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  1. 1. Cell Replication <ul><li>The Cell Cycle, Mitosis, Meiosis, and Basic Inheritance </li></ul>Figure 12.1
  2. 2. <ul><li>Unicellular organisms </li></ul><ul><ul><li>Reproduce by cell division </li></ul></ul><ul><li>Multicellular organisms depend on cell division for </li></ul><ul><ul><li>a. Development from a fertilized cell </li></ul></ul><ul><ul><li>b. Growth </li></ul></ul><ul><ul><li>c. Repair </li></ul></ul>100 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). Figure 12.2 A 20 µm 200 µm (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM). Figure 12.2 B, C
  3. 3. <ul><li>Cell division results in genetically identical daughter cells </li></ul><ul><li>The DNA molecules in a cell </li></ul><ul><ul><li>Are packaged into chromosomes </li></ul></ul><ul><li>A cell’s endowment of DNA, its genetic information </li></ul><ul><ul><li>Is called its genome </li></ul></ul>
  4. 4. <ul><li>Eukaryotic chromosomes </li></ul><ul><ul><li>Consist of chromatin, a complex of DNA and protein that condenses during cell division </li></ul></ul><ul><li>In animals </li></ul><ul><ul><li>Somatic cells have two sets of chromosomes (diploid) </li></ul></ul><ul><ul><li>Gametes have one set of chromosomes (haploid) </li></ul></ul>
  5. 5. <ul><li>In preparation for cell division </li></ul><ul><ul><li>DNA is replicated and the chromosomes condense </li></ul></ul><ul><li>Each duplicated chromosome </li></ul><ul><ul><li>Has two sister chromatids, which separate during cell division </li></ul></ul>0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centromeres Sister chromatids A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Figure 12.4
  6. 6. <ul><li>Eukaryotic cell division consists of </li></ul><ul><ul><li>Mitosis, the division of the nucleus </li></ul></ul><ul><ul><li>Cytokinesis, the division of the cytoplasm </li></ul></ul><ul><li>In meiosis </li></ul><ul><ul><li>Sex cells are produced after a reduction in chromosome number </li></ul></ul>
  7. 7. Phases of the Cell Cycle <ul><li>The cell cycle consists of </li></ul><ul><ul><li>The mitotic phase </li></ul></ul><ul><ul><li>Interphase </li></ul></ul>INTERPHASE G 1 S (DNA synthesis) G 2 Cytokinesis Mitosis MITOTIC (M) PHASE Figure 12.5
  8. 8. <ul><li>The mitotic phase </li></ul><ul><ul><li>Is made up of mitosis and cytokinesis </li></ul></ul><ul><li>Interphase can be divided into subphases </li></ul><ul><ul><li>G 1 phase </li></ul></ul><ul><ul><li>S phase </li></ul></ul><ul><ul><li>G 2 phase </li></ul></ul>
  9. 9. <ul><li>Mitosis consists of five distinct phases </li></ul><ul><ul><li>Prophase </li></ul></ul><ul><ul><li>Prometaphase </li></ul></ul>G 2 OF INTERPHASE PROPHASE PROMETAPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule Figure 12.6 Nonkinetochore microtubules
  10. 10. <ul><ul><li>Metaphase </li></ul></ul><ul><ul><li>Anaphase </li></ul></ul><ul><ul><li>Telophase </li></ul></ul>Centrosome at one spindle pole Daughter chromosomes METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope forming Figure 12.6
  11. 11. The Mitotic Spindle: A Closer Look <ul><li>The mitotic spindle </li></ul><ul><ul><li>Is an apparatus of microtubules that controls chromosome movement during mitosis </li></ul></ul><ul><li>The spindle arises from the centrosomes </li></ul><ul><ul><li>And includes spindle microtubules and asters </li></ul></ul>Centrosome Aster Sister chromatids Metaphase Plate Kinetochores Overlapping nonkinetochore microtubules Kinetochores microtubules Centrosome Chromosomes Microtubules 0.5 µm 1 µm Figure 12.7
  12. 12. <ul><li>In anaphase, sister chromatids separate </li></ul><ul><ul><li>And move along the kinetochore microtubules toward opposite ends of the cell </li></ul></ul>EXPERIMENT 1 The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow). Spindle pole Kinetochore Figure 12.8
  13. 13. <ul><li>Nonkinetechore microtubules from opposite poles </li></ul><ul><ul><li>Overlap and push against each other, elongating the cell </li></ul></ul><ul><li>In telophase </li></ul><ul><ul><li>Genetically identical daughter nuclei form at opposite ends of the cell </li></ul></ul>
  14. 14. Cytokinesis: A Closer Look <ul><li>In animal cells </li></ul><ul><ul><li>Cytokinesis occurs by a process known as cleavage, forming a cleavage furrow </li></ul></ul>Cleavage furrow Contractile ring of microfilaments Daughter cells 100 µm (a) Cleavage of an animal cell (SEM) Figure 12.9 A
  15. 15. <ul><li>In plant cells, during cytokinesis </li></ul><ul><ul><li>A cell plate forms </li></ul></ul>Daughter cells 1 µm Vesicles forming cell plate Wall of patent cell Cell plate New cell wall (b) Cell plate formation in a plant cell (SEM) Figure 12.9 B
  16. 16. <ul><li>Mitosis in a plant cell </li></ul>1 Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is staring to from. Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelop will fragment. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their kinetochore microtubles shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell. 2 3 4 5 Nucleus Nucleolus Chromosome Chromatine condensing Figure 12.10
  17. 17. Binary Fission <ul><li>Prokaryotes (bacteria) </li></ul><ul><ul><li>Reproduce by a type of cell division called binary fission </li></ul></ul>Origin of replication E. coli cell Bacterial Chromosome Cell wall Plasma Membrane Two copies of origin Origin Origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. 1 Replication continues. One copy of the origin is now at each end of the cell. 2 Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. 3
  18. 18. <ul><li>The cell cycle is regulated by a molecular control system </li></ul><ul><li>The frequency of cell division </li></ul><ul><ul><li>Varies with the type of cell </li></ul></ul><ul><li>These cell cycle differences </li></ul><ul><ul><li>Result from regulation at the molecular level </li></ul></ul>
  19. 19. The Cell Cycle Control System <ul><li>The sequential events of the cell cycle </li></ul><ul><ul><li>Are directed by a distinct cell cycle control system, which is similar to a clock </li></ul></ul>Figure 12.14 Control system G 2 checkpoint M checkpoint G 1 checkpoint G 1 S G 2 M
  20. 20. <ul><li>The clock has specific checkpoints </li></ul><ul><ul><li>Where the cell cycle stops until a go-ahead signal is received </li></ul></ul>G 1 checkpoint G 1 G 1 G 0 (a) If a cell receives a go-ahead signal at the G 1 checkpoint, the cell continues      on in the cell cycle. (b) If a cell does not receive a go-ahead signal at the G 1 checkpoint, the cell exits the cell cycle and goes into G 0 , a nondividing state. Figure 12.15 A, B
  21. 21. The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases <ul><li>Two types of regulatory proteins are involved in cell cycle control </li></ul><ul><li>Cyclins and cyclin-dependent kinases (Cdks) </li></ul>
  22. 22. <ul><li>The activity of cyclins and Cdks </li></ul><ul><ul><li>Fluctuates during the cell cycle </li></ul></ul>During G 1 , conditions in the cell favor degradation of cyclin, and the Cdk component of MPF is recycled. 5 During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G 1 phase. 4 Accumulated cyclin molecules combine with recycled Cdk mol- ecules, producing enough molecules of MPF to pass the G 2 checkpoint and initiate the events of mitosis. 2 Synthesis of cyclin begins in late S phase and continues through G 2 . Because cyclin is protected from degradation during this stage, it accumulates. 1 Cdk Cdk G 2 checkpoint Cyclin MPF Cyclin is degraded Degraded Cyclin G 1 G 2 S M G 1 G 1 S G 2 G 2 S M M MPF activity Cyclin Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle (b) Molecular mechanisms that help regulate the cell cycle MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase. 3 Figure 12.16 A, B M
  23. 23. Stop and Go Signs: Internal and External Signals at the Checkpoints <ul><li>Both internal and external signals </li></ul><ul><ul><li>Control the cell cycle checkpoints </li></ul></ul>
  24. 24. <ul><li>Cancer cells </li></ul><ul><ul><li>Exhibit neither density-dependent inhibition nor anchorage dependence </li></ul></ul>25 µm Cancer cells do not exhibit anchorage dependence or density-dependent inhibition. Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells. (b) Figure 12.18 B
  25. 25. Loss of Cell Cycle Controls in Cancer Cells <ul><li>Cancer cells </li></ul><ul><ul><li>Do not respond normally to the body’s control mechanisms </li></ul></ul><ul><ul><li>Form tumors </li></ul></ul>
  26. 26. <ul><li>Malignant tumors invade surrounding tissues and can metastasize </li></ul><ul><ul><li>Exporting cancer cells to other parts of the body where they may form secondary tumors </li></ul></ul>Figure 12.19 Cancer cells invade neighboring tissue. 2 A small percentage of cancer cells may survive and establish a new tumor in another part of the body. 4 Cancer cells spread through lymph and blood vessels to other parts of the body. 3 A tumor grows from a single cancer cell. 1 Tumor Glandular tissue Cancer cell Blood vessel Lymph vessel Metastatic Tumor
  27. 27. Meiosis <ul><li>Overview: Hereditary Similarity and Variation </li></ul><ul><li>Living organisms </li></ul><ul><ul><li>Are distinguished by their ability to reproduce their own kind </li></ul></ul>
  28. 28. <ul><li>Heredity </li></ul><ul><ul><li>Is the transmission of traits from one generation to the next </li></ul></ul><ul><li>Variation </li></ul><ul><ul><li>Shows that offspring differ somewhat in appearance from parents and siblings </li></ul></ul><ul><li>Genetics </li></ul><ul><ul><li>Is the scientific study of heredity and hereditary variation </li></ul></ul>Figure 13.1
  29. 29. <ul><li>Offspring acquire genes from parents by inheriting chromosomes </li></ul><ul><li>Genes </li></ul><ul><ul><li>Are the units of heredity </li></ul></ul><ul><ul><li>Are segments of DNA </li></ul></ul><ul><li>Each gene in an organism’s DNA </li></ul><ul><ul><li>Has a specific locus on a certain chromosome </li></ul></ul><ul><li>We inherit </li></ul><ul><ul><li>One set of chromosomes from our mother and one set from our father </li></ul></ul>
  30. 30. Comparison of Asexual and Sexual Reproduction <ul><li>In asexual reproduction </li></ul><ul><ul><li>One parent produces genetically identical offspring by mitosis </li></ul></ul>Figure 13.2 Parent Bud 0.5 mm
  31. 31. <ul><li>In sexual reproduction </li></ul><ul><ul><li>Two parents give rise to offspring that have unique combinations of genes inherited from the two parents </li></ul></ul><ul><li>Fertilization and meiosis alternate in sexual life cycles </li></ul><ul><li>A life cycle </li></ul><ul><ul><li>Is the generation-to-generation sequence of stages in the reproductive history of an organism </li></ul></ul>
  32. 32. Sets of Chromosomes in Human Cells <ul><li>In humans </li></ul><ul><ul><li>Each somatic cell has 46 chromosomes, made up of two sets (diploid 2 n = 46) </li></ul></ul><ul><ul><li>One set of chromosomes comes from each parent (haploid) </li></ul></ul>
  33. 33. <ul><li>A karyotype </li></ul><ul><ul><li>Is an ordered, visual representation of the chromosomes in a cell </li></ul></ul>5 µ m Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3
  34. 34. <ul><li>Homologous chromosomes </li></ul><ul><ul><li>Are the two chromosomes composing a pair </li></ul></ul><ul><ul><li>Have the same characteristics </li></ul></ul><ul><ul><li>May also be called autosomes </li></ul></ul><ul><li>Sex chromosomes </li></ul><ul><ul><li>Are distinct from each other in their characteristics </li></ul></ul><ul><ul><li>Are represented as X and Y </li></ul></ul><ul><ul><li>Determine the sex of the individual, XX being female, XY being male </li></ul></ul>
  35. 35. <ul><li>In a cell in which DNA synthesis has occurred </li></ul><ul><ul><li>All the chromosomes are duplicated and thus each consists of two identical sister chromatids </li></ul></ul>Key Maternal set of chromosomes ( n = 3) Paternal set of chromosomes ( n = 3) 2 n = 6 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere Figure 13.4
  36. 36. <ul><li>Unlike somatic cells </li></ul><ul><ul><li>Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes </li></ul></ul><ul><li>At sexual maturity </li></ul><ul><ul><li>The ovaries and testes produce haploid gametes by meiosis </li></ul></ul><ul><li>During fertilization </li></ul><ul><ul><li>These gametes, sperm and ovum, fuse, forming a diploid zygote </li></ul></ul><ul><li>The zygote </li></ul><ul><ul><li>Develops into an adult organism </li></ul></ul>
  37. 37. <ul><li>The human life cycle </li></ul>Figure 13.5 Key Haploid ( n ) Diploid (2 n ) Haploid gametes ( n = 23) Ovum ( n ) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2 n = 46) Mitosis and development Multicellular diploid adults (2 n = 46)
  38. 38. <ul><li>In animals </li></ul><ul><ul><li>Meiosis occurs during gamete formation </li></ul></ul><ul><ul><li>Gametes are the only haploid cells </li></ul></ul>Gametes Figure 13.6 A Diploid multicellular organism Key MEIOSIS FERTILIZATION n n n 2n 2n Zygote Haploid Diploid Mitosis (a) Animals
  39. 39. <ul><li>Plants and some algae </li></ul><ul><ul><li>Exhibit an alternation of generations </li></ul></ul><ul><ul><li>The life cycle includes both diploid and haploid multicellular stages </li></ul></ul>MEIOSIS FERTILIZATION n n n n n 2n 2n Haploid multicellular organism (gametophyte) Mitosis Mitosis Spores Gametes Mitosis Zygote Diploid multicellular organism (sporophyte) (b) Plants and some algae Figure 13.6 B
  40. 40. <ul><li>In most fungi and some protists </li></ul><ul><ul><li>Meiosis produces haploid cells that give rise to a haploid multicellular adult organism </li></ul></ul><ul><ul><li>The haploid adult carries out mitosis, producing cells that will become gametes </li></ul></ul>MEIOSIS FERTILIZATION n n n n n 2n Haploid multicellular organism Mitosis Mitosis Gametes Zygote (c) Most fungi and some protists Figure 13.6 C
  41. 41. <ul><li>Meiosis reduces the number of chromosome sets from diploid to haploid </li></ul><ul><li>Meiosis </li></ul><ul><ul><li>Takes place in two sets of divisions, meiosis I and meiosis II </li></ul></ul>
  42. 42. The Stages of Meiosis <ul><li>An overview of meiosis </li></ul>Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous chromosomes separate Haploid cells with replicated chromosomes Sister chromatids separate Haploid cells with unreplicated chromosomes Meiosis I Meiosis II
  43. 43. <ul><li>Meiosis I </li></ul><ul><ul><li>Reduces the number of chromosomes from diploid to haploid </li></ul></ul><ul><li>Meiosis II </li></ul><ul><ul><li>Produces four haploid daughter cells </li></ul></ul>
  44. 44. <ul><li>Interphase and meiosis I </li></ul>Figure 13.8 Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2 n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I
  45. 45. <ul><li>Telophase I, cytokinesis, and meiosis II </li></ul>TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8
  46. 46. A Comparison of Mitosis and Meiosis <ul><li>Meiosis and mitosis can be distinguished from mitosis </li></ul><ul><ul><li>By three events in Meiosis l </li></ul></ul><ul><li>a. Synapsis and crossing over Homologous chromosomes physically connect and exchange genetic information </li></ul><ul><li>b. Tetrads on the metaphase plate </li></ul><ul><li>At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates </li></ul><ul><li>c. Separation of homologues </li></ul><ul><ul><li>At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell </li></ul></ul><ul><ul><li>In anaphase II of meiosis, the sister chromatids separate </li></ul></ul>
  47. 47. <ul><li>A comparison of mitosis and meiosis </li></ul>Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n n n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2 n 2 n Daughter cells of mitosis 2 n = 6
  48. 48. <ul><li>Genetic variation produced in sexual life cycles contributes to greater variation </li></ul><ul><li>Reshuffling of genetic material in meiosis </li></ul><ul><ul><li>Produces genetic variation </li></ul></ul><ul><li>In species that produce sexually </li></ul><ul><ul><li>The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation </li></ul></ul>
  49. 49. Independent Assortment of Chromosomes <ul><li>Homologous pairs of chromosomes </li></ul><ul><ul><li>Orient randomly at metaphase I of meiosis </li></ul></ul>
  50. 50. <ul><li>In independent assortment </li></ul><ul><ul><li>Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs </li></ul></ul>Figure 13.10 Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4
  51. 51. Crossing Over <ul><li>Crossing over </li></ul><ul><ul><li>Produces recombinant chromosomes that carry genes derived from two different parents </li></ul></ul> Variation Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes
  52. 52. Random Fertilization <ul><li>The fusion of gametes </li></ul><ul><ul><li>Will produce a zygote with any of about 64 trillion diploid combinations </li></ul></ul>Number of children from one couple without two exactly the same: 10 2017
  53. 53. <ul><li>Mutations </li></ul><ul><ul><li>Are the original source of genetic variation </li></ul></ul><ul><li>Sexual reproduction </li></ul><ul><ul><li>Produces new combinations of variant genes, adding more genetic diversity </li></ul></ul>
  54. 62. Wild dogs 1 Aardwolf African wild dog Arctic fox Argentine gray fox Black-backed jackal Blanford’s fox Bat-eared fox Bush dog Wild dogs
  55. 63. Wild dogs 2 Wild dogs Arctic wolf Cape fox Corsac fox Coyote Crab-eating fox Culpeo fox Dhole Fennec fox
  56. 64. Wild dogs Arctic fox Dingo Dingo Ethiopian wolf Falkland Island’s fox Golden jackal Tibetan sand fox Gray wolf
  57. 65. Wild dogs 4 Wild dogs Gray fox Hoary zorro Kit fox Maned wolf Mexican gray wolf Raccoon dog Sand fox Small-eared dog
  58. 66. Wild dogs 5 Wild dogs Pale fox Pampas fox Red fox Red wolf Sechuan zorro Timber wolf Iberian wolf
  59. 67. Q 910 Savolainen, et.al The origin of the domestic dog from wolves has been established … we examined the mitochondrial DNA (mtDNA) sequence variation among 654 domestic dogs representing all major dog populations worldwide … suggesting a common origin from a single gene pool for all dog populations . Q 910 Savolainen, et.al., ‘Genetic Evidence for an East Asian origin of Domestic Dogs,’ Science , Vol 298:5598, 22 Nov 2002, pp 1610-1613.