1. Meiosis and Sexual Life Cycles
Chapter 13:
Meiosis and Sexual Life Cycles

2. Figure 13.1
Figure 13.1 What accounts for family resemblance?

3. Asexual vs. Sexual reproduction
The Birds and the Bees
Asexual vs. Sexual reproduction

4. 0.5 mm Parent Bud (a) Hydra (b) Redwoods
Figure 13.2
0.5 mm
Parent
Bud
Figure 13.2 Asexual reproduction in two multicellular organisms.
(a) Hydra
(b) Redwoods

6. Chromosome arrangement
Somatic cells – typical body cells
46 chromosomes in 23 pairs (humans)
The chromosomes are not normally paired up
Each pair is called homologous chromosomes
Gametes – sex cells
23 chromosomes (humans)
One member from each pair

7. Homologous chromosomes
22 of the pairs (autosomes) are “true homologues”
One of each came from mom and dad
Identical in length and type of genes carried
Genes on each are slightly different
Sex chromosomes (23rd pair) don’t match up exactly (X vs Y)

8. Figure 13.3a
Figure 13.3 RESEARCH METHOD: Preparing a karyotype

9. Pair of homologous duplicated chromosomes
Figure 13.3b
5 m
Pair of homologous duplicated chromosomes
Centromere
Sister chromatids
Figure 13.3 RESEARCH METHOD: Preparing a karyotype
Metaphase chromosome

10. Maternal set of chromosomes (n  3)
Figure 13.4
Key
Key
Maternal set of chromosomes (n  3)
2n  6
Paternal set of chromosomes (n  3)
Sister chromatids of one duplicated chromosome
Centromere
Figure 13.4 Describing chromosomes.
Two nonsister chromatids in a homologous pair
Pair of homologous chromosomes (one from each set)

11. Mitosis and development Multicellular diploid adults (2n  46)
Figure 13.5
Key
Haploid (n)
Diploid (2n)
Egg (n)
Haploid gametes (n  23)
Sperm (n)
Ovary
Testis
Mitosis and development
Diploid zygote (2n  46)
Multicellular diploid adults (2n  46)
MEIOSIS
FERTILIZATION
The Human Life Cycle
Figure 13.5 The human life cycle.

12. Sexual Life Cycles Figure 13.6 Key Haploid (n)
Haploid multi- cellular organism (gametophyte)
Haploid unicellular or multicellular organism
Diploid (2n) n
Gametes n n
Mitosis n
Mitosis
Mitosis n
Mitosis n n n n n
MEIOSIS
FERTILIZATION
Spores n n
Gametes n
Gametes
MEIOSIS
FERTILIZATION
Zygote
MEIOSIS
FERTILIZATION 2n 2n 2n
Diploid multicellular organism (sporophyte) 2n
Zygote
Diploid multicellular organism 2n
Figure 13.6 Three types of sexual life cycles.
Mitosis
Mitosis
Zygote
(a) Animals (Including Humans)
(b) Plants and some algae
(c) Most fungi and some protists

13. Meiosis Reduces chromsome number Produces haploid cells
Results in four daughter cells
All are haploid
All are genetically unique from each other and from the parent
DNA replicated before meiosis begins
Two stages
Meiosis I: separates homologous chromosomes
Meiosis II: separates sister chromatids

14. How does meiosis reduce chromosome number?
2 divisions

15. Interphase Pair of homologous chromosomes in diploid parent cell
Figure
Interphase
Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number.

16. Figure
Interphase
Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Meiosis I 1
Homologous chromosomes separate
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number.
Haploid cells with duplicated chromosomes

17. Interphase Meiosis I Meiosis II
Figure
Interphase
Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Meiosis I 1
Homologous chromosomes separate
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number.
Haploid cells with duplicated chromosomes
Meiosis II 2
Sister chromatids separate
Haploid cells with unduplicated chromosomes

18. Figure 13.8 Exploring: Meiosis in an Animal Cell
MEIOSIS I: Separates homologous chromosomes
MEIOSIS I: Separates sister chromatids
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II and Cytokinesis
Centrosome (with centriole pair)
Sister chromatids remain attached
Sister chromatids
Chiasmata
Centromere (with kinetochore)
Spindle
Metaphase plate
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
Cleavage furrow
Homologous chromosomes separate
Fragments of nuclear envelope
Sister chromatids separate
Haploid daughter cells forming
Homologous chromosomes
Microtubule attached to kinetochore
Each pair of homologous chromosomes separates.
Two haploid cells form; each chromosome still consists of two sister chromatids.
Figure 13.8 Exploring: Meiosis in an Animal Cell
Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example.
Chromosomes line up by homologous pairs.

19. Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I
Figure 13.8a
Telophase I and Cytokinesis
Prophase I
Metaphase I
Anaphase I
Centrosome (with centriole pair)
Sister chromatids remain attached
Sister chromatids
Chiasmata
Centromere (with kinetochore)
Spindle
Metaphase plate
Cleavage furrow
Homologous chromosomes separate
Homologous chromosomes
Fragments of nuclear envelope
Figure 13.8 Exploring: Meiosis in an Animal Cell
Microtubule attached to kinetochore
Each pair of homologous chromosomes separates.
Two haploid cells form; each chromosome still consists of two sister chromatids.
Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example.
Chromosomes line up by homologous pairs.

20. Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II
Figure 13.8b
Telophase II and Cytokinesis
Prophase II
Metaphase II
Anaphase II
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids separate
Haploid daughter cells forming
Figure 13.8 Exploring: Meiosis in an Animal Cell

21. Chromosome duplication Chromosome duplication Duplicated chromosome
Figure 13.9a
MITOSIS
MEIOSIS
Parent cell
Chiasma
MEIOSIS I
Prophase
Prophase I
Chromosome duplication
Chromosome duplication
Duplicated chromosome
Homologous chromosome pair
2n  6
Metaphase
Metaphase I
Anaphase Telophase
Anaphase I
Telophase I
Figure 13.9 A comparison of mitosis and meiosis in diploid cells.
Daughter cells of meiosis I
Haploid n  3 2n 2n
MEIOSIS II
Daughter cells of mitosis n n n n
Daughter cells of meiosis II

22. SUMMARY Property Mitosis Meiosis
Figure 13.9b
SUMMARY
Property
Mitosis
Meiosis
DNA replication
Occurs during interphase before mitosis begins
Occurs during interphase before meiosis I begins
Number of divisions
One, including prophase, metaphase, anaphase, and telophase
Two, each including prophase, metaphase, anaphase, and telophase
Synapsis of homologous chromosomes
Does not occur
Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion
Number of daughter cells and genetic composition
Two, each diploid (2n) and genetically identical to the parent cell
Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other
Figure 13.9 A comparison of mitosis and meiosis in diploid cells.
Role in the animal body
Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

23. Sexual Reproduction leads to genetic variation!
3 processes that contribute to variation
Independent assortment of chromosomes
Crossing over
Random fertilization

24. Two equally probable arrangements of chromosomes at metaphase I
Figure
1. Independent Assortment
Possibility 1
Possibility 2
Two equally probable arrangements of chromosomes at metaphase I
Each homologous pair lines up independently of the other pairs at the Metaphase Plate.
Figure The independent assortment of homologous chromosomes in meiosis.

25. Two equally probable arrangements of chromosomes at metaphase I
Figure
Possibility 1
Possibility 2
Two equally probable arrangements of chromosomes at metaphase I
Metaphase II
Figure The independent assortment of homologous chromosomes in meiosis.

26. Two equally probable arrangements of chromosomes at metaphase I
Figure
Possibility 1
Possibility 2
Two equally probable arrangements of chromosomes at metaphase I
Metaphase II
Daughter cells
Combination 1
Combination 2
Combination 3
Combination 4
Figure The independent assortment of homologous chromosomes in meiosis.

27. 2. Crossing Over

28. Nonsister chromatids held together during synapsis
Figure
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Figure The results of crossing over during meiosis.

29. Nonsister chromatids held together during synapsis
Figure
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Figure The results of crossing over during meiosis.

30. Nonsister chromatids held together during synapsis
Figure
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis.

31. Nonsister chromatids held together during synapsis
Figure
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis.
Anaphase II

32. Nonsister chromatids held together during synapsis
Figure
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis.
Anaphase II
Daughter cells
Recombinant chromosomes

33. Which sperm ends up fertilizing which egg is “random”
3. Random fertilization
Which sperm ends up fertilizing which egg is “random”
In humans:
There are 23 pairs of chromosomes, so there are 223 combinations during independent assortment
223 =

34. Variation in Humans Independent Assortment
There are 23 pairs of chromosomes, so there are 223 possible combinations during independent assortment
223 = 8,388,608 combinations of chromosomes from one meiotic event
Random fertilization
8,388,608 x 8,388,608 = 70,368,744,177,664
That’s 70.3 TRILLION possible combinations of sperm and eggs
Crossing over
Makes the variation essentially limitless

35. Darwin vs. Mendel