1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis Overview: Hereditary Similarity and Variation Living organisms – Are distinguished by their ability to reproduce their own kind

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heredity – Is the transmission of traits from one generation to the next Variation – Shows that offspring differ somewhat in appearance from parents and siblings Figure 13.1

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetics – Is the scientific study of heredity and hereditary variation

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Acquiring Traits Offspring acquire genes from parents by inheriting chromosomes Genes are packaged into gametes (sperm and egg)

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Genes Genes – Program the specific traits that emerge as we develop – Are the units of heredity – Written in the language of DNA

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of genes Each gene in an organism’s DNA – Has a specific locus (location) on a certain chromosome We inherit – One set of chromosomes from our mother and one set from our father

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Asexual and Sexual Reproduction In asexual reproduction – One parent produces genetically identical offspring by mitosis – Clone – genetically identical offspring Figure 13.2 Parent Bud 0.5 mm

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Types of reproduction In sexual reproduction – Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual life cycles Fertilization and meiosis alternate in sexual life cycles A life cycle – The sequence of stages from conception to production of its own offspring

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sets of Chromosomes in Human Cells In humans – Each somatic cell (two chromosomes of each type) has 46 chromosomes, made up of two sets – One set of chromosomes comes from each parent

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Homologous chromosomes – Are the two chromosomes composing a pair – Have the same characteristics Genes are located at the same locus

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex chromosomes – Are distinct from each other in their characteristics – Are represented as X and Y – Determine the sex of the individual, XX being female, XY being male – All other chromosomes are called autosomes

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 µm Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3 A karyotype – Is an ordered, visual representation of the chromosomes in a cell

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A diploid cell (2n) – Has two sets of each of its chromosomes – In a human has 46 chromosomes (2n = 46)

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Unlike somatic cells – Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes Abbreviated as n

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In a cell in which DNA synthesis has occurred – All the chromosomes are duplicated and thus each consists of two identical sister chromatids Figure 13.4 Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Behavior of Chromosome Sets in the Human Life Cycle At sexual maturity – The ovaries and testes produce haploid gametes by meiosis

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings During fertilization – These gametes, sperm and ovum, fuse, forming a diploid zygote The zygote – Develops into an adult organism

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) The human life cycle

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Variety of Sexual Life Cycles The three main types of sexual life cycles – Differ in the timing of meiosis and fertilization

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Variety of sexual lifecycles In animals – Meiosis occurs during gamete formation – Gametes are the only haploid cells Gametes Figure 13.6 A Diploid multicellular organism Key MEIOSIS FERTILIZATION n n n 2n Zygote Haploid Diploid Mitosis (a) Animals

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSISFERTILIZATION n n n n n 2n 3.Haploid multicellular organism (gametophyte ) Mitosis Spores Gametes Mitosis Zygote (b) Plants and some algae Figure 13.6 B Variety of sexual lifecycles Plants and some algae – Exhibit an alternation of generations – The life cycle includes both diploid and haploid multicellular stages 1.Multicellular diploid sporophyte stage produces haploid spores 2.Spores undergo mitosis 4.Produces gametes by mitosis 5.Gametes fuse and form a diploid zygote

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS FERTILIZATION n n n n n 2n Haploid multicellular organism Mitosis Gametes Zygote (c) Most fungi and some protists Figure 13.6 C Variety of sexual lifecycles In most fungi and some protists – Meiosis produces haploid cells that give rise to a haploid multicellular adult organism – The haploid adult carries out mitosis, producing cells that will become gametes

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid Meiosis – Takes place in two sets of divisions, meiosis I and meiosis II

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Meiosis An overview of meiosis Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous chromosomes separate Haploid cells with replicated chromosomes Sister chromatids separate Haploid cells with unreplicated chromosomes Meiosis I Meiosis II

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis is Reduction/Division Meiosis I - Reduction – Reduces the number of chromosomes from diploid to haploid Meiosis II - Division – Produces four haploid daughter cells

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Meiosis Interphase and meiosis I Figure 13.8

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8 Meiosis Telophase I, cytokinesis, and meiosis II

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Comparison of Mitosis and Meiosis Meiosis can be distinguished from mitosis – By three events in Meiosis l Crossing over Formation of tetrads Separation of homologous chromosomes

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis vs. Mitosis Synapsis and crossing over – Homologous chromosomes physically connect and exchange genetic information

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis vs. Mitosis Tetrads on the metaphase plate – At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis vs. Mitosis Separation of homologues – At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell – In anaphase II of meiosis, the sister chromatids separate

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n nn Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n2n2n2n Daughter cells of mitosis 2n = 6 A comparison of mitosis and meiosis

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Variation through sexual reproduction Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution Reshuffling of genetic material in meiosis – Produces genetic variation

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Origins of Genetic Variation Among Offspring In species that produce sexually – The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Independent Assortment of Chromosomes Homologous pairs of chromosomes – Orient randomly at metaphase I of meiosis

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Independent assortment of chromosomes In independent assortment – Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Figure 13.10 Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 4

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Crossing Over Crossing over – Produces recombinant chromosomes that carry genes derived from two different parents Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Random Fertilization The fusion of gametes – Will produce a zygote with any of about 64 trillion diploid combinations

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Significance of Genetic Variation Within Populations Genetic variation – Is the raw material for evolution by natural selection

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutations – Are the original source of genetic variation Sexual reproduction – Produces new combinations of variant genes, adding more genetic diversity

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex Cell Formation Spermatogenesis Process takes about 74 hours. Males produce sperm. From one germ cell, 4 cells will be produced that will become sperm.

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sperm Development Males contain germ cells called spermatogonium within their testes. These cells will undergo both divisions of meiosis to produce sperm cells with half the number of chromosomes. This division begins when signaled by hormones. Men produce sperm throughout their life time.

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex Cell Formation Oogenesis In females, the cell divisions at the end of meiosis I and meiosis II are uneven, so that a single cell, which becomes an egg, receives most of the cytoplasm. Cytoplasm contains nutrients and we want that egg to have lots of nutrients. The three other cells do not form eggs and are called polar bodies Takes 15-30 yrs.