Chapter 9

1. Chapter 9 The passage of life’s organization and information from one generation to the next One way, but are there others? How do organisms pass genetic information? Are the contributions the same from males and females? What kinds of mishaps occur and where do they originate?

2. General Life Strategies Asexual reproduction corms fragmentation bulbs • No exchange of genetic material • Offspring are genetically identical to parents • No time ‘wasted” finding a mate • No courtship

3. Figure 9.8 Bacterial Duplication

4. Some Interesting Strategies The life cycle of aphids can involve a mix of parthenogenetic (asexual) and sexual reproduction. Parthenogenetic reproduction provides the development of young from unfertilized eggs. The young are female and genetically identical to the parent. Eggs typically hatch in spring and develop into wingless females which then produce live young. After some generations of parthenogenesis, winged reproductive males and females are produced which mate and lay eggs.

5. Another Interesting Organism • In approximately 15 of the Cnemidophorus species there are no males. They reproduce by parthenogenesis. • Parthenogenesis is rare in vertebrates. The offspring of parthenogenic lizards are clones, identical to the mother.

6. Human Cloning 1998 Mice United Nations (Nov. 20, 2001) - A key General Assembly committee backed a resolution calling for a treaty to ban the cloning of human beings, saying it was "contrary to human dignity.“ Under the draft resolution, a group would meet twice next year to define what should be negotiated in an international convention to ban reproductive cloning. 1997 Dolly 2000 Monkey Business

7. Box 9.3, Figure 1 But something else is happening: genetic recombination BACTERIAL CONJUGATION AND RECOMBINATION 1. Hfr cells containgenes that allow themto transfer some or all of their chromosometo another cell. Hfr cell Normal cell 2. Conjugation tubeconnects Hfr cell to normal cell. Copy of Hfr chromosome begins to move to recipient cell. Conjugation tube 3. Homologous sections of chromosome synapse. 4. Cells separate. Section of Hfr chromosome integrates into recipient chromosome by crossing over.

8. Figure 9.9 Some comparisons between asexual and sexual reproduction Asexual reproduction Sexual reproduction Generation 1 Generation 2 Generation 3 So, what good are males???

9. Genetic Recombination: Sexual Reproduction • What are the benefits? • Two copies of each gene (provides instructions) • “Sharing” of beneficial genes • “Infinite” number of combinations (variation)

10. Genetic Recombination: Sexual Reproduction • What are the Costs? • Courtship expenses • Two parents investing resources • “Complicated” process to make gametes • Dangerous!

11. Genetic Recombination: Sexual Reproduction • What are the Costs? • Courtship expenses • Two parents investing resources • “Complicated” process to make gametes • Dangerous!

12. Genetic Recombination: Sexual Reproduction • What are the Costs? • Courtship expenses • Two parents investing resources • “Complicated” process to make gametes • Dangerous!

13. Genetic Recombination: Sexual Reproduction • What are the Costs? • Courtship expenses • Two parents investing resources • “Complicated” process to make gametes • Dangerous!

14. Life Cycle Strategies Involving Sexual Reproduction • Diploid Dominant (two copies of each chromosome) • Haploid Dominant (one copy of each chromosome) • Alteration of Generations

15. Figure 9.7a MEIOSIS: 2n >> n Haploidgametes (n) Diploid adult MITOSIS FERTILIZATION Diploidzygote Diploid dominant 2n

16. Figure 9.7b Haploid dominant MEIOSIS MITOSIS Haploid cell Diploid cell Haploid adult MITOSIS FERTILIZATION Haploid gametes

17. Figure 9.7c, upper Alternation of generations MEIOSIS MITOSIS Haploid cells Diploid plant Haploid gametes Haploid plant Diploid cell MITOSIS MITOSIS FERTILIZATIION

18. Figure 9.10a Evidence for the benefits of sexual reproduction: resistance Snails subject to parasitism by trematode worms (Lively)

19. Figure 9.10b Are genetically diverse populations more resistant to parasites? 0.40 0.30 0.20 0.15 Male frequency 0.10 0.05 0.01 0.00 0.00 0.05 0.15 0.30 0.50 Frequency of infection by parasites

20. Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms • Meiosis reduces the chromosome number by half, enabling sexual recombination to occur. • Meiosis of diploid cells produces haploid daughter cells, which may function as gametes. (Fig. 9.2a-c, 9.3)

21. Figure 9.2c A full complement of chromosomes is restored during fertilization. Femalegamete n = 23 in humans Malegamete n = 23 in humans Fertilization Diploid offspringcontains homologouspair of chromosomes

22. Figure 9.2a Each chromosome replicates prior to undergoing meiosis. Maternalchromosome Paternal chromosome (n = 23 in humans) (n = 23 in humans) Duplication in S phase Sister chromatids Centromere Homologous pair of premeiotic chromosomes

23. Figure 9.2b During meiosis, chromosome number in each cell is reduced. Parent cellcontainshomologouspair of chromosomes MEIOSIS I Homologs separate at meiosis I Daughtercellscontainjust one homolog MEIOSIS II Sisterchromatids separate at meiosis II Four daughter cells contain one chromosome each. These cells become gametes.

24. Figure 9.3, left PRIOR TO MEIOSIS MEIOSIS I Homologous chromosomes separate. Chromosomes replicate, forming sister chromatids. Tetrad (4 chromatids from homologous chromosomes) Sister chromatids Chiasma 1. Chromosomesreplicate inparent cell. 2. Synapsis of homologous chromosomes. Crossing over of non-sister chromatids. 3. Tetrads migrate to middle of cell. 4. Homologsseparate.

25. Figure 9.3, right MEIOSIS II Sister chromatids separate 6. Chromosomes begin moving to middle of cell. 7. Chromosomes line up at middle of cell. 8. Sister chromatids separate. 9. Cell division results in four daughter cells. 5. Cell divides.

26. Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms • Meiosis reduces the chromosome number by half, enabling sexual recombination to occur. • Gametes undergo fertilization, restoring the diploid number of chromosomes in the zygote. 23 pairs of chromosomes in humans But what about the difference in size between the egg and sperm? Can be “extrachromosomal” factors in cytoplasm of egg: Mitochondria, chloroplasts, infectious agents, chemicals

27. Box 9.1 Figure 1

28. Figure 9.1a,b 12 types of chromosomes in the lubber grasshopper e b k a d j X i h f c g Each type of chromosome has two homologs. e k b d a j f X h i c g

29. Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms • Meiosis and fertilization introduce genetic variation in several ways: Independent assortment of homologous pairs at metaphase I: • Each homologous pair can orient in either of two ways at the plane of cell division. (Fig. 9.5a,b) • The total number of possible outcomes = 2n (n = number of haploid chromosomes). (Fig. 9.6) • Crossing over between homologous chromosomes at prophase I.

30. Figure 9.5a Hypothetical example Eye color Hair color Gene thatcontributes to blueeyes Gene thatcontributes to browneyes Gene thatcontributes to black hair Gene thatcontributes to red hair Maternalchromosome Paternalchromosome Maternalchromosome Paternalchromosome

31. Figure 9.5b During meiosis I, tetrads can line up two different waysbefore the homologs separate. OR Brown eyesBlack hair Blue eyesRed hair Brown eyesRed hair Blue eyesBlack hair

32. Figure 9.6 Crossing over EVEN SELF-FERTILIZATION LEADS TO GENETICALLY VARIABLE OFFSPRING because of crossing over 2. Crossingover duringmeiosis I. 3. Homologs separate.(Pairing of chromosomes depends on independentassortment.) 1. Parent cellwith four chromosomes. 5. Offspring produced by selfing (only some of the possibilities shown.) 4. Gametes produced by meiosis II.

33. Box 9.2, Figure 1a,b: Crossing over involves breakage and reunion of chromatids Shape of chromosome 9 varies in two maize strains Knob No knob Long Short Strain 2 Strain 1 Genes on chromosome 9 also vary Colored kernels Colorless kernels Starchy kernels Waxy kernels Strain 1 Strain 2

34. Box 9.2, Figure 1c Predictions of crossing over hypothesis If crossing over results in exchange ofgenetic material between twochromosomes, the products ofmeiosis will look like this: Products of meiosis Long withknob Short withknob Longwithno knob Shortwith no knob Chromosome shape: Traits contributedto offspring: Colored, waxy kernels Colored, starchykernels Colorless, waxykernels Colorless, starchykernels Experimental results support these predictions

35. Figure 9.4c

36. Figure 9.4b

37. Figure 9.4d

38. The Consequences of Meiotic Mistakes • Nondisjunctions occur when homologous chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II. • Fertilization can result in embryos that are 2n + 1 (a “trisomy”) or 2n - 1. (Fig. 9.11) • Abnormal copy numbers of one or more chromosomes is usually, but not always, fatal (Example: Down syndrome). (Fig. 9.12) • Human survivors: trisomics = 13, 18, 21

39. Figure 9.11 NONDISJUNCTION at Meiosis I: most common cause, weak meiosis I alignment checkpoint in females??? n + 1 n + 1 n – 1 2n = 4n = 2 n – 1 1. Meiosis I starts normally. Tetrads line up in middle of cell. 2. Then one set of homologs does notseparate (= nondisjunction). 4. All gametes have an abnormal number of chromosomes--either one too many or one too few. 3. Meiosis II occurs normally.

40. Figure 9.12 146 12300 11600 11200 1880 1290 1100 28 32 37 42 Incidence of Down syndromeper number of births 20 24 47 Age of mother (years)

41. Other Consequences of Meiosis • Polyploidy can occur when whole sets of chromosomes fail to separate at meiosis I or II. • The resulting 2n gametes, if fertilized by normal sperm, create 3n zygotes (triploid). • Organisms with an odd number of chromosome sets cannot produce viable gametes (Example: seedless fruits). 3n = 2X1 chromosome separation at meiosis I = unbalanced gametes, undeveloped seeds

42. So where does this take us? • How do mitosis and meiosis figure into the passage of genetic information? • What are “patterns of inheritance”? • How do genes determine organismic characteristics