1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetics – Is the scientific study of heredity and hereditary variation – Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes Genes – Are the units of heredity – Are segments of DNA

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

5. 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

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

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A karyotype – Is an ordered, visual representation of the chromosomes in a cell

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Homologous chromosomes – Are the two chromosomes composing a pair – Have the same characteristics – May also be called autosomes

11. 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

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

13. 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

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.3 Pair of homologous duplicated chromosomes Centromere Sister chromatids Metaphase chromosome 5  m APPLICATION TECHNIQUE

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.3a

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.3b Pair of homologous duplicated chromosomes Centromere Sister chromatids Metaphase chromosome 5  m

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.3c 5  m

18. 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 At sexual maturity – The ovaries and testes produce haploid gametes by meiosis

19. 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

20. 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

21. 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

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSISFERTILIZATION n n n n n 2n Haploid multicellular organism (gametophyte) Mitosis Spores Gametes Mitosis Zygote Diploid multicellular organism (sporophyte) (b) Plants and some algae Figure 13.6 B Plants and some algae – Exhibit an alternation of generations – The life cycle includes both diploid and haploid multicellular stages

24. 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 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

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

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

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Division in meiosis I occurs in four phases – Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis © 2011 Pearson Education, Inc.

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.8 MEIOSIS I: Separates homologous chromosomes Prophase I Metaphase IAnaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example. Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Sister chromatids remain attached Homologous chromosomes separate Each pair of homologous chromosomes separates. Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids. MEIOSIS I: Separates sister chromatids Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.8a Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example. Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Sister chromatids remain attached Homologous chromosomes separate Each pair of homologous chromosomes separates. Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids.

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prophase I Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair up, aligned gene by gene © 2011 Pearson Education, Inc.

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In crossing over, nonsister chromatids exchange DNA segments Each pair of chromosomes forms a tetrad, a group of four chromatids Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred © 2011 Pearson Education, Inc.

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome © 2011 Pearson Education, Inc.

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Anaphase I In anaphase I, pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole © 2011 Pearson Education, Inc.

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Telophase I and Cytokinesis In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells © 2011 Pearson Education, Inc.

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated © 2011 Pearson Education, Inc.

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Division in meiosis II also occurs in four phases – Prophase II – Metaphase II – Anaphase II – Telophase II and cytokinesis Meiosis II is very similar to mitosis © 2011 Pearson Education, Inc.

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.8b Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prophase II In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate © 2011 Pearson Education, Inc.

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Metaphase II In metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles © 2011 Pearson Education, Inc.

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Anaphase II In anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles © 2011 Pearson Education, Inc.

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Comparison of Mitosis and Meiosis Meiosis and mitosis can be distinguished from mitosis – By three events in Meiosis l

42. 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

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

44. 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

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Independent Assortment of Chromosomes Homologous pairs of chromosomes – Orient randomly at metaphase I of meiosis In independent assortment – Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Crossing over – Produces recombinant chromosomes that carry genes derived from two different parents

46. 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

47. 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

48. 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