Cell Reproduction


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Cell Reproduction

  1. 1. Cell Reproduction Mitosis & Meiosis
  2. 2. Why Cells Divide <ul><li>Surface Area/ Volume Ratio </li></ul><ul><ul><li>As the cell grows, the volume increases at a greater rate than the surface area </li></ul></ul><ul><ul><li>Can't take in enough nutrients, or remove wastes </li></ul></ul><ul><ul><li>Therefore the cell must grow or divide </li></ul></ul><ul><li>Growth and Repair </li></ul><ul><ul><li>Replace worn or damaged cells </li></ul></ul><ul><ul><li>Frequency of replacement varies: </li></ul></ul><ul><ul><ul><li>bacteria ~ every 20 minutes </li></ul></ul></ul><ul><ul><ul><li>human cells ~ every 18-22 hours </li></ul></ul></ul><ul><ul><li>Many cells in the body don't divide </li></ul></ul>
  3. 3. Cell Division
  4. 4. Cellular Reproduction <ul><li>When the parent cell divides, it forms new daughter cells </li></ul><ul><li>Organisms reproduce in two ways: </li></ul><ul><ul><li>Asexual Reproduction </li></ul></ul><ul><ul><li>Sexual Reproduction </li></ul></ul>
  5. 5. Sexual vs. Asexual Reproduction <ul><li>Asexual Reproduction </li></ul><ul><ul><li>production of offspring from one parent </li></ul></ul><ul><ul><li>therefore genetic material is identical to parent </li></ul></ul><ul><li>Sexual Reproduction </li></ul><ul><ul><li>formation of a new individual from the union of 2 cells </li></ul></ul><ul><ul><li>2 parents, therefore offspring have some hereditary material from each </li></ul></ul>
  6. 6. Types of Asexual Reproduction <ul><li>Binary Fission </li></ul><ul><ul><li>simplest form; the cell splits in 2 </li></ul></ul><ul><li>Spore Formation </li></ul><ul><ul><li>Begins with replication </li></ul></ul><ul><ul><li>Spores can remain inactive until conditions are favorable </li></ul></ul><ul><ul><li>molds, fungi </li></ul></ul><ul><li>Yeast reproduce by budding </li></ul><ul><li>Vegetative Propagation </li></ul><ul><ul><li>Some plants, e.g. strawberries </li></ul></ul><ul><li>Regeneration </li></ul><ul><ul><li>planaria, star fish, etc. </li></ul></ul>
  7. 7. Cell Division in Prokaryotes <ul><li>Binary Fission: </li></ul><ul><li>The simplest form of cell division </li></ul><ul><li>The cell splits in 2 </li></ul>
  8. 8. The Process of Binary Fission <ul><li>First the single circular chromosome duplicates = Replication </li></ul><ul><li>Both chromosomes attach to sites on the cell membrane </li></ul><ul><li>As the cell grows, a new membrane forms between attachment sites </li></ul><ul><li>Membrane pinches off and the new cells separate </li></ul>
  9. 9. Sexual Reproduction <ul><li>The joining of 2 specialized sex cells called gametes </li></ul><ul><ul><li>male = sperm </li></ul></ul><ul><ul><li>female = ovum </li></ul></ul><ul><li>Process of combining gametes = fertilization </li></ul><ul><li>Fertilization produces a zygote </li></ul><ul><ul><li>has characteristics of both parents </li></ul></ul>
  10. 10. Human Sexual Reproduction <ul><li>Male testis produces sperm </li></ul><ul><li>Female ovary produces ova </li></ul><ul><li>Each has 23 chromosomes </li></ul><ul><li>Unite to form a zygote with 46 chromosomes </li></ul><ul><ul><li>23 pair </li></ul></ul><ul><li>Develops into a fetus </li></ul>
  11. 11. Cell Division <ul><li>All types of reproduction require cell division </li></ul><ul><li>2 processes can be used to divide the cell’s nuclear material: </li></ul><ul><li>Mitosis </li></ul><ul><ul><li>Occurs in somatic cells (body cells) in eukaryotes </li></ul></ul><ul><ul><li>As a result of mitosis each daughter cell receives an exact copy of the chromosomes present in the parent cell </li></ul></ul><ul><li>Meiosis </li></ul><ul><ul><li>Occurs in gametes (sex cells) </li></ul></ul><ul><ul><li>As a result each daughter cell receives 1 of each pair of chromosomes present in the parent cell </li></ul></ul>
  12. 12. Mitosis <ul><li>Cell division in eukaryotic cells involves nuclear division called mitosis </li></ul><ul><li>Occurs in somatic cells </li></ul><ul><ul><li>body cells; not sex cells </li></ul></ul><ul><li>As a result of mitosis, each daughter cell receives an exact copy of the chromosomes present in the parent cell </li></ul><ul><li>Chromosomes contain genetic material </li></ul><ul><ul><li>DNA </li></ul></ul>
  13. 13. Chromosomes <ul><li>During cell division in eukaryotic cells, the DNA is coiled into chromosomes </li></ul><ul><li>Every body cell of the same type of organism has the same number of chromosomes </li></ul><ul><ul><li>humans = 46 </li></ul></ul><ul><ul><li>goldfish = 94 </li></ul></ul><ul><ul><li>mosquito = 6 </li></ul></ul>
  14. 14. Chromosome Structure <ul><li>Each chromosome is formed from two joined strands called chromatids </li></ul><ul><li>Each chromatid is alike </li></ul><ul><ul><li>has a long arm & a short arm </li></ul></ul><ul><ul><li>joined at the centromere </li></ul></ul><ul><li>Chromosomes contain DNA and associated proreins </li></ul>
  15. 15. Chromosomal Proteins <ul><li>Each chromosome is a single DNA molecule and associated proteins </li></ul><ul><li>Histones – </li></ul><ul><ul><li>One type of chromosomal protein </li></ul></ul><ul><ul><li>The DNA wraps tightly around the histones </li></ul></ul><ul><ul><li>Histones help maintain the shape of the chromosome </li></ul></ul><ul><li>Nonhistone proteins - </li></ul><ul><ul><li>control the activity of specific regions of DNA </li></ul></ul>
  16. 16. Picturing Chromosome Structure
  17. 17. Visualizing Chromosomes
  18. 18. Chromosome Make-up <ul><li>Chromosomes of somatic cells are in pairs </li></ul><ul><ul><li>One of each pair comes from mother, one from father </li></ul></ul><ul><li>The 2 chromosomes in a pair are homologous </li></ul><ul><ul><li>Alike in appearance and type of genetic information carried </li></ul></ul><ul><li>Humans have 23 pairs of chromosomes </li></ul><ul><ul><li>22 pairs of autosomes </li></ul></ul><ul><ul><ul><li>Autosomes are all but the sex </li></ul></ul></ul><ul><ul><li>2 sex chromosomes ( X & Y) </li></ul></ul>
  19. 19. Sex Chromosomes <ul><li>Determine the sex of the organism </li></ul><ul><li>Also carry other genetic information </li></ul><ul><li>In humans, either X or Y </li></ul><ul><li>Females are XX, males are XY </li></ul><ul><li>Thus the male determines the sex of the offspring </li></ul>
  20. 20. Haploid vs. Diploid <ul><li>Cells with two copies of each chromosome = diploid </li></ul><ul><li>Autosomal cells are diploid </li></ul><ul><li>Gametes (sex cells) have only one of each type of chromosome </li></ul><ul><li>Cells with one copy of each chromosome = haploid </li></ul>
  21. 21. Karyotypes <ul><li>A picture of paired human chromosomes </li></ul><ul><li>Used to to detect certain genetic diseases </li></ul>
  22. 23. Mitosis <ul><li>The process of dividing the nuclear material in a somatic cell in eukaryotes </li></ul><ul><li>Necessary for cell division </li></ul>
  23. 24. Preparation for Mitosis <ul><li>Interphase </li></ul><ul><ul><li>The time between the formation of a cell through mitosis and the next mitosis </li></ul></ul><ul><li>Most of the cell cycle is interphase </li></ul><ul><li>During this phase cell prepares by: </li></ul><ul><ul><li>replicating genetic material </li></ul></ul><ul><ul><li>producing organelles </li></ul></ul><ul><ul><li>assembling structures needed for mitosis </li></ul></ul>
  24. 25. Chromosomes & Interphase <ul><li>During interphase chromosomes cannot be distinguished under the light microscope </li></ul><ul><ul><li>They appear as chromatin </li></ul></ul><ul><li>At the start of mitosis, the chromatin thickens, and chromosomes become visible </li></ul>
  25. 26. The Cell Cycle <ul><li>The sequence of cell growth and division </li></ul><ul><li>The cell cycle can last several hours to several days </li></ul><ul><li>Can be affected by environmental factors, like temperature </li></ul><ul><li>Has 4 stages: </li></ul><ul><ul><li>mitosis & division of cytoplasm ( cytokinesis ) </li></ul></ul><ul><ul><li>The other 3 are part of interphase : </li></ul></ul><ul><ul><ul><li>G1 </li></ul></ul></ul><ul><ul><ul><li>S </li></ul></ul></ul><ul><ul><ul><li>G2 </li></ul></ul></ul>
  26. 27. Picturing the Cell Cycle
  27. 28. G1 - Growth <ul><li>After mitosis, a period of intense cellular activity and growth </li></ul><ul><li>The cell doubles in size </li></ul><ul><li>Enzyme production is high </li></ul><ul><li>Cells that stop growing remain in G1 </li></ul>
  28. 29. S- Synthesis <ul><li>  Cells that divide enter S, or synthesis, phase </li></ul><ul><li>  The chromosomes replicate </li></ul>
  29. 30. G2 – Further Growth <ul><li>A second period of growth </li></ul><ul><li>Structures used in mitosis are assembled </li></ul>
  30. 31. The Phases of Mitosis <ul><li>Mitosis is actually a continuous process </li></ul><ul><li>But we divide it into 4 phases: </li></ul><ul><ul><li>Prophase </li></ul></ul><ul><ul><li>Metaphase </li></ul></ul><ul><ul><li>Anaphase </li></ul></ul><ul><ul><li>Telophase </li></ul></ul>
  31. 32. Prophase <ul><li>60% of the period of mitosis is prophase </li></ul><ul><li>Divided into 3 parts: early, middle, & late </li></ul><ul><li>Chromosomes begin to coil into short rods </li></ul><ul><li>Nucleoli break down & begin to disappear </li></ul><ul><li>2 pairs of dark spots called centrosomes appear outside the nuclear membrane </li></ul><ul><ul><li>In animal cells, the centrosomes contain centrioles , formed from microtubules </li></ul></ul><ul><ul><li>Plant cells have no centrioles </li></ul></ul><ul><li>The centrosomes move to opposite sides of the cell </li></ul>
  32. 33. Mid Prophase <ul><li>At the beginning of mid-prophase spindle fibers form between the centrioles </li></ul><ul><li>Additional fibers radiating out from each centriole form the aster </li></ul><ul><li>The nuclear membrane has broken down and disappeared </li></ul>
  33. 34. The Mitotic Spindle <ul><li>Spindle fibers made of microtubules radiate from the centrosomes </li></ul><ul><li>This array of spindle fibers = the mitotic spindle </li></ul><ul><li>2 types of spindle fibers: </li></ul><ul><li>Kinetechore fibers </li></ul><ul><ul><li>Attach to a disk-shaped protein called a kinetecore </li></ul></ul><ul><ul><li>Found in the centromere of each chromosome </li></ul></ul><ul><ul><li>Extend from the kinetechore of each chromatid to one of the centrosomes </li></ul></ul><ul><li>Polar fibers </li></ul><ul><ul><li>Extend across the dividing cell from one centrosome to the other </li></ul></ul>
  34. 36. Late Prophase <ul><li>The centrosome pairs are at opposite ends of the cell </li></ul><ul><li>The centosomes are fully formed </li></ul><ul><li>Chromosomes are attached to the centrosomes by spindle fibers </li></ul><ul><li>Other spindle fibers stretch across the cell from one centriole to the other </li></ul>
  35. 38. Metaphase <ul><li>The chromosomes are pushed and pulled by spindle fibers along cell's the midplane </li></ul><ul><ul><li>called the equator </li></ul></ul>
  36. 39. Anaphase <ul><li>Begins with the separation of chromatids in each chromosome </li></ul><ul><li>Spindle fibers appear to shorten, pulling the chromatids apart at the centomere </li></ul><ul><li>Each chromatid is now a chromosome </li></ul><ul><li>2 sets of separated chromosomes then move through the cytoplasm to opposite poles of the cell </li></ul>
  37. 41. Telophase <ul><li>The last stage of mitosis </li></ul><ul><li>After the individual chromosomes have reached opposite poles of the cell, spindles disappear </li></ul><ul><li>A nuclear membrane forms around each set of chromosomes </li></ul><ul><li>Chromosomes return to a thread-like mass </li></ul><ul><li>Centrioles duplicate </li></ul><ul><ul><li>2 centrioles formed in each daughter cell </li></ul></ul><ul><li>Nucleoli re-form within each newly formed nucleus </li></ul>
  38. 43. Cytokinesis <ul><li>The division of the cytoplasm </li></ul><ul><li>Follows mitosis </li></ul><ul><li>  Cytokinesis begins during telophase </li></ul><ul><li>In animal cells, the cell membrane pinches together </li></ul><ul><li>The area that pinches in and separates is called the cleavage furrow </li></ul><ul><li>In plants, a cell plate is formed, dividing the two halves </li></ul>
  39. 44. Picturing Cytokinesis
  40. 47. Chromosome Number <ul><li>Cells formed thru mitosis have the same number of chromosomes as the parent cells </li></ul><ul><li>If combined in sexual reproduction, the offspring would have 2x chromosomes! </li></ul><ul><li>Therefore gametes have only half the number of chromosomes of somatic cells </li></ul><ul><ul><li>Gametes = sex cells </li></ul></ul>
  41. 48. Meiosis <ul><li>Gametes need another process for nuclear division </li></ul><ul><li>Meiosis reduces the number of chromosomes to 1/2 the number in somatic cells </li></ul>
  42. 49. Meiosis I & II <ul><li>Forming haploid daughter cells from diploid parent cells requires two successive cell divisions: </li></ul><ul><ul><li>First = Meiosis I – </li></ul></ul><ul><ul><ul><li>homologous chromosomes separate </li></ul></ul></ul><ul><ul><li>Second = Meiosis II </li></ul></ul><ul><ul><ul><li>chromatids of each chromosome separate </li></ul></ul></ul>
  43. 50. Meiosis I <ul><li>Preceded by replication of DNA that forms the chromosome </li></ul><ul><li>Synapsis = pairing of homologous chromosomes </li></ul><ul><li>Each pair of homologous chromosomes twists around each other, forming a structure called a tetrad </li></ul><ul><li>Meiosis can be divided into same 4 phases as mitosis: </li></ul><ul><ul><li>Prophase, Metaphase, Anaphase, Telophase </li></ul></ul>
  44. 51. Prophase I <ul><li>  Chromatin begins to coil into short rods </li></ul><ul><li>  Homologous chromosomes are formed </li></ul><ul><li>  Spindle fibers appear </li></ul><ul><li>  Nucleoli break down </li></ul><ul><li>By the end, the nuclear membrane has dissolved, and tetrads are visible </li></ul>
  45. 52. Crossing Over <ul><li>During synapsis , (prophase I) the chromatids of homologous pairs twist around each other </li></ul><ul><li>A portion of one chromatid may break off and reattach, “trading” with the same piece from its homologous partner </li></ul><ul><li>The exchange of genes by reciprocal segments of homologous chromosomes during meiosis = “ crossing-over ” </li></ul>
  46. 53. <ul><li>Crossing over causes exchange of genetic material between maternal & paternal chromosomes </li></ul><ul><li>Results in genetic recombination </li></ul><ul><li>Genetic recombination is less likely in genes that are closer together. </li></ul>
  47. 54. Chromosome Mapping <ul><li>The likelihood that recombination will occur due to crossing-over depends on the genes’ distance from each other on the chromosome </li></ul><ul><li>Scientists can determine how frequently genes for particular traits occur together in offspring </li></ul><ul><li>This can be used to create a map of the chromosome </li></ul><ul><li>1% recombination (crossing-over) = 1 map unit </li></ul>
  48. 55. Metaphase I <ul><li>Tetrads line up along the equator of the cell </li></ul><ul><li>Each tetrad becomes attached to spindle fibers </li></ul>
  49. 56. Anaphase I <ul><li>Homologous chromosomes that form each tetrad are pulled apart in pairs </li></ul><ul><li>One pair goes to one end of the cell, the other to opposite end </li></ul>
  50. 57. Telophase I <ul><li>Chromosomes reach ends of the cell </li></ul><ul><li>Cell divides into 2 daughter cells </li></ul>
  51. 59. Independent Assortment <ul><li>During Anaphase I, one member of each homologous chromosome pair moves to one end of the cell, the other moves to the opposite end </li></ul><ul><li>The separation of homologous chromosomes is random </li></ul><ul><li>More, or fewer maternal (or paternal) chromosomes may end up on one side or the other </li></ul><ul><li>Each separation is independent of the others </li></ul><ul><li>This is the principal of independent assortment of chromosomes </li></ul><ul><li>Results in genetic variation </li></ul>
  52. 61. Meiosis I Summary <ul><li>Meiosis I is a Reductive Division </li></ul><ul><li>It reduces the number of chromosomes from diploid &quot;2n&quot; to haploid &quot;n&quot; </li></ul>
  53. 62. Meiosis II <ul><li>Similar to mitosis but not preceded by replication of DNA </li></ul><ul><li>4 Stages: </li></ul><ul><ul><li>Prophase II </li></ul></ul><ul><ul><li>Metaphase II </li></ul></ul><ul><ul><li>Anaphase II </li></ul></ul><ul><ul><li>Telophase II </li></ul></ul>
  54. 63. Prophase II & Metaphase II <ul><li>Prophase II </li></ul><ul><ul><li>A new spindle forms around paired chromatids </li></ul></ul><ul><li>Metaphase II </li></ul><ul><ul><li>Chromosomes line up along the equator </li></ul></ul><ul><ul><li>They are attached at the centromere to spindle fibers </li></ul></ul>
  55. 64. Anaphase II <ul><li>Centromeres duplicate & the chromatids separate </li></ul><ul><li>Resulting single chromatids move to opposite poles </li></ul><ul><li>Chromatids are now called chromosomes </li></ul>
  56. 65. Telophase II <ul><li>  A nuclear membrane forms around each set of chromosomes </li></ul><ul><li>  The spindle breaks down and cytokinesis occurs </li></ul><ul><li>  Result: 4 haploid daughter cells </li></ul>
  57. 67. Males vs. Females <ul><li>In males, all 4 daughter cells differentiate to become sperm </li></ul><ul><li>  In females, the cytoplasm divides unevenly in Meiosis I </li></ul><ul><ul><li>The smaller cell = first polar body </li></ul></ul><ul><ul><li>doesn't survive </li></ul></ul><ul><li>  In Meiosis II, the division is again unequal </li></ul><ul><ul><li>smaller half is second polar body </li></ul></ul><ul><li>So only 1 of 4 daughter cells survives </li></ul><ul><ul><li>rich in cytoplasm, has many nutrients to nourish the young organism </li></ul></ul>
  58. 68. Comparing Mitosis & Meiosis <ul><li>MEIOSIS </li></ul><ul><li>2 </li></ul><ul><li>4 </li></ul><ul><li>diploid </li></ul><ul><li>haploid </li></ul><ul><li>different </li></ul><ul><li>MITOSIS </li></ul><ul><li>1 </li></ul><ul><li>2 </li></ul><ul><li>diploid </li></ul><ul><li>diploid </li></ul><ul><li>identical </li></ul># of nuclear divisions: # of daughter cells: Parent cell type: Daughter cell type: Genetic likeness to parent:
  59. 70. Control of Cell Division <ul><li>Timing and rate of cell division varies in different cell types </li></ul><ul><li>Control of rate of division is critical </li></ul><ul><li>Some cells require regulatory substances to begin division = growth factors </li></ul>
  60. 71. Effect of Growth Factors
  61. 72. Effect of Density <ul><li>Density of cells also effects the rate of division </li></ul><ul><li>Crowding inhibits cell division </li></ul>
  62. 73. The Restriction Point <ul><li>A crucial checkpoint occurs late in the G1 phase of the cell cycle </li></ul><ul><li>Point of decision to divide = restriction point </li></ul><ul><li>Cell cannot turn back after this point </li></ul><ul><li>If it is “yes,” cell goes to S phase and copies DNA </li></ul><ul><li>If “no,” it goes to non-dividing state (G 0 ) </li></ul><ul><ul><li>Most cells are in G 0 </li></ul></ul>
  63. 74. MPF <ul><li>After S, the cell will enter G2 </li></ul><ul><li>The “OK” signal that causes the cell to proceed from G2 to mitosis = mitosis promoting factor (MPF) </li></ul><ul><ul><li>A complex of proteins </li></ul></ul><ul><li>MPF is an enzyme </li></ul><ul><ul><li>Protein kinase </li></ul></ul>
  64. 75. Abnormal Cell Division <ul><li>Cancer cells do not respond normally to the body’s control mechanisms for cell division </li></ul><ul><li>Cancer cells divide excessively </li></ul><ul><li>Can invade other body tissues </li></ul><ul><li>When a cell divides abnormally = transformed </li></ul><ul><li>Abnormal cells are usually destroyed by the immune system </li></ul>
  65. 76. Cancer <ul><li>If abnormal cells are not destroyed and reproduce, they may form a mass of abnormal cells = tumor </li></ul><ul><ul><li>Benign tumor = abnormal cells remain at the original site </li></ul></ul><ul><ul><li>Malignant tumor = cells spread to other parts of the body </li></ul></ul><ul><li>Metastasis = spread of cancer cells in the body </li></ul>
  66. 77. Breast Cancer Cell