1. Meiosis

2. meiosis Nuclear division Sexual reproduction, genetic variation
Reduces the chromosome number
Diploid-2n, haploid-n, 46, 23
Gamete – sex cell, haploid
Zygote – fused gametes, diploid

3. Karyotype 46 chromosomes 44 autosomes 2 sex chromosomes

4. Homologous chromosomes
Look alike, have similar banding pattern, same length and centromere position
One from mom, one from dad
Alleles – alternate forms of a gene, have genes for same traits, just code for differences.

5. Meiosis I and II 2 nuclear divisions, Produce 4 haploid cells
Not genetically identical
Differences from mitosis
Homologous chromosomes line up together during meiosis 1
Called synapsis, results in a bivalent (aka tetrad)
No interphase between phases

6. Genetic Variation
Crossing over – exchange of genetic material between non-sister chromatids of a bivalent (2-3 times)
Nucleoprotein lattice forms
Chiasmata – regions where non-sister chromatids are held together due to crossing over.
Results in different sequence of alleles and are different from their parents.

7. Independent assortment
Homologous chromosomes separate in a random order. (8,388,608 possible chromosomal combinations)

8. Fertilization Random… Significance of genetic variation:
In changing environments, sexual reproduction is an advantage

9. Phases of meiosis Prophase 1 Spindle fibers form, centrosomes migrate
Nuclear envelopes fragments, nucleolus gone
Chromosomes condense
Homologous chromosomes line up, synapsis forming bivalents
Crossing over occurs

10. Metaphase 1 Homologous chromosomes move to equator
Kinetochores are seen outside of centromere and connected to spindles
Bivalents independently align in middle

11. Anaphase 1
Homologous chromosomes separate from each other and move to opposite poles

12. Telophase 1 Spindle disappears
Nuclear envelope appears (not necessary by end of telophase to move on)
Cytokinesis may or may not happen during this phase

13. interkinesis
Between divisions
Like interphase with no “s” phase

14. Meiosis II
Similar to Mitosis in division except ends with 4 haploid daughter cells
Haploid cells become gametes, in plants = spores

15. Comparison of Mitosis and Meiosis
1 nuclear division
Results in 2 daughter cells
Daughter cells genetically identical to parent
Daughter cells have same # of chromosomes as parent (diploid)
2 nuclear divisions
Results in 4 daughter cells
Daughter cells not genetically identical to parent
Daughter cells have half the # of chromosomes as parent (haploid)

16. Comparison of Mitosis and Meiosis
Occurs in all tissues for growth and repair, common
Occurs only in reproductive organs and produces gametes

17. Human life cycle Gametogenesis – formation of gametes
Spermatogenesis – formation of sperm in testes, male meiosis
Oogenesis – egg production in ovary, female meiosis

18. spermatogenesis
Testes contain stem cells – spermatogonia which keep the testes supplied with primary spermatocytes (2n - 46) that undergo spermatogenesis
Primary spermatocytes form secondary spermatocytes (n- 23), which form spermatids (n-23), which differentiate into 4 viable sperm

19. oogenesis Ovaries have stem cells called oogonia
Produce primary oocytes (2n-46) during fetal development and undergoes oogenesis during puberty
Forms 1 secondary oocyte (n-23), which receives the most cytoplasm, and a polar body
Secondary oocyte begin meiosis II but stops at metaphase II. It leaves ovary and moves to oviduct where it awaits sperm.
If no sperm enters, it disintergrates.
If sperm enter oocyte, Meiosis II continues and another polar body is formed.