1. Cell Division
Part 2

2. Sexual Reproduction
Most common way for eukaryotic organisms to produce offspring
Organisms make gametes or spores with one set of all genes
Two gametes fuse during fertilization to generate a new organism - zygote
Spores germinate to develop either a gametophyte (plants) or new haploid organism (fungi)

3. Gametes Simple eukaryotes are isogamous
gametes are morphologically similar
Most eukaryotes are heterogamous
gametes are morphologically different
Sperm
small and motile
Oocytes or ova (Eggs)
large and nonmotile
Store nutrients & developmental information
Microspores (Pollen)
Macrospores (Ovules)

4. Meiosis Produces Haploid Gametes
Gametogenesis or Sporulation
Chromosome number reduced to ½ original
2n  n
give a single set of all parental chromosomes
Chromosomes sorted to assure each chromosome (& its genes) is represented

5. genetically identical genetically distinct
Mitosis vs Meiosis
Makes 2
genetically identical
diploid daughter cells
Makes 1-4
genetically distinct
haploid gametes

6. Meiosis After a cell has gone through G1, S, & G2
Two successive divisions
Meiosis I & II
Each subdivided into
Prophase (I & II)
Metaphase (I & II)
Anaphase (I & II)
Telophase (I & II)

7. Meiosis Prophase I subdivided into Leptotena Zygotena Pachytena
Diplotena
Diakinesis

8. Periods of Prophase I Figure 3.11 A total of 4 chromatids
tetrads
A total of 4
chromatids
Figure 3.11
A recognition
process

9. Synaptonemal Complex

10. Periods of Prophase I 2 bivalents A tetrad
A physical exchange of chromosome pieces

11. Stages of Meiosis I

12. Tetrads organized along the metaphase plate
Homologous chromosomes aligned side by side (tetrads)
One homolog linked to one pole
The other homolog is linked to the opposite pole
Assortment of homologues is independent
Figure 3.13

13. Stages of Meiosis I Homologous pairs of sister chromatids separate
Centromeres remain between sister chromatid pairs

14. Meiosis Telophase I of meiosis I is followed by meiosis II
Each daughter cell has only one member of each homologous pair
Each homolog = 2 recombinant sister chromatids
genetic content is still 2n
Meiosis II reduces genetic content to n

15. 1 of each type of chromosome (n) in each gamete
Stages of Meiosis II
1 of each type of chromosome (n) in each gamete

16. Spermatogenesis Production of sperm In testes
Diploid spermatogonium cell divides mitotically  two cells
One remains a spermatogonial cell
Other becomes 1o spermatocyte
1o spermatocyte goes through meiosis I and II

17. Spermatogenesis Figure 3.14 (a) Meiois II  four haploid spermatids
Meiois I  two 2 spermatocytes
Figure 3.14 (a)

18. Oogenesis Production of egg cells (ova) In ovaries
Early in development, diploid oogonia produce diploid 1o oocytes
In humans ~ 106 1o occytes/ovary produced before birth

19. Mammalian Oogenesis 1o oocytes initiate meiosis I
Then stop in prophase I until sexual maturity (puberty)
At puberty, 1o oocytes are activated & progress through meiosis I
Cell division is asymmetric 
1 large 2o oocyte
1 small polar body

20. 2o oocyte enters meiosis II arrests in metaphase II until ovulation
Mammalian Oogenesis
2o oocyte enters meiosis II arrests in metaphase II until ovulation
If 2o oocyte is fertilized meiosis II is completed
A haploid egg & a 2nd polar body produced
haploid egg & sperm nuclei fuse to create a diploid zygote

21. Cell divisions in oogenesis is asymmetric
Figure 3.14 (b)

22. y r r y Y
Heterozygous diploid
cell (YyRr) undergoing meiosis Y R R y Y y Y R r r R
Meiosis I y y y Y Y y Y Y R R r r r r R R
Meiosis II y y y y Y Y Y Y R R r r r r R R Ry : rY ry : RY

23. XY system 3-78 Humans have 46 chromosomes
44 autosomes
2 sex chromosomes
Males contain one X & one Y chromosome
termed heterogametic
Females have two X chromosomes
termed homogametic
The Y chromosome determines maleness
3-78

24. Sitting on the toilet reading (SIT)

25. Real Y Chromosome Map

26. Normal Human Karyotypes
Female 46, XX
Male 46, XY

27. Aneuploidies of the Sex Chromosomes
47, XXY
45, X
Klinefelter syndrome
Turner syndrome

28. ZW System sex chromosomes designated Z & W
Males have two Z chromosomes
are homogametic
Females have Z & W chromosomes
are heterogametic
and some fish

29. XO System In some invertebrates (insects, nematodes)
Males are XO and females are XX
In some insects (fruit fly)
Males are XY and females are XX
Y chromosome does not determines maleness
Sex determined by the ratio of the # of X chromosomes to the # of autosome pairs (X/A)
If X/A =< 0.5, fly is male
If X/A = >1.0, fly is female

30. Haplo-Diplo System 3-81 Bees and some relatives Males are haploid
Known as drones
Produced from unfertilized eggs
Females are diploid
Include worker bees and queen bees
Produced from fertilized eggs
3-81