1. Copyright © 2012 Pearson Education, Inc. Chapter 19 Chromosomes and Cell Division Betty McGuire Cornell University Lecture Presentation

2. Copyright © 2012 Pearson Education, Inc. Chromosomes and Cell Division  Two types of cell division  Form of chromosomes  The cell cycle  Mitosis: Creation of genetically identical diploid body cells  Cytokinesis  Karyotypes  Meiosis: Creation of haploid gametes

3. Copyright © 2012 Pearson Education, Inc. Two Types of Cell Division  The human life cycle has two types of cell division  Meiosis  Mitosis

4. Copyright © 2012 Pearson Education, Inc. Two Types of Cell Division  Meiosis  Gives rise to gametes that have half the number of chromosomes as the original cell  In females  Occurs in ovaries  Produces eggs  In males  Occurs in testes  Produces sperm

5. Copyright © 2012 Pearson Education, Inc. Two Types of Cell Division  Mitosis  Results in identical body cells  Occurs during growth and repair

6. Copyright © 2012 Pearson Education, Inc. [INSERT FIGURE 19.1 ON THIS SLIDE]

7. Copyright © 2012 Pearson Education, Inc. Two Types of Cell Division Web Activity: The Human Life Cycle

8. Copyright © 2012 Pearson Education, Inc. Form of Chromosomes  Chromosomes  Location: nucleus  Structure and function: each is a tightly coiled combination of a DNA molecule and specialized proteins called histones  DNA contains genetic information, which directs body development and maintenance  Histones help with support and control of gene activity

9. Copyright © 2012 Pearson Education, Inc. Form of Chromosomes  Gene  Specific segment of the DNA  Directs synthesis of a protein, which plays a structural or functional role in the cell

10. Copyright © 2012 Pearson Education, Inc. Form of Chromosomes  Somatic cells  All cells except eggs and sperm  In humans, have 46 chromosomes  Two sets of 23 chromosomes  One set of 23 from each parent

11. Copyright © 2012 Pearson Education, Inc. Form of Chromosomes  Each somatic cell contains two chromosomes with genes for the same traits  Called homologous pairs of chromosomes  One chromosome of the pair is from the mother  One chromosome of the pair is from the father

12. Copyright © 2012 Pearson Education, Inc. Form of Chromosomes  Diploid  A cell with two sets of chromosomes (2n)  Genes also occur in pairs in diploid cells  Members of each gene pair are located at the same position on homologous chromosomes

13. Copyright © 2012 Pearson Education, Inc. Form of Chromosomes  Of the 23 pairs of chromosomes  Sex chromosomes make up one pair  Determine gender  Two types: X and Y  XX = genetic female  XY = genetic male  Autosomes make up 22 pairs  Determine expression of most of a person’s inherited characteristics

14. Copyright © 2012 Pearson Education, Inc. The Cell Cycle  Mitosis  The process of cell division for body cells  When one nucleus divides into two daughter nuclei with the same number and kinds of chromosomes

15. Copyright © 2012 Pearson Education, Inc. The Cell Cycle  Cell cycle  Predictable set of events that a cell goes through from its origin to its own division into two daughter cells  Two major phases  Interphase  Cell division

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17. The Cell Cycle  Interphase  Period of growth and preparation for cell division; not a “resting period”  Three phases  G 1  S  G 2

18. Copyright © 2012 Pearson Education, Inc. The Cell Cycle  Interphase  G 1  First “gap”  Time of major growth before DNA synthesis begins  Chromosomes consist of a strand of DNA and proteins

19. Copyright © 2012 Pearson Education, Inc. The Cell Cycle  Interphase  S  DNA is replicated, and the two copies of the chromosome, called chromatids, remain attached at the centromere  Two attached chromatids are genetically identical  Called sister chromatids  Growth continues

20. Copyright © 2012 Pearson Education, Inc. The Cell Cycle  Interphase  G 2  Second “gap”  Period after DNA is synthesized and before mitosis begins  Growth continues

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22. The Cell Cycle  Cell division consists of two processes  Mitosis  Division of the nucleus  Cytokinesis  Division of the cytoplasm

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24. The Cell Cycle  Developing embryo and fetus  Body cells divide continually  Children  Body cells divide in growth and repair  Adults  Some cells lose ability to divide (most neurons)  Some cells stop dividing but retain ability to divide should the need arise (liver cells)  Some cells actively divide throughout life (skin cells)

25. Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells  Mitosis occurs in four phases  Prophase  Metaphase  Anaphase  Telophase

26. Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells  Prophase  Chromatin condenses and forms chromosomes as DNA wraps around histones  Nuclear membrane begins to break down  Mitotic spindle forms

27. Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells  Metaphase  Chromosomes attach to mitotic spindles and form a line at the center of the cell  Alignment ensures each daughter cell receives one chromatid from each of the 46 chromosomes

28. Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells  Anaphase  Sister chromatids of each chromosome begin to separate, splitting at centromere  Now separate entities, the sister chromatids are considered chromosomes

29. Copyright © 2012 Pearson Education, Inc. Mitosis: Creation of Genetically Identical Diploid Body Cells  Telophase  Nuclear envelope forms around each group of chromosomes at each pole  Mitotic spindle disassembles

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31. Mitosis: Creation of Genetically Identical Diploid Body Cells Web Activity: Mitosis

32. Copyright © 2012 Pearson Education, Inc. Cytokinesis  Cytokinesis  Division of the cytoplasm  Begins during telophase  Ring of microfilaments contracts at midline of cell, eventually pinching cell in two

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34. Karyotypes  Karyotype  Constructed by arranging chromosomes from photographs based on size and centromere location  Can be examined for defects in number and structure of chromosomes

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36. Meiosis: Creation of Haploid Gametes  Gametes  Differ from somatic cells  Haploid (n)  Have only one member of each homologous pair of chromosomes  23 chromosomes  Formed by meiosis  Two divisions that result in up to four haploid daughter cells

37. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Functions of meiosis  Keeps the number of chromosomes in a body cell constant from generation to generation  Increases genetic variability in population

38. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Meiosis involves two cell divisions  Meiosis I – separates homologues  Meiosis II – separates sister chromatids  Each division has its own  Prophase  Metaphase  Anaphase  Telophase

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40. Meiosis: Creation of Haploid Gametes  Meiosis I  Preceded by interphase, during which each chromosome was copied and now consists of two attached chromatids  Reduction division because it produces two haploid cells  Each daughter cell has 23 chromosomes  One member of each homologous pair  Each chromosome consists of two sister chromatids

41. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Prophase I  Chromosomes condense  Homologous chromosomes pair  Phenomenon called synapsis

42. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Metaphase I  Matched homologous pairs line up at the midline of cell and attach to spindle fibers

43. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Anaphase I  Homologous pairs of chromosomes separate and move to opposite ends of cell  Each homologue still consists of two sister chromatids

44. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Telophase I  Nuclear envelope forms around chromosomes at each end of cell  Cytokinesis occurs to form two haploid cells

45. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Interkinesis  Brief interphase-like period before meiosis II  Differs from mitotic interphase in that there is no replication of DNA

46. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Meiosis II  Prophase II  Chromosomes condense  Metaphase II  Chromosomes line up along midline of cell

47. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Meiosis II (cont.)  Anaphase II  Centromere holding two sister chromatids separates  Chromatids of each pair now considered chromosomes  Chromosomes move to opposite poles of cell

48. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Meiosis II (cont.)  Telophase II  Nuclear membrane forms around chromosomes at each end of cell  Cytokinesis occurs in both daughter cells, forming four haploid daughter cells

49. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  The important feature of meiosis II  It separates the two sister chromatids of each chromosome

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52. Meiosis: Creation of Haploid Gametes Web Activity: Comparing Mitosis and Meiosis

53. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Following meiosis, changes in shape and function of the haploid cells result in functional gametes  Spermatogenesis produces four sperm cells specialized for transporting the male’s genetic information to the egg  Oogenesis produces up to three polar bodies and one ovum packed with nutrients to nourish the early embryo

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55. Meiosis: Creation of Haploid Gametes  During meiosis, genetic variation is created through  Crossing over  Independent assortment

56. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Crossing over  Occurs when corresponding pieces of chromatids of maternal and paternal homologues are exchanged during synapsis  Prophase I

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58. Meiosis: Creation of Haploid Gametes  Independent assortment  The relative positioning of homologous maternal and paternal chromosomes with respect to poles of the cell is random  Members of each homologous pair orient independently of other pairs  Metaphase I

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61. Meiosis: Creation of Haploid Gametes  Nondisjunction  Failure of homologous chromosomes to separate during meiosis I or of sister chromatids to separate during meiosis II

62. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Nondisjunction results in too many, or too few, chromosomes in a cell  The imbalance of chromosome numbers usually causes abnormalities in development and miscarriage

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64. Meiosis: Creation of Haploid Gametes  Trisomy  Three representatives of a chromosome in a cell  Monosomy  Only one representative of a chromosome in a cell

65. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Down syndrome  An infant born with three copies of chromosome 21  Risk of having a baby with Down syndrome increases with maternal age  Multiple physical and mental abnormalities

66. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Nondisjunction also can occur with sex chromosomes (X and Y)  Nondisjunction of sex chromosomes during sperm formation  Resulting sperm will carry both X and Y or no sex chromosome at all  Nondisjunction of sex chromosomes during egg formation  Resulting egg will have two X chromosomes or none at all

67. Copyright © 2012 Pearson Education, Inc. Meiosis: Creation of Haploid Gametes  Turner syndrome  XO  Klinefelter syndrome  XXY

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