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2. Resources Chapter Presentation Visual Concepts Transparencies Standardized Test Prep

3. Meiosis and Sexual Reproduction Chapter 7 Table of Contents Section 1 Meiosis Section 2 Sexual Reproduction

4. Chapter 7 Section 1 Nature of Matter Objectives • Summarizethe events that occur during meiosis. • Relatecrossing-over, independent assortment, and random fertilization to genetic variation. • Comparespermatogenesis and oogenesis.

5. Chapter 7 Section 1 Nature of Matter Formation of Haploid Cells • Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells, such as gametes or spores. • Meiosis involves two divisions of the nucleus—meiosis I and meiosis II. • Before meiosis begins, the DNA in the original cell is replicated. Thus, meiosis starts with homologous chromosomes.

6. Chapter 7 Section 1 Nature of Matter Formation of Haploid Cells, continued • The eight stages of meiosis are: Prophase I The nuclear envelope breaks down. Homologous chromosomes pair. Crossing-over occurs when portions of a chromatid on one homologous chromosome are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome. Metaphase I The pairs of homologous chromosomes are moved by the spindle to the equator of the cell. Anaphase I The chromosomes of each pair are pulled to opposite poles of the cell by the spindle fibers.

7. Chapter 7 Section 1 Nature of Matter Formation of Haploid Cells, continued Telophase I Individual chromosomes gather at each of the poles. In most organisms, the cytokinesis occurs. Prophase II A new spindle forms around the chromosomes. Metaphase II The chromosomes line up along the equator and are attached at their centromeres to spindle fibers. Anaphase II The centromeres divide, and the chromatids (now called chromosomes) move to opposite poles of the cell. Telophase II A nuclear envelope forms around each set of chromosomes, and the cell undergoes cytokinesis.

8. Chapter 7 Section 1 Nature of Matter Stages of Meiosis

9. Chapter 7 Section 1 Nature of Matter Meiosis

10. Chapter 7 Section 1 Nature of Matter Comparing Meiosis and Mitosis

11. Chapter 7 Section 1 Nature of Matter Comparing the Results of Meiosis and Mitosis

12. Chapter 7 Section 1 Nature of Matter Meiosis and Genetic Variation • Meiosis is an important process that allows for the rapid generation of new genetic combinations. • Three mechanisms make key contributions to this genetic variation: 1. independent assortment 2. crossing-over 3. random fertilization

13. Chapter 7 Section 1 Nature of Matter Meiosis and Genetic Variation, continued Independent Assortment The random distribution of homologous chromosomes during meiosis is called independent assortment.

14. Chapter 7 Section 1 Nature of Matter Independent Assortment

15. Chapter 7 Section 1 Nature of Matter Meiosis and Genetic Variation, continued Crossing-Over and Random Fertilization • The DNA exchange that occurs during crossing-over adds even more recombination to the independent assortment of chromosomes that occurs later in meiosis. • Thus, the number of genetic combinations that can occur among gametes is practically unlimited. • Furthermore, the zygote that forms a new individual is created by the random joining of two gametes.

16. Chapter 7 Section 1 Nature of Matter Crossing-Over of Chromosomes

17. Chapter 7 Section 1 Nature of Matter Tetrads and Crossing Over of Genetic Material

18. Chapter 7 Section 1 Nature of Matter Meiosis and Genetic Variation, continued Importance of Genetic Variation • Meiosis and the joining of gametes are essential to evolution. No genetic process generates variation more quickly. • The pace of evolution is sped up by genetic recombination. The combination of genes from two organisms results in a third type, not identical to either parent.

19. Chapter 7 Section 1 Nature of Matter Meiosis and Gamete Formation Meiosis in Males • The process by which sperm are produced in male animals is called spermatogenesis. • Spermatogenesis occurs in the testes (male reproductive organs), and produces male gametes called sperm.

20. Chapter 7 Section 1 Nature of Matter Formation of Sperm

21. Chapter 7 Section 1 Nature of Matter Meiosis and Gamete Formation, continued Meiosis in Females • The process by which gametes are produced in female animals is called oogenesis. • Oogenesis occurs in the ovaries (female reproductive organs) and produces female gametes called ova.

22. Chapter 7 Section 1 Nature of Matter Formation of the Egg Cell

23. Chapter 7 Section 1 Nature of Matter Meiosis in Male and Female Animals

24. Chapter 7 Section 2 Sexual Reproduction Objectives • Differentiatebetween asexual and sexual reproduction. • Identifythree types of asexual reproduction. • Evaluatethe relative genetic and evolutionary advantages and disadvantages of asexual and sexual reproduction. • Differentiatebetween the three major sexual life cycles found in eukaryotes.

25. Chapter 7 Section 2 Sexual Reproduction Sexual and Asexual Reproduction • In asexual reproduction a single parent passes copies of all of its genes to each of its offspring; there is no fusion of haploid cells such as gametes. • An individual produced by asexual reproduction is a clone, an organism that is genetically identical to its parent. • In contrast, in sexual reproduction two parents each form reproductive cells that have one-half the number of chromosomes.

26. Chapter 7 Section 2 Sexual Reproduction Sexual Reproduction

27. Chapter 7 Section 2 Sexual Reproduction Sexual and Asexual Reproduction, continued Types of Asexual Reproduction • During fission, a parent separates into two or more individuals of about equal size. • Some multicellular eukaryotes undergo fragmentation, a type of reproduction in which the body breaks into several pieces. • Other organisms undergo budding, in which new individuals split off from existing ones.

28. Chapter 7 Section 2 Sexual Reproduction Sexual and Asexual Reproduction, continued Genetic Diversity • Asexual reproduction allows organisms to produce many offspring in a short period of time, without using energy to produce gametes or to find a mate. However, the DNA of these organisms varies little between individuals. • Sexual reproduction provides a powerful means of quickly making different combinations of genes among individuals. Such genetic diversity is the raw material for evolution.

29. Chapter 7 Section 2 Sexual Reproduction Sexual and Asexual Reproduction, continued Evolution of Sexual Reproduction • Only diploid cells can repair certain kinds of chromosome damage, such as breaks in both strands of DNA. • Thus the process of meiosis and the pairing of homologous chromosomes may have allowed early protistan cells to repair damaged DNA.

30. Chapter 7 Section 2 Sexual Reproduction Sexual Life Cycles in Eukaryotes • The entire span in the life of an organism from one generation to the next is called a life cycle. • The life cycles of all sexually reproducing organisms follows a basic pattern of alternation between the diploid and haploid chromosome numbers. • Eukaryotes that undergo sexual reproduction can have one of three types of sexual life cycles: haploid, diploid, or alternation of generations.

31. Chapter 7 Section 2 Sexual Reproduction Sexual Life Cycles in Eukaryotes, continued Haploid Life Cycle • The haploid life cycle is the simplest of sexual life cycles. • In this life cycle, haploid cells occupy the major portion of the life cycle. • This type of life cycle is found in many protists, as well as in some fungi and algae.

32. Chapter 7 Section 2 Sexual Reproduction Haploid Life Cycle

33. Chapter 7 Section 2 Sexual Reproduction Sexual Life Cycles in Eukaryotes, continued Diploid Life Cycle • The outstanding characteristic of the diploid life cycle is that adult individuals are diploid, each individual inheriting chromosomes from two parents. • In most animals, including humans, a diploid reproductive cell undergoes meiosis to produce gametes. • The gametes join in a process called fertilization, which results in a diploid zygote.

34. Chapter 7 Section 2 Sexual Reproduction Diploid Life Cycle

35. Chapter 7 Section 2 Sexual Reproduction Fertilization

36. Chapter 7 Section 2 Sexual Reproduction Sexual Life Cycles in Eukaryotes, continued Alternation of Generations • Plants, algae, and some protists have a life cycle that regularly alternates between a haploid phase and a diploid phase. • In plants, the diploid phase in the life cycle that produces spores is called asporophyte. • Spore-forming cells in the sporophyte undergo meiosis to produce spores, haploid reproductive cell produced by meiosis that is capable of developing into an adult without fusing with another cell.

37. Chapter 7 Section 2 Sexual Reproduction Sexual Life Cycles in Eukaryotes, continued Alternation of Generations • In the life cycle of a plant, the gametophyte is the haploid phase that produces gametes by mitosis. • The gametophyte produces gametes that fuse and give rise to the diploid phase. • Thus, the sporophyte and gametophyte generations take turns, or alternate, in the life cycle.

38. Chapter 7 Section 2 Sexual Reproduction Alternation of Generations

39. Chapter 7 Section 2 Sexual Reproduction Alternation of Generations

40. Standardized Test Prep Chapter 7 Multiple Choice Use the figure below and your knowledge of science to answer questions 1–3.

41. Standardized Test Prep Chapter 7 Multiple Choice, continued 1. The process that contributes to the production of the sperm and egg is A. budding. B. fragmentation. C. meiosis. D. mitosis.

42. Standardized Test Prep Chapter 7 Multiple Choice, continued 1. The process that contributes to the production of the sperm and egg is A. budding. B. fragmentation. C. meiosis. D. mitosis.

43. Standardized Test Prep Chapter 7 Multiple Choice, continued 2. The process that contributes to the formation of the embryo from the zygote is F. oogenesis. G. fission. H. meiosis. J. mitosis.

44. Standardized Test Prep Chapter 7 Multiple Choice, continued 2. The process that contributes to the formation of the embryo from the zygote is F. oogenesis. G. fission. H. meiosis. J. mitosis.

45. Standardized Test Prep Chapter 7 Multiple Choice, continued 3. Which type of eukaryotic life cycle does the diagram represent? A. asexual B. haploid C. diploid D. Alternation of generations

46. Standardized Test Prep Chapter 7 Multiple Choice, continued 3. Which type of eukaryotic life cycle does the diagram represent? A. asexual B. haploid C. diploid D. Alternation of generations