1. © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey Chapter 8 The Cellular Basis of Reproduction and Inheritance

2.  Cancer cells –start out as normal body cells, –undergo genetic mutations, –lose the ability to control the tempo of their own division, and –run amok, causing disease. Introduction © 2012 Pearson Education, Inc.

3.  In a healthy body, cell division allows for –growth, –the replacement of damaged cells, and –development from an embryo into an adult.  In sexually reproducing organisms, eggs and sperm result from –mitosis and –meiosis. Introduction © 2012 Pearson Education, Inc.

4. Figure 8.0_2 Chapter 8: Big Ideas Cell Division and Reproduction The Eukaryotic Cell Cycle and Mitosis Meiosis and Crossing Over Alterations of Chromosome Number and Structure

5. Figure 8.0_3

6. CELL DIVISION AND REPRODUCTION © 2012 Pearson Education, Inc.

7. 8.1 Cell division plays many important roles in the lives of organisms  Organisms reproduce their own kind, a key characteristic of life.  Cell division –is reproduction at the cellular level, –requires the duplication of chromosomes, and –sorts new sets of chromosomes into the resulting pair of daughter cells. © 2012 Pearson Education, Inc.

8.  Cell division is used –for reproduction of single-celled organisms, –growth of multicellular organisms from a fertilized egg into an adult, –repair and replacement of cells, and –sperm and egg production. 8.1 Cell division plays many important roles in the lives of organisms © 2012 Pearson Education, Inc.

9. 8.1 Cell division plays many important roles in the lives of organisms  Living organisms reproduce by two methods. –Asexual reproduction –produces offspring that are identical to the original cell or organism and –involves inheritance of all genes from one parent. –Sexual reproduction –produces offspring that are similar to the parents, but show variations in traits and –involves inheritance of unique sets of genes from two parents. © 2012 Pearson Education, Inc.

11. THE EUKARYOTIC CELL CYCLE AND MITOSIS © 2012 Pearson Education, Inc.

12.  Eukaryotic cells –are more complex and larger than prokaryotic cells, –have more genes, and –store most of their genes on multiple chromosomes within the nucleus. 8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division © 2012 Pearson Education, Inc.

13.  Eukaryotic chromosomes are composed of chromatin consisting of –one long DNA molecule and –proteins that help maintain the chromosome structure and control the activity of its genes.  To prepare for division, the chromatin becomes –highly compact and –visible with a microscope. 8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division © 2012 Pearson Education, Inc.

14. Figure 8.3A

15. Figure 8.3B Sister chromatids Chromosomes Centromere Chromosome duplication Sister chromatids Chromosome distribution to the daughter cells DNA molecules

16.  Before a eukaryotic cell begins to divide, it duplicates all of its chromosomes, resulting in –two copies called sister chromatids –joined together by a narrowed “waist” called the centromere.  When a cell divides, the sister chromatids –separate from each other, now called chromosomes, and –sort into separate daughter cells. 8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division © 2012 Pearson Education, Inc.

17. Figure 8.3B_1 Chromosomes Centromere Chromosome duplication Sister chromatids Chromosome distribution to the daughter cells DNA molecules

18.  The cell cycle is an ordered sequence of events that extends –from the time a cell is first formed from a dividing parent cell –until its own division. 8.4 The cell cycle multiplies cells © 2012 Pearson Education, Inc.

19.  The cell cycle consists of two stages, characterized as follows: 1.Interphase: duplication of cell contents –G 1 —growth, increase in cytoplasm –S—duplication of chromosomes –G 2 —growth, preparation for division 2.Mitotic phase: division –Mitosis—division of the nucleus –Cytokinesis—division of cytoplasm 8.4 The cell cycle multiplies cells © 2012 Pearson Education, Inc.

20. Figure 8.4 G 1 (first gap) S (DNA synthesis) G 2 (second gap) M Cytokinesis Mitosis I N T E R P H A S E PH A SE T T MI O IC

21.  Mitosis progresses through a series of stages: –prophase, –prometaphase, –metaphase, –anaphase, and –telophase.  Cytokinesis often overlaps telophase. 8.5 Cell division is a continuum of dynamic changes © 2012 Pearson Education, Inc.

22. Figure 8.5_1 INTERPHASE MITOSIS Prophase Prometaphase Centrosome Early mitotic spindle Chromatin Fragments of the nuclear envelope Kinetochore Centrosomes (with centriole pairs) Centrioles Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Centromere Spindle microtubules

23. Figure 8.5_left INTERPHASE MITOSIS ProphasePrometaphase Centrosome Early mitotic spindle Chromatin Fragments of the nuclear envelope Kinetochore Centrosomes (with centriole pairs) Centrioles Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Centromere Spindle microtubules

24.  A mitotic spindle is –required to divide the chromosomes, –composed of microtubules, and –produced by centrosomes, structures in the cytoplasm that –organize microtubule arrangement and –contain a pair of centrioles in animal cells. 8.5 Cell division is a continuum of dynamic changes © 2012 Pearson Education, Inc.

25.  Interphase –The cytoplasmic contents double, –two centrosomes form, –chromosomes duplicate in the nucleus during the S phase, and –nucleoli, sites of ribosome assembly, are visible. 8.5 Cell division is a continuum of dynamic changes © 2012 Pearson Education, Inc.

26. Figure 8.5_2 INTERPHASE

27.  Prophase –In the cytoplasm microtubules begin to emerge from centrosomes, forming the spindle. –In the nucleus –chromosomes coil and become compact and –nucleoli disappear. 8.5 Cell division is a continuum of dynamic changes © 2012 Pearson Education, Inc.

28. Figure 8.5_3 Prophase

29.  Prometaphase –Spindle microtubules reach chromosomes, where they –attach at kinetochores on the centromeres of sister chromatids and –move chromosomes to the center of the cell through associated protein “motors.” –Other microtubules meet those from the opposite poles. –The nuclear envelope disappears. 8.5 Cell division is a continuum of dynamic changes © 2012 Pearson Education, Inc.

30. Figure 8.5_4 Prometaphase

31. Figure 8.5_left INTERPHASE MITOSIS ProphasePrometaphase Centrosome Early mitotic spindle Chromatin Fragments of the nuclear envelope Kinetochore Centrosomes (with centriole pairs) Centrioles Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Centromere Spindle microtubules

32. Figure 8.5_5 MITOSIS Anaphase Metaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Nuclear envelope forming Daughter chromosomes Mitotic spindle

33. Figure 8.5_right MITOSIS Anaphase Metaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Nuclear envelope forming Daughter chromosomes Mitotic spindle

34.  Metaphase –The mitotic spindle is fully formed. –Chromosomes align at the cell equator. –Kinetochores of sister chromatids are facing the opposite poles of the spindle. 8.5 Cell division is a continuum of dynamic changes © 2012 Pearson Education, Inc.

35. Figure 8.5_6 Metaphase

36.  Anaphase –Sister chromatids separate at the centromeres. –Daughter chromosomes are moved to opposite poles of the cell as –motor proteins move the chromosomes along the spindle microtubules and –kinetochore microtubules shorten. –The cell elongates due to lengthening of nonkinetochore microtubules. 8.5 Cell division is a continuum of dynamic changes © 2012 Pearson Education, Inc.

37. Figure 8.5_7 Anaphase

38.  Telophase –The cell continues to elongate. –The nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei. –Chromatin uncoils and nucleoli reappear. –The spindle disappears. 8.5 Cell division is a continuum of dynamic changes © 2012 Pearson Education, Inc.

39. Figure 8.5_8 Telophase and Cytokinesis

40. Figure 8.5_right MITOSIS Anaphase Metaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Nuclear envelope forming Daughter chromosomes Mitotic spindle

41.  During cytokinesis, the cytoplasm is divided into separate cells.  The process of cytokinesis differs in animal and plant cells. 8.5 Cell division is a continuum of dynamic changes © 2012 Pearson Education, Inc.

42.  In animal cells, cytokinesis occurs as 1.a cleavage furrow forms from a contracting ring of microfilaments, interacting with myosin, and 2.the cleavage furrow deepens to separate the contents into two cells. 8.6 Cytokinesis differs for plant and animal cells © 2012 Pearson Education, Inc.

43. Figure 8.6A Cytokinesis Cleavage furrow Contracting ring of microfilaments Daughter cells Cleavage furrow

44.  In plant cells, cytokinesis occurs as 1.a cell plate forms in the middle, from vesicles containing cell wall material, 2.the cell plate grows outward to reach the edges, dividing the contents into two cells, 3.each cell now possesses a plasma membrane and cell wall. 8.6 Cytokinesis differs for plant and animal cells © 2012 Pearson Education, Inc.

45. Figure 8.6B Cytokinesis Cell wall of the parent cell Daughter nucleus Cell wall Plasma membrane Vesicles containing cell wall material Cell plate forming New cell wall Cell plate Daughter cells

46.  The cells within an organism’s body divide and develop at different rates.  Cell division is controlled by –the presence of essential nutrients, –growth factors, proteins that stimulate division, –density-dependent inhibition, in which crowded cells stop dividing, and –anchorage dependence, the need for cells to be in contact with a solid surface to divide. 8.7 Anchorage, cell density, and chemical growth factors affect cell division © 2012 Pearson Education, Inc.

47. Figure 8.7A Cultured cells suspended in liquid The addition of growth factor

48. Figure 8.7B Anchorage Single layer of cells Removal of cells Restoration of single layer by cell division

49.  The cell cycle control system is a cycling set of molecules in the cell that –triggers and –coordinates key events in the cell cycle.  Checkpoints in the cell cycle can –stop an event or –signal an event to proceed. 8.8 Growth factors signal the cell cycle control system © 2012 Pearson Education, Inc.

50.  There are three major checkpoints in the cell cycle. 1.G 1 checkpoint –allows entry into the S phase or –causes the cell to leave the cycle, entering a nondividing G 0 phase. 2.G 2 checkpoint, and 3.M checkpoint.  Research on the control of the cell cycle is one of the hottest areas in biology today. 8.8 Growth factors signal the cell cycle control system © 2012 Pearson Education, Inc.

51. Figure 8.8A G0G0 G 1 checkpoint G1G1 S M G2G2 Control system M checkpoint G 2 checkpoint

52. Figure 8.8B Receptor protein Signal transduction pathway Growth factor Relay proteins Plasma membrane E XTRACELLULAR F LUID C YTOPLASM G 1 checkpoint G1G1 S M G2G2 Control system

53.  Cancer currently claims the lives of 20% of the people in the United States and other industrialized nations.  Cancer cells escape controls on the cell cycle.  Cancer cells –divide rapidly, often in the absence of growth factors, –spread to other tissues through the circulatory system, and –grow without being inhibited by other cells. 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors © 2012 Pearson Education, Inc.

54.  A tumor is an abnormally growing mass of body cells. –Benign tumors remain at the original site. –Malignant tumors spread to other locations, called metastasis. 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors © 2012 Pearson Education, Inc.

55. Figure 8.9 Tumor Glandular tissue GrowthInvasionMetastasis Lymph vessels Blood vessel Tumor in another part of the body

56.  Cancers are named according to the organ or tissue in which they originate. –Carcinomas arise in external or internal body coverings. –Sarcomas arise in supportive and connective tissue. –Leukemias and lymphomas arise from blood-forming tissues. 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors © 2012 Pearson Education, Inc.

57.  Cancer treatments –Localized tumors can be –removed surgically and/or –treated with concentrated beams of high-energy radiation. –Chemotherapy is used for metastatic tumors. 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors © 2012 Pearson Education, Inc.

58.  When the cell cycle operates normally, mitosis produces genetically identical cells for –growth, –replacement of damaged and lost cells, and –asexual reproduction. 8.10 Review: Mitosis provides for growth, cell replacement, and asexual reproduction © 2012 Pearson Education, Inc.

59. Figure 8.10A

60. MEIOSIS AND CROSSING OVER © 2012 Pearson Education, Inc.

61.  In humans, somatic cells have –23 pairs of homologous chromosomes and –one member of each pair from each parent.  The human sex chromosomes X and Y differ in size and genetic composition.  The other 22 pairs of chromosomes are autosomes with the same size and genetic composition. 8.11 Chromosomes are matched in homologous pairs © 2012 Pearson Education, Inc.

62.  Homologous chromosomes are matched in –length, –centromere position, and –gene locations.  A locus (plural, loci) is the position of a gene.  Different versions of a gene may be found at the same locus on maternal and paternal chromosomes. 8.11 Chromosomes are matched in homologous pairs © 2012 Pearson Education, Inc.

63. Figure 8.11 Pair of homologous chromosomes Locus Centromere Sister chromatids One duplicated chromosome

64.  An organism’s life cycle is the sequence of stages leading –from the adults of one generation –to the adults of the next.  Humans and many animals and plants are diploid, with body cells that have –two sets of chromosomes, –one from each parent. 8.12 Gametes have a single set of chromosomes © 2012 Pearson Education, Inc.

65.  Meiosis is a process that converts diploid nuclei to haploid nuclei. –Diploid cells have two homologous sets of chromosomes. –Haploid cells have one set of chromosomes. –Meiosis occurs in the sex organs, producing gametes—sperm and eggs.  Fertilization is the union of sperm and egg.  The zygote has a diploid chromosome number, one set from each parent. 8.12 Gametes have a single set of chromosomes © 2012 Pearson Education, Inc.

66. Figure 8.12A Haploid gametes (n  23) Egg cell Sperm cell Fertilization n n Meiosis Ovary Testis Diploid zygote (2n  46) 2n2n Mitosis Key Haploid stage (n) Diploid stage (2n) Multicellular diploid adults (2n  46)

67.  All sexual life cycles include an alternation between –a diploid stage and –a haploid stage.  Producing haploid gametes prevents the chromosome number from doubling in every generation. 8.12 Gametes have a single set of chromosomes © 2012 Pearson Education, Inc.

68. Figure 8.12B A pair of homologous chromosomes in a diploid parent cell A pair of duplicated homologous chromosomes Sister chromatids 1 2 3 I NTERPHASE M EIOSIS I M EIOSIS II

69.  Meiosis is a type of cell division that produces haploid gametes in diploid organisms.  Two haploid gametes combine in fertilization to restore the diploid state in the zygote. 8.13 Meiosis reduces the chromosome number from diploid to haploid © 2012 Pearson Education, Inc.

70.  Meiosis and mitosis are preceded by the duplication of chromosomes. However, –meiosis is followed by two consecutive cell divisions and –mitosis is followed by only one cell division.  Because in meiosis, one duplication of chromosomes is followed by two divisions, each of the four daughter cells produced has a haploid set of chromosomes. 8.13 Meiosis reduces the chromosome number from diploid to haploid © 2012 Pearson Education, Inc.

71.  Meiosis I – Prophase I – events occurring in the nucleus. –Chromosomes coil and become compact. –Homologous chromosomes come together as pairs by synapsis. –Each pair, with four chromatids, is called a tetrad. –Nonsister chromatids exchange genetic material by crossing over. 8.13 Meiosis reduces the chromosome number from diploid to haploid © 2012 Pearson Education, Inc.

72. Figure 8.13_left Centrosomes (with centriole pairs) Centrioles Sites of crossing over Spindle Tetrad Nuclear envelope Chromatin Sister chromatids Fragments of the nuclear envelope Centromere (with a kinetochore) Spindle microtubules attached to a kinetochore Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Chromosomes duplicate Prophase I Metaphase I Anaphase I I NTERPHASE: M EIOSIS I : Homologous chromosomes separate

73.  Meiosis I – Metaphase I – Tetrads align at the cell equator.  Meiosis I – Anaphase I – Homologous pairs separate and move toward opposite poles of the cell. 8.13 Meiosis reduces the chromosome number from diploid to haploid © 2012 Pearson Education, Inc.

74.  Meiosis I – Telophase I –Duplicated chromosomes have reached the poles. –A nuclear envelope re-forms around chromosomes in some species. –Each nucleus has the haploid number of chromosomes. 8.13 Meiosis reduces the chromosome number from diploid to haploid © 2012 Pearson Education, Inc.

75.  Meiosis II follows meiosis I without chromosome duplication.  Each of the two haploid products enters meiosis II.  Meiosis II – Prophase II –Chromosomes coil and become compact (if uncoiled after telophase I ). –Nuclear envelope, if re-formed, breaks up again. 8.13 Meiosis reduces the chromosome number from diploid to haploid © 2012 Pearson Education, Inc.

76. Figure 8.13_right Cleavage furrow Telophase I and CytokinesisProphase II Metaphase II Anaphase II M EIOSIS II : Sister chromatids separate Sister chromatids separate Haploid daughter cells forming Telophase II and Cytokinesis

77. Figure 8.13_5 Two lily cells undergo meiosis II

78.  Meiosis II – Metaphase II – Duplicated chromosomes align at the cell equator.  Meiosis II – Anaphase II –Sister chromatids separate and –chromosomes move toward opposite poles. 8.13 Meiosis reduces the chromosome number from diploid to haploid © 2012 Pearson Education, Inc.

79.  Meiosis II – Telophase II –Chromosomes have reached the poles of the cell. –A nuclear envelope forms around each set of chromosomes. –With cytokinesis, four haploid cells are produced. 8.13 Meiosis reduces the chromosome number from diploid to haploid © 2012 Pearson Education, Inc.

80.  Mitosis and meiosis both –begin with diploid parent cells that –have chromosomes duplicated during the previous interphase.  However the end products differ. –Mitosis produces two genetically identical diploid somatic daughter cells. –Meiosis produces four genetically unique haploid gametes. 8.14 Mitosis and meiosis have important similarities and differences © 2012 Pearson Education, Inc.

81. Figure 8.14 Prophase Metaphase Duplicated chromosome (two sister chromatids) M ITOSIS Parent cell (before chromosome duplication) Chromosome duplication Chromosome duplication Site of crossing over 2n  4 Chromosomes align at the metaphase plate Tetrads (homologous pairs) align at the metaphase plate Tetrad formed by synapsis of homologous chromosomes Metaphase I Prophase I M EIOSIS I Anaphase Telophase Sister chromatids separate during anaphase 2n2n 2n2n Daughter cells of mitosis No further chromosomal duplication; sister chromatids separate during anaphase II nnnn Daughter cells of meiosis II Daughter cells of meiosis I Haploid n  2 Anaphase I Telophase I Homologous chromosomes separate during anaphase I ; sister chromatids remain together M EIOSIS II

82. Figure 8.14_1 Prophase Metaphase M ITOSIS Parent cell (before chromosome duplication) Chromosome duplication Chromosome duplication Site of crossing over 2n  4 Chromosomes align at the metaphase plate Tetrads (homologous pairs) align at the metaphase plate Tetrad Metaphase I Prophase I M EIOSIS I

83. Figure 8.14_2 Metaphase M ITOSIS Chromosomes align at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase 2n2n 2n2n Daughter cells of mitosis

84. Figure 8.14_3 Tetrads (homologous pairs) align at the metaphase plate Metaphase I No further chromosomal duplication; sister chromatids separate during anaphase II n Daughter cells of meiosis II Daughter cells of meiosis I Haploid n  2 Anaphase I Telophase I Homologous chromosomes separate during anaphase I ; sister chromatids remain together M EIOSIS II M EIOSIS I nnn

85.  Genetic variation in gametes results from –independent orientation at metaphase I and –random fertilization. 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring © 2012 Pearson Education, Inc.

86.  Independent orientation at metaphase I –Each pair of chromosomes independently aligns at the cell equator. –There is an equal probability of the maternal or paternal chromosome facing a given pole. –The number of combinations for chromosomes packaged into gametes is 2 n where n = haploid number of chromosomes. 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring © 2012 Pearson Education, Inc.

87.  Random fertilization – The combination of each unique sperm with each unique egg increases genetic variability. 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring © 2012 Pearson Education, Inc.

88. Figure 8.15_s1 Possibility A Two equally probable arrangements of chromosomes at metaphase I Possibility B

89. Figure 8.15_s2 Possibility A Two equally probable arrangements of chromosomes at metaphase I Possibility B Metaphase II

90. Figure 8.15_s3 Possibility A Two equally probable arrangements of chromosomes at metaphase I Possibility B Metaphase II Gametes Combination 3Combination 4Combination 2Combination 1

91. 8.16 Homologous chromosomes may carry different versions of genes  Separation of homologous chromosomes during meiosis can lead to genetic differences between gametes. –Homologous chromosomes may have different versions of a gene at the same locus. –One version was inherited from the maternal parent and the other came from the paternal parent. –Since homologues move to opposite poles during anaphase I, gametes will receive either the maternal or paternal version of the gene. © 2012 Pearson Education, Inc.

92. Figure 8.16 Coat-color genes Eye-color genes BrownBlack Meiosis WhitePink Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes White coat (c); pink eyes (e) Brown coat (C); black eyes (E) EC e c e E E ec c C C

93.  Genetic recombination is the production of new combinations of genes due to crossing over.  Crossing over is an exchange of corresponding segments between separate (nonsister) chromatids on homologous chromosomes. –Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over. –Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids. 8.17 Crossing over further increases genetic variability © 2012 Pearson Education, Inc.

94. Figure 8.17A Chiasma Tetrad

95. Figure 8.17B_1 Tetrad (pair of homologous chromosomes in synapsis) Breakage of homologous chromatids Joining of homologous chromatids 1 2 C ce E C ce E C ce E Chiasma

96. Figure 8.17B_2 Separation of homologous chromosomes at anaphase I 3 C E C e Chiasma c c E e C E ce

97. Figure 8.17B_3 Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types CE e C 4 E c c e EC eC cE ce

98. ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE © 2012 Pearson Education, Inc.

99.  A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs.  Karyotypes –are often produced from dividing cells arrested at metaphase of mitosis and –allow for the observation of –homologous chromosome pairs, –chromosome number, and –chromosome structure. 8.18 A karyotype is a photographic inventory of an individual’s chromosomes © 2012 Pearson Education, Inc.

100. Figure 8.18_s5 Centromere Sister chromatids Pair of homologous chromosomes 5 Sex chromosomes

101.  Trisomy 21 –involves the inheritance of three copies of chromosome 21 and –is the most common human chromosome abnormality. 8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome © 2012 Pearson Education, Inc.

102.  Trisomy 21, called Down syndrome, produces a characteristic set of symptoms, which include: –mental retardation, –characteristic facial features, –short stature, –heart defects, –susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and –shortened life span.  The incidence increases with the age of the mother. 8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome © 2012 Pearson Education, Inc.

103. Figure 8.19A Trisomy 21

104. Figure 8.19B Age of mother 50454035302520 0 10 20 30 40 50 60 70 80 90 Infants with Down syndrome (per 1,000 births)

105.  Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during –meiosis I, if both members of a homologous pair go to one pole or –meiosis II if both sister chromatids go to one pole.  Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes. 8.20 Accidents during meiosis can alter chromosome number © 2012 Pearson Education, Inc.

106. Figure 8.20A_s1 Nondisjunction M EIOSIS I

107. Figure 8.20A_s2 Nondisjunction M EIOSIS I M EIOSIS II Normal meiosis II

108. Figure 8.20A_s3 Nondisjunction M EIOSIS I M EIOSIS II Normal meiosis II Gametes Number of chromosomes Abnormal gametes n  1 n  1

109. Figure 8.20B_s1 Normal meiosis I M EIOSIS I

110. Figure 8.20B_s2 Normal meiosis I M EIOSIS I M EIOSIS II Nondisjunction

111. Figure 8.20B_s3 Normal meiosis I M EIOSIS I M EIOSIS II Nondisjunction Abnormal gametesNormal gametes n  1n  1 nn

112.  Sex chromosome abnormalities tend to be less severe, perhaps because of –the small size of the Y chromosome or –X-chromosome inactivation. 8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival © 2012 Pearson Education, Inc.

113.  Chromosome breakage can lead to rearrangements that can produce –genetic disorders or, –if changes occur in somatic cells, cancer. 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer © 2012 Pearson Education, Inc.

114.  These rearrangements may include –a deletion, the loss of a chromosome segment, –a duplication, the repeat of a chromosome segment, –an inversion, the reversal of a chromosome segment, or –a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal. 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer © 2012 Pearson Education, Inc.

115.  Chronic myelogenous leukemia (CML) –is one of the most common leukemias, –affects cells that give rise to white blood cells (leukocytes), and –results from part of chromosome 22 switching places with a small fragment from a tip of chromosome 9. 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer © 2012 Pearson Education, Inc.

116. Figure 8.23A Deletion Duplication Inversion Reciprocal translocation Homologous chromosomes Nonhomologous chromosomes

117. Figure 8.23B Chromosome 9 Chromosome 22 Reciprocal translocation “Philadelphia chromosome” Activated cancer-causing gene