1. AP Biology Chapter 13: Meiosis/Heredity

2. Objectives: Meiosis: Consists of two groups of cell divisions Meiosis I and Meiosis II Meiosis reduces chromosome number from diploid to haploid Mitosis vs. Meiosis Genetic Variation Why Cells Divide

3. Meiosis Meiosis is similar to Mitosis. The major distinction is that meiosis consists on two groups of divisions Meiosis I and Meiosis II In Meiosis I, homologous chromosomes pair at the metaphase plate and migrate to poles of the cell. (2n) In Meiosis II, chromosomes spread across the metaphase plate and sister chromatids separate and migrate to poles of the cell. (1n)

4. Homologous chromosomes Paternal chromosome Maternal chromosome

5. Duplicating chromosome

6. Meiotic Stages: Prophase I: Begins like prophase of mitosis. Nucleolus disappears Chromosome become visible Nuclear envelope breaks down Unlike mitosis, the homologous chromosomes pair (tetrads). The process is called synapsis

7. Condensing chromosomes

8. Homologous chromosomes Sister chromatids

9. Meiotic Stages Prophase I cont: During synapsis, corresponding regions of non-sister chromatids form a close association called chiasmata. Chiasmata are sites where genetic material is exchanged between the non-sister homologous chromatids. This is called crossing over.

10. Homologous chromosomes Non-sister chromatids Sister chromatids Site of crossing over

11. Spindle microtubules Nuclear envelope

12. Spindle microtubules

13. Homologous chromosomes Centromere

14. Meiotic Stages Metaphase I: Homologous pairs of chromosomes are spread across the metaphase plate. Microtubules extend from the poles and attach to the kinetochores of each homologous pair.

15. Spindle microtubules Homologous chromosomes

16. Meiotic Stages Anaphase I: Homologous pairs within tetrads uncouple as they are pulled to opposite poles.

17. Sister chromatids Chromosome Paternal genetic material Maternal genetic material

19. Meiotic Stages Telophase I: The chromosomes have reached their respective poles, and a nuclear membrane forms around them. Each pole of the cell will form a new nucleus that will have half the number of chromosomes, but each will contain two tetrads.

20. Daughter cell

22. Meiotic Stages Prophase II: The nuclear envelope disappears and the spindle develops. There are no chiasmata and no crossing over in prophase II

23. Daughter cell Spindle microtubules Chromosomes

24. Spindle microtubules Chromosomes

25. Meiotic Stages Metaphase II: The chromosome align along the metaphase plate. They are not arranged in tetrads as in Metaphase I. Meiosis II is similar to Mitosis

26. Sister chromatids separate

27. Meiotic Stages Anaphase II: Each chromosome is pulled apart into two chromatids by the microtubules of the spindle apparatus. The chromatids migrate to separate poles of the cell.

28. Daughter cell divides

29. Meiotic Stages Telophase II: The nuclear envelope reappears at each pole and cytokinesis occurs. The end result of telophase II is four haploid cells.

30. Four haploid cells

31. p254-255

32. Gametes

33. The Variety of Life Cycles Animal: In animals, including humans gametes are the only haploid cells. Meiosis occurs during gamete production. (production of sperm and egg cells) Fertilization produces a diploid zygote that divides by mitosis (joining of sperm and egg cells restores the 2n condition).

34. The Variety of Life Cycles In other organisms, such as plants, meiosis produces spores. Spores are haploid that divide by mitosis and become a multicellular haploid structure, the gametophyte. Gametes (“sperm and egg”) are produced by the gametophyte, since the organisms is already haploid.

35. The Variety of Life Cycles The gametes then fuse and produce a diploid cell that grows by mitosis to become the sporophyte (diploid). Sporophytes produces haploid spores, and the cycle repeats.

36. Page 252

37. Genetic Variation In mitosis, every daughter cell is exactly like the parent cell. Meiosis and sexual reproduction, however, result in a rearrangement of the genetic material. This reassortment (chiasmata), is called genetic recombination. There are 3 types of genetic recombination: There are 3 types of genetic recombination: Crossing over Crossing over Independent assortment Independent assortment Random joining Random joining

38. Crossing Over During prophase I, non- sister chromatids of homologous chromosomes exchange pieces of genetic material. As a result, each homologue no longer entirely represents a single parent p259

39. Independent Assortment During metaphase I, tetrads of homologous chromosomes separate into chromosomes that go to opposite poles. Which chromosome goes where depends on the orientation of the tetrad. This orientation is random for each tetrad. (note p258) The assortment of chromosomes is random and independent of each other.

40. p258

41. Random Joining of Gametes Fertilization – random joining Which sperm fertilizes which egg is to a large degree a random event. In many cases, however, this event may be affected by the genetic composition of a gamete. For example, some sperm may be faster swimmers and have a better chance of fertilizing the egg.

42. Sources of Genetic Variation AP Exam: The 2 main sources of genetic variation are a result of: SEXUAL REPRODUCTION MUTATION

43. Lab 3B.1 – Meiosis I Each student (or pair of students) have a set of cell outlines (3 papers) and bag of chromosome beads plus other items…spindle centriole / spindle… Using the outlines and beads… walk through Meiosis I while reading procedure… sketch as you go in the margin on the procedure… Share what your cells look like with neighboring students. Are they the same? How are they different? What caused it?

44. Meiosis II Continue with the steps of Meiosis II Sketch each step in the margin of worksheet Repeat the process with the accessory structures in your bag this time! Spindle; Centriole; blue beads are potential mutations that you can use as well