOSIS
Prophase
Metaphase
Anaphase T
elophase
Homologous
chromosomes
Chromosome
replication
Maternal
homologue
Two
daughter
cells
(each 2n)
Homologous chromosomes
do not pair.
Individual homologues align
on metaphase plate.
Sister chromatids separate, cytokinesis occurs, and two cells result, each containing the original number of homologues.

32. each containing half the original number of homologues.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
MEIOSIS II
Prophase II
Metaphase II
Anaphase II
Telophase II
Four
daughter
cells
(each n)
Chromosomes align, sister chromatids separate, and four haploid cells result,
each containing half the original number of homologues.