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Chapter 9 The passage of lifes organization and information from one generation to the next One way, but are there others? How do organisms pass genetic.

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Presentation on theme: "Chapter 9 The passage of lifes organization and information from one generation to the next One way, but are there others? How do organisms pass genetic."— Presentation transcript:

1 Chapter 9 The passage of lifes 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 bulbs fragmentation 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 n In approximately 15 of the Cnemidophorus species there are no males. They reproduce by parthenogenesis. n Parthenogenesis is rare in vertebrates. The offspring of parthenogenic lizards are clones, identical to the mother.

6 Human Cloning 1997 Dolly 1998 Mice 2000 Monkey Business 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.

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

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

9 Genetic Recombination: Sexual Reproduction n 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 n What are the Costs? Courtship expenses Two parents investing resources Complicated process to make gametes Dangerous!

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

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

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

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

15 Diploid adult MITOSIS FERTILIZATION MEIOSIS: 2n >> n Haploid gametes (n) Diploid zygote Figure 9.7a Diploid dominant 2n

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

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

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

19 Male frequency Frequency of infection by parasites Figure 9.10b Are genetically diverse populations more resistant to parasites?

20 Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms n 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 A full complement of chromosomes is restored during fertilization. Female gamete n = 23 in humans Fertilization Diploid offspring contains homologous pair of chromosomes Male gamete n = 23 in humans Figure 9.2c

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

23 During meiosis, chromosome number in each cell is reduced. Parent cell contains homologous pair of chromosomes MEIOSIS I Homologs separate at meiosis I Sister chromatids separate at meiosis II Daughter cells contain just one homolog Four daughter cells contain one chromosome each. These cells become gametes. MEIOSIS II Figure 9.2b

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

25 MEIOSIS II Sister chromatids separate 5. Cell divides. 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. Figure 9.3, right

26 Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms n Meiosis reduces the chromosome number by half, enabling sexual recombination to occur. Gametes undergo fertilization, restoring the diploid number of chromosomes in the zygote. 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 23 pairs of chromosomes in humans

27 Box 9.1 Figure 1

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

29 Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms n 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 = 2 n (n = number of haploid chromosomes). (Fig. 9.6) Crossing over between homologous chromosomes at prophase I.

30 Hypothetical example Eye color Gene that contributes to brown eyes Gene that contributes to blue eyes Maternal chromosome Paternal chromosome Hair color Gene that contributes to black hair Gene that contributes to red hair Maternal chromosome Paternal chromosome Figure 9.5a

31 During meiosis I, tetrads can line up two different ways before the homologs separate. OR Brown eyes Black hair Blue eyes Red hair Blue eyes Black hair Brown eyes Red hair Figure 9.5b

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

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

34 Predictions of crossing over hypothesis Products of meiosis Chromosome shape: Traits contributed to offspring: Long with knob Short with knob Colored, waxy kernels Long with no knob Short with no knob Colored, starchy kernels Colorless, waxy kernels Colorless, starchy kernels If crossing over results in exchange of genetic material between two chromosomes, the products of meiosis will look like this: Experimental results support these predictions Box 9.2, Figure 1c

35 Figure 9.4c

36 Figure 9.4b

37 Figure 9.4d

38 The Consequences of Meiotic Mistakes n 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 n + 1 n – 1 1. Meiosis I starts normally. Tetrads line up in middle of cell. 2. Then one set of homologs does not separate (= nondisjunction). 3. Meiosis II occurs normally. 4. All gametes have an abnormal number of chromosomes--either one too many or one too few. NONDISJUNCTION at Meiosis I: most common cause, weak meiosis I alignment checkpoint in females??? 2n = 4 n = 2 Figure 9.11

40 Incidence of Down syndrome per number of births Age of mother (years) Figure 9.12

41 Other Consequences of Meiosis n 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? n How do mitosis and meiosis figure into the passage of genetic information? n What are patterns of inheritance? n How do genes determine organismic characteristics


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