Cell Division Notes part 3:

1. Cell Division Notes part 3: Meiosis

2. Meiosis • Meiosis is a type of cell division that is used to produce genetically unique gametes, or sex cells • Male gametes are sperm cells; female gametes are eggs • Gametes have only half the original number of chromosomes; this is known as being haploid (1n) versus the diploid (2n) body cells • n = number of chromosomes • Example: Human diploid number is 46; human haploid number is 23

3. Meiosis results in 4 cells with half of the original chromosomes as the parent cell

4. 2 rounds of division • Meiosis requires 2 rounds of division (MI and MII) • Homologous chromosomes are separated in the first round of division • Homologous chromosomes are matching pairs • Sister chromatids are separated in the second round of division Meiosis I Meiosis II

5. Steps of Meiosis I • STEP 1: PROPHASE I • Chromatin condenses; chromosomes become visible. Nuclear membrane breaks down; centrioles move apart. • Homologous pairs come together—form tetrads. • Crossing over occurs—homologous chromosomes “swap” genetic material

6. Steps of Meiosis I • STEP 2: METAPHASE I • Centrioles are at opposite ends of cell. • Spindle fibers attach to sister chromatids at kinetochore of the centromere. • Homologous chromosomes pair up at center of cell.

7. Steps of Meiosis I • STEP 3: ANAPHASE I • Homologous pairs are pulled apart, move to opposite ends of the cell by spindle fibers. (they are still in an “X” shape)

8. Steps of Meiosis I • STEP 4: TELOPHASE I/ CYTOKINESIS I (aka – INTERKINESIS) • Cell pinches into 2 cells

9. Steps of Meiosis II • STEP 5: PROPHASE II • 2 new cells, each with half the original number of chromosomes (haploid) and sister chromatids still attached. • Centrioles double and move apart; spindle fibers form again. • No nuclear membrane.

10. Steps of Meiosis II • STEP 6: METAPHASE IICentrioles are at opposite ends of cell. • Spindle fibers attach to sister chromatids at kinetochores of the centromere. • Sister chromatids line up at center of cell.

11. Steps of Meiosis II • STEP 7: ANAPHASE II • Sister chromatids are pulled apart, move to opposite ends of cell by spindle fibers.

12. Steps of Meiosis II • STEP 8: TELOPHASE II/CYTOKINESIS II • Nuclear membrane forms around chromosomes at each end of cell. • Each of the 2 cells pinch into 2 cells, making 4 haploid gametes.

13. Meiosis Animation • Stages of Meiosis • Images of Nondisjunction in Meiosis • When chromosomes do not separate as they should

14. Genetic Variation • 3 sources of genetic variation: many possible combinations of genes in sexual reproduction 1. Crossing over: swapping of genes in Prophase I of Meiosis; homologous chromosomes trade chromosome parts

15. Genetic Variation 2. Law of independent assortment: homologous chromosomes line up randomly and randomly separate to each new cell (some of mom’s chromosomes, some of dad’s) Animation

16. Genetic Variation 3. Random fertilization: many sperm could fertilize each egg, but only one will—the possibilities for fertilization are almost endless A zygote is formed by the random union of independently-produced gametes. For humans, the number of different gametes is 223 * 223, or 8,388,6082, giving 70,368,744,177,664 (70 trillion) possible combinations!

17. Sperm vs. egg cells • Sperm cells are much smaller than egg cells, and have a tail-like cell part called a flagellum. • In humans, both sperm and egg cells begin to develop by meiosis at puberty. • Sperm form in the testes; eggs form in the ovaries.