Introduction to Meiosis: Hereditary Similarity and Variation in Living Organisms

1. Chapter 13 Meiosis and Sexual Life Cycles

2. Overview: Hereditary Similarity and Variation • Living organisms • Are distinguished by their ability to reproduce their own kind

3. Figure 13.1 • Heredity • Is the transmission of traits from one generation to the next • Variation • Shows that offspring differ somewhat in appearance from parents and siblings

4. Genetics • Is the scientific study of heredity and hereditary variation

5. Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

6. Inheritance of Genes • Genes • Are the units of heredity • Are segments of DNA

7. Each gene in an organism’s DNA • Has a specific locus on a certain chromosome • We inherit • One set of chromosomes from our mother and one set from our father

8. Parent Bud 0.5 mm Figure 13.2 Comparison of Asexual and Sexual Reproduction • In asexual reproduction • One parent produces genetically identical offspring by mitosis

9. In sexual reproduction • Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

10. Concept 13.2: Fertilization and meiosis alternate in sexual life cycles • A life cycle • Is the generation-to-generation sequence of stages in the reproductive history of an organism

11. Sets of Chromosomes in Human Cells • In humans • Each somatic cell has 46 chromosomes, made up of two sets • One set of chromosomes comes from each parent

12. Pair of homologous chromosomes 5 µm Centromere Sister chromatids Figure 13.3 • A karyotype • Is an ordered, visual representation of the chromosomes in a cell

13. Homologous chromosomes • Are the two chromosomes composing a pair • Have the same characteristics • May also be called autosomes

14. 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

15. A diploid cell • Has two sets of each of its chromosomes • In a human has 46 chromosomes (2n = 46)

16. Figure 13.4 • In a cell in which DNA synthesis has occurred • All the chromosomes are duplicated and thus each consists of two identical sister chromatids Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

17. Unlike somatic cells • Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes

18. Behavior of Chromosome Sets in the Human Life Cycle • At sexual maturity • The ovaries and testes produce haploid gametes by meiosis

19. During fertilization • These gametes, sperm and ovum, fuse, forming a diploid zygote • The zygote • Develops into an adult organism

20. Key Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm Cell (n) FERTILIZATION MEIOSIS Diploid zygote (2n = 46) Ovary Testis Mitosis and development Multicellular diploid adults (2n = 46) Figure 13.5 • The human life cycle

21. 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

22. Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Figure 13.7 Haploid cells with unreplicated chromosomes The Stages of Meiosis • An overview of meiosis

23. Meiosis I • Reduces the number of chromosomes from diploid to haploid • Meiosis II • Produces four haploid daughter cells

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

25. MEIOSIS II: Separates sister chromatids TELOPHASE II AND CYTOKINESIS TELOPHASE I AND CYTOKINESIS METAPHASE II ANAPHASE II PROPHASE II Cleavage furrow Haploid daughter cells forming Sister chromatids separate Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Figure 13.8 • Telophase I, cytokinesis, and meiosis II

26. A Comparison of Mitosis and Meiosis • Meiosis and mitosis can be distinguished from mitosis • By three events in Meiosis l

27. Synapsis and crossing over • Homologous chromosomes physically connect and exchange genetic information

28. Tetrads on the metaphase plate • At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned down the center (equator)

29. 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

30. MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Chromosome replication Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) 2n = 6 Tetrads positioned at the metaphase plate Chromosomes positioned at the metaphase plate Metaphase I Metaphase Sister chromatids separate during anaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II Figure 13.9 • A comparison of mitosis and meiosis

31. Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution • Reshuffling of genetic material in meiosis • Produces genetic variation

32. 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

33. Independent Assortment of Chromosomes • Homologous pairs of chromosomes • Orient randomly at metaphase I of meiosis

34. Key Maternal set of chromosomes Possibility 1 Possibility 2 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 • In independent assortment • Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Figure 13.10

35. Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes Figure 13.11 Crossing Over • Crossing over • Produces recombinant chromosomes that carry genes derived from two different parents

36. Random Fertilization • The fusion of gametes • Will produce a zygote with any of about 64 trillion diploid combinations

37. Evolutionary Significance of Genetic Variation Within Populations • Genetic variation • Is the raw material for evolution by natural selection

38. Mutations • Are the original source of genetic variation • Sexual reproduction • Produces new combinations of variant genes, adding more genetic diversity