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Chromosomes and Cell Division: Understanding Mitosis and Meiosis

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  1. Chapter 19 Chromosomes and Cell Division Lecture Presentation Betty McGuireCornell University

  2. 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. Two Types of Cell Division • The human life cycle has two types of cell division • Meiosis • Mitosis

  4. 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. Two Types of Cell Division • Mitosis • Results in identical body cells • Occurs during growth and repair

  6. [INSERT FIGURE 19.1 ON THIS SLIDE]

  7. Two Types of Cell Division Web Activity: The Human Life Cycle

  8. 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. 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. 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. 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. 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. 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. 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. 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

  16. The Cell Cycle • Interphase • Period of growth and preparation for cell division; not a “resting period” • Three phases • G1 • S • G2

  17. The Cell Cycle • Interphase • G1 • First “gap” • Time of major growth before DNA synthesis begins • Chromosomes consist of a strand of DNA and proteins

  18. 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

  19. The Cell Cycle • Interphase • G2 • Second “gap” • Period after DNA is synthesized and before mitosis begins • Growth continues

  20. The Cell Cycle • Cell division consists of two processes • Mitosis • Division of the nucleus • Cytokinesis • Division of the cytoplasm

  21. 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)

  22. Mitosis: Creation of Genetically Identical Diploid Body Cells • Mitosis occurs in four phases • Prophase • Metaphase • Anaphase • Telophase

  23. 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

  24. 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

  25. 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

  26. Mitosis: Creation of Genetically Identical Diploid Body Cells • Telophase • Nuclear envelope forms around each group of chromosomes at each pole • Mitotic spindle disassembles

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

  28. Cytokinesis • Cytokinesis • Division of the cytoplasm • Begins during telophase • Ring of microfilaments contracts at midline of cell, eventually pinching cell in two

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. Meiosis: Creation of Haploid Gametes • Prophase I • Chromosomes condense • Homologous chromosomes pair • Phenomenon called synapsis

  35. Meiosis: Creation of Haploid Gametes • Metaphase I • Matched homologous pairs line up at the midline of cell and attach to spindle fibers

  36. 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

  37. Meiosis: Creation of Haploid Gametes • Telophase I • Nuclear envelope forms around chromosomes at each end of cell • Cytokinesis occurs to form two haploid cells

  38. 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

  39. Meiosis: Creation of Haploid Gametes • Meiosis II • Prophase II • Chromosomes condense • Metaphase II • Chromosomes line up along midline of cell

  40. 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

  41. 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

  42. Meiosis: Creation of Haploid Gametes • The important feature of meiosis II • It separates the two sister chromatids of each chromosome

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