1. Meiosis

2.  Two Types of Diploid Cells In Sexual Organisms Somatic GERM • Body Cells • Diploid • Divide by mitosis to make exact copies • In every tissue except reproductive tissue • “Germinating” (reproductive) cells • Diploid • (Most) Divide by meiosis to make gametes (sex cells) (sperm & ova) • In testes & ovaries

3. Mitosis vs. Meiosis

4. What is an “n”? Haploid (n) is the number of chromosomes in a gamete. Diploid (2n) – number of chromosomes in a somatic cell. You have one copy from the mother, and one copy from the Father.

5. Meiosis is part of SEXUAL Reproduction (requiring 2 parents)

6.  In Meiosis: ONEdiploid, germ cell divides to become FOUR (4) daughter cells • (*circle*) … diploid / haploid • (*circle*) … somatic / gamete

7.  In order to go from diploid to haploid, Meiosis Involves Two Divisions Meiosis I Meiosis II • Splits homologous pairs of chromosomes • Splits sister chromatidsapart

8. Time To Name The Phases • PROPHASE I • METAPHASE I • ANAPHASE I • TELOPHASE I • PROPHASE II • METAPHASE II • ANAPHASE II • TELOPHASE II

9. Make 2 Tables TABLE I

10. Make 2 Tables TABLE II

11.  PROPHASE I • Nuclear envelope disintegrates • Centrioles migrate, produce spindle fibers • Homologous chromosomes pair up • Homologous pairsCROSS OVER!

12.  PROPHASE I CROSSING OVER Occurs when homologous chromosomes exchange segments of genes

13.  METAPHASE I • Homologous pairs of chromosomes line up along metaphase plate

14.  ANAPHASE I • Homologous pairs pulled apart to opposite poles

15.  TELOPHASE I & CYTOKINESIS • Chromosomes (no longer homologous pairs!) prepare for meiosis II • Cytoplasm begins to split into first 2 (of 4) daughter cells

16.  TELOPHASE I & CYTOKINESIS NOTE: At the end of meiosis I, daughter cells are already Haploid! (because homologous pairs were separated)

17.  2 haploid daughter cells then go through MEIOSIS II In Meiosis II, 2 haploid cells will separate sister chromatids to make 4 gametes!

18.  PROPHASE II • Centrioles migrate, produce spindle fibers Just like in mitosis!

19.  METAPHASE II • Chromosomes line up along metaphase plate Just like in mitosis!

20.  ANAPHASE II • Sister chromatids pulled apart to opposite poles Just like in mitosis!

21.  TELOPHASE II & CYTOKINESIS • Nuclear envelopes reform around 4 daughter cells • Cytoplasm splits to separate the 4 daughter cells • Each of the 4 daughter cells is a new gamete withgenetic information different from either parent!

22. Is This Meiosis I or Meiosis II? Why?

23.  Is This Meiosis I or Meiosis II? Why?

24. Meiosis II Meiosis I Check – Point: At what point do the cells become haploid?

25. Mitosis v. Meiosis

26. 200 to 400 million sperm per ejaculate! But what happens to the other 3 eggs? The largest ovum absorbs the others! Only one wins per cycle, usually

27. Finally, Fertilization • Male and female gametes unite • Fusion of two haploid nuclei produces diploid nucleus • Egg + Sperm = Zygote(new life in one diploid cell)

28. Meiosis and Sexual Reproduction Guarantee Variation and Diversity!

29. Meiosis and Sexual Reproduction Guarantee Variation and Diversity! In Three Ways • Independent Assortment • Homologous Chromosomes separate randomly • Crossing Over • Homologous pairs exchange gene segments • Random Fertilization • Which sperm meets which egg? (game of chance)