1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 13 Meiosis and Sexual Life Cycles

2. Vocabulary Review A species is distinguished by their ability to reproduce and produce fertile offspring. Genetics is the scientific study of heredity and variation Heredity is the transmission of traits from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

3. Vocabulary Review Genes are the units of heredity, and are made up of segments of DNA Gametes: Sex Cells (sperm and eggs) Somatic cells (any cell other than a gamete) In humans, somatic cells have 23 pairs of chromosomes Locus: specific location of a gene on a certain chromosome Clone: an individual or group of genetically identical individuals from the same parent Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

4. Vocabulary Review Asexual reproduction: one parent produces genetically identical offspring by mitosis Sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Life cycle: the generation-to-generation sequence of stages in the reproductive history of an organism Video: Hydra Budding Video: Hydra Budding Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

5. Vocabulary Review Homologous chromosomes: (Homologs) one from each parent, carry genes controlling the same traits Allele: a gene with homologs, the genes control the same traits, but influence the trait differently Karyotype: a picture of pairs of chromosomes from a cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

6. Fig. 13-3 APPLICATION TECHNIQUE Pair of homologous replicated chromosomes 5 µm Centromere Sister chromatids Metaphase chromosome

7. Fig. 13-3b TECHNIQUE Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome 5 µm

8. Sex chromosomes: carry genes that distinguish gender, 2 types = X and Y, in humans the 23 pair Autosomes: the chromosomes that do not determine sex, in humans pairs 1 to 22 Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome – not homologous Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Vocabulary Review

9. Diploid (2n): a cell with two sets of chromosomes, in humans diploid number is 46 (2n = 46) Haploid (n): A cell with a single set of chromosomes, in animals this only applies to gametes (sperm or egg) For humans, the haploid number is 23 (n = 23) (22 autosomes + 1 sex chromosome) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

10. Vocabulary Review Fertilization: the union of gametes (the sperm and the egg) Zygote: a fertilized egg with one set of chromosomes from each parent Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

11. A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell and each other Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

12. Fig. 13-4 Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Centromere Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) A cell after S-phase

13. After meiosis is complete in a female, the egg (ovum), carries an X sex chromosome In males, the sperm cell sex chromosome may be either X or Y After fertilization the diploid zygote produces somatic cells by mitosis and develops into an adult Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

14. At sexual maturity in animals, the ovaries and testes produce haploid gametes Gametes are the only types of animal cells produced by meiosis, rather than mitosis Meiosis results in one set of chromosomes in each gamete Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

15. Fig. 13-5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Egg (n) Sperm (n) MEIOSISFERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

16. The Previous Slides Summarize What We Should Already Know About Meiosis and Sexual Life Cycles! However, the sexual life cycles in non-animals can vary from what we are familiar with! Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

17. Fig. 13-6 Key Haploid (n) Diploid (2n) n n Gametes n n n Mitosis MEIOSIS FERTILIZATION MEIOSIS 2n2n 2n2n Zygote 2n2n Mitosis Diploid multicellular organism (a) Animals Spores Diploid multicellular organism (sporophyte) (b) Plants and some algae 2n2n Mitosis Gametes Mitosis n n n Zygote FERTILIZATION n n n Mitosis Zygote (c) Most fungi and some protists MEIOSIS FERTILIZATION 2n2n Gametes n n Mitosis Haploid multi- cellular organism (gametophyte) Haploid unicellular or multicellular organism

18. The Variety of Sexual Life Cycles The alternation of meiosis and fertilization is common to all organisms that reproduce sexually The three main types of sexual life cycles differ in the timing of meiosis and fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

19. The Variety of Sexual Life Cycles Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis However, only diploid cells can undergo meiosis In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

20. New Vocabulary Sporophyte Gametophyte Alternation of Generations Cohesins Synapsis (Synaptonemal Complex) Chiasmata Kinetochore Tetrad Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

21. Animals

22. In animals, meiosis produces gametes, which undergo no further cell division before fertilization Gametes are the only haploid cells in animals Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

23. Fig. 13-6a Key Haploid (n) Diploid (2n) Gametes n n n 2n2n2n2n Zygote MEIOSISFERTILIZATION Mitosis Diploid multicellular organism (a) Animals

24. Plants and Some Protists (Algae)

25. Plants and some algae exhibit an alternation of generations This life cycle includes both a diploid and haploid multicellular stage The diploid organism, called the sporophyte, makes haploid spores by meiosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

26. Each spore grows by mitosis into a haploid organism called a gametophyte A gametophyte makes haploid gametes by mitosis Fertilization of gametes results in a diploid sporophyte Spores and Gametophytes are both haploid. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

27. Fig. 13-6b Key Haploid (n) Diploid (2n) n n n n n 2n2n 2n2n Mitosis Zygote Spores Gametes MEIOSISFERTILIZATION Diploid multicellular organism (sporophyte) Haploid multi- cellular organism (gametophyte) (b) Plants and some algae

28. Fungi and Some Protists

29. In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage The zygote produces haploid cells by meiosis Each haploid cell grows by mitosis into a haploid multicellular organism The haploid adult produces gametes by mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

30. Fig. 13-6c Key Haploid (n) Diploid (2n) Mitosis Gametes Zygote Haploid unicellular or multicellular organism MEIOSIS FERTILIZATION n n n n n 2n2n (c) Most fungi and some protists

31. Three events are unique to meiosis, and all three occur in meiosis l: –Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information –At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes –At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

32. Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I Protein complexes called cohesins are responsible for this cohesion In mitosis, cohesins are cleaved at the end of metaphase In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

33. Genetic variation produced in sexual life cycles contributes to evolution Mutations (changes in an organism’s DNA) are the original source of genetic diversity Mutations create different versions of genes called alleles Reshuffling of alleles during sexual reproduction produces genetic variation Crossing over produces recombinant chromosomes, which combine genes inherited from each parent Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

34. Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Three mechanisms contribute to genetic variation: – Independent assortment of chromosomes – Crossing over – Random fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

35. The number of combinations possible when chromosomes assort independently into gametes is 2 n, where n is the haploid number For humans (n = 23), there are more than 8 million (2 23 ) possible combinations of chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

36. Fig. 13-12-1 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis

37. Fig. 13-12-2 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere TEM

38. Fig. 13-12-3 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I TEM

39. Fig. 13-12-4 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Anaphase II TEM

40. Fig. 13-12-5 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Anaphase II Daughter cells Recombinant chromosomes TEM

41. Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

42. Crossing over adds even more variation Each zygote has a unique genetic identity Animation: Genetic Variation Animation: Genetic Variation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

43. The Evolutionary Significance of Genetic Variation Within Populations Natural selection results in the accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which originates from mutations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings