1. Sexual Reproduction and Meiosis
Chapter 11 1

2. 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
Allows offspring to inherit genetic material from 2 parents

4. 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 more diploid cells
multicellularity
Later in the life cycle, some of these diploid cells undergo meiosis to produce haploid gametes
(sperm & eggs)

6. Meiosis DNA replicates (chromosome # doubled) Meiosis I Stages:
Prophase I – synapsis occurs
Metaphase I – homologous chromosome pairs line up (4 chromosomes)
Anaphase I – homologous chromosomes seperate
Telophase I
Results in 2 cells, but still diploid number
Meiosis II
Stages
Prophase II
Metaphase II
Anaphase II – sister chromosomes seperate
Telophase II
Results in 4 cells, haploid number, one of each homologous pair in every cell

7. Features of Meiosis Meiosis includes 2 rounds of division
Meiosis I and meiosis II
Each has prophase, metaphase, anaphase, and telophase stages
Synapsis (crossing-over)
During early prophase I
Homologous chromosomes become closely associated (eventually swapping DNA)
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
Meiosis II is very much like mitosis (but half the DNA)

10. Prophase I
Chromosomes coil tighter and become visible, nuclear envelope disappears, spindle forms
Each chromosome composed of 2 sister chromatids
Synapsis (crossing-over)
Homologues become closely associated
Crossing over occurs between nonsister chromatids
Nonsister chromatids remain attached at chiasmata
Chiasmata move to the end of the chromosome arm before metaphase I

12. Crossing over Occurs between nonsister chromatids
Allows the maternal and paternal homologues to exchange chromosomal material (recombination)
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. Metaphase I
Terminal chiasmata hold homologues together following crossing over
Microtubules from opposite poles attach to each homologue (not each sister chromatid)
Homologues are aligned at the metaphase plate side-by-side
Orientation of each pair of homologues on the spindle is random
(This sets up independent assortment)

14. Many random combinations of the homologous pairing of 3 chromosomes
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Many random combinations of the homologous pairing of 3 chromosomes

15. 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 occurs

16. 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

17. Meiosis II Resembles a mitotic division
But with a haploid rather than diploid cell
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

18. Prophase II 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, removing the need for
Nuclear envelope breakdown.

19. Metaphase II 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 chromatids, as in mitosis.

20. Anaphase II When microtubules shorten
In anaphase II, sister chromatids
are pulled to opposite poles
of the cells, as in mitosis.

21. 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.

22. Final result of Meiosis
Four cells containing haploid sets of chromosomes
In animals, these cells develop directly into gametes
In Spermtogenesis: 4 sperm
In Oogenesis: 1 egg, 3 polar bodies
In plants, fungi, and many protists, haploid cells then divide mitotically
Produce greater number of gametes

23. 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

24. 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

25. Meiosis I Mitosis Metaphase I Crossovers and sister chromatid
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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.

26. Homologous chromosomes pair; synapsis and crossing over occur.
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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.

27. each containing half the original number of homologues.
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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.