Cellular Division copyright cmassengale
Cell Division <ul><li>All cells are derived from   pre-existing   cells </li></ul><ul><li>New cells are produced for  grow...
Keeping Cells Identical <ul><li>The  instructions for making cell parts  are encoded in the  DNA , so each new cell must g...
DNA Replication <ul><li>DNA must be copied or  replicated  before cell division  </li></ul><ul><li>Each new cell will then...
Identical Daughter Cells Parent Cell Two identical daughter cells copyright cmassengale
Chromosomes copyright cmassengale
Prokaryotic Chromosome <ul><li>The DNA of prokaryotes (bacteria) is  one, circular chromosome   attached to the inside of ...
Eukaryotic Chromosomes <ul><li>All  eukaryotic  cells store genetic information in chromosomes </li></ul><ul><li>Most euka...
Eukaryotic Chromosomes <ul><li>Each chromosome is composed of a  single,   tightly coiled DNA  molecule </li></ul><ul><li>...
Compacting DNA into Chromosomes <ul><li>DNA is tightly coiled around  proteins called histones </li></ul>copyright cmassen...
Chromosomes in Dividing Cells <ul><li>Duplicated chromosomes are called  chromatids  & are held together by the  centromer...
Karyotype <ul><li>A  picture  of the chromosomes from a human cell  arranged in pairs by size </li></ul><ul><li>First 22 p...
Boy or Girl? Y - Chromosome X - Chromosome The Y Chromosome Decides copyright cmassengale
Cell Reproduction copyright cmassengale
Types of Cell Reproduction <ul><li>Asexual reproduction  involves a  single cell  dividing to make  2 new, identical daugh...
Cell Division in Prokaryotes copyright cmassengale
Cell Division in Prokaryotes <ul><li>Prokaryotes such as  bacteria  divide into 2 identical cells by the process of  binar...
Prokaryotic Cell Undergoing Binary Fission copyright cmassengale
Animation of Binary Fission copyright cmassengale
The Cell Cycle copyright cmassengale
Five Phases of the Cell Cycle <ul><li>G 1  - primary growth phase </li></ul><ul><li>S – synthesis; DNA replicated </li></u...
Cell Cycle copyright cmassengale
Interphase - G 1  Stage <ul><li>1 st  growth stage  after cell division </li></ul><ul><li>Cells  mature  by making more cy...
Interphase – S Stage <ul><li>Synthesis  stage </li></ul><ul><li>DNA  is copied or  replicated </li></ul>Two identical copi...
Interphase – G 2  Stage <ul><li>2 nd  Growth  Stage </li></ul><ul><li>Occurs  after DNA has been copied </li></ul><ul><li>...
What’s Happening in Interphase? What the cell looks like Animal Cell What’s occurring copyright cmassengale
Sketch the Cell Cycle Daughter Cells DNA Copied Cells Mature Cells prepare for Division Cell Divides into Identical cells ...
Mitosis copyright cmassengale
Mitosis <ul><li>Division of the  nucleus </li></ul><ul><li>Also called  karyokinesis </li></ul><ul><li>Only occurs in  euk...
Four Mitotic Stages <ul><li>Pro phase </li></ul><ul><li>Meta phase </li></ul><ul><li>Ana phase </li></ul><ul><li>Telo phas...
Early Prophase <ul><li>Chromatin  in nucleus condenses to form  visible chromosomes </li></ul><ul><li>Mitotic spindle  for...
Late Prophase <ul><li>Nuclear membrane & nucleolus  are broken down </li></ul><ul><li>Chromosomes  continue condensing & a...
Late Prophase Nucleus & Nucleolus have disintegrated Chromosomes  copyright cmassengale
Spindle Fiber attached to Chromosome Kinetochore Fiber Chromosome copyright cmassengale
Review of Prophase What the cell looks like What’s happening copyright cmassengale
Spindle Fibers <ul><li>The mitotic  spindle  form from the  microtubules   in plants  and  centrioles in animal cells </li...
Sketch The Spindle copyright cmassengale
Metaphase <ul><ul><li>Chromosomes, attached to the  kinetochore fibers , move to the center of the cell </li></ul></ul><ul...
Metaphase Chromosomes lined at the Equator Asters at the poles Spindle Fibers copyright cmassengale
Metaphase Aster Chromosomes at Equator copyright cmassengale
Review of Metaphase What the cell looks like What’s occurring copyright cmassengale
Anaphase <ul><li>Occurs  rapidly </li></ul><ul><li>Sister chromatids  are pulled apart to  opposite poles  of the cell by ...
Anaphase Sister Chromatids being separated copyright cmassengale
Anaphase Review What the cell looks like What’s occurring copyright cmassengale
Telophase <ul><ul><li>Sister chromatids at  opposite poles </li></ul></ul><ul><ul><li>Spindle  disassembles </li></ul></ul...
Comparison of Anaphase & Telophase copyright cmassengale
Cytokinesis <ul><li>Means division of the cytoplasm </li></ul><ul><li>Division  of cell into two, identical halves called ...
Cytokinesis Cleavage furrow in animal cell Cell plate in plant cell copyright cmassengale
Mitotic Stages copyright cmassengale
Daughter Cells of Mitosis <ul><li>Have the  same number of chromosomes as each other   and as the parent cell  from which ...
Identical Daughter Cells Chromosome number the  same , but cells  smaller  than parent cell What is the 2n or diploid numb...
Review of Mitosis copyright cmassengale
Draw & Learn these Stages copyright cmassengale
Draw & Learn these Stages copyright cmassengale
Name the Mitotic Stages: Interphase Prophase Metaphase Anaphase Telophase Name this? Name this? copyright cmassengale
Eukaryotic Cell Division <ul><li>Used for  growth and repair </li></ul><ul><li>Produce two new cells  identical to the ori...
Mitosis Animation Name each stage as you see it occur? copyright cmassengale
Mitosis in Onion Root Tips Do you see any stages of mitosis? copyright cmassengale
Test Yourself over Mitosis copyright cmassengale
Mitosis Quiz copyright cmassengale
Mitosis Quiz copyright cmassengale
Name the Stages of Mitosis: Interphase Early prophase Mid-Prophase Late Prophase Metaphase Late Anaphase Early Anaphase Ea...
Identify the Stages Early, Middle, & Late Prophase  Late Prophase Metaphase Anaphase Late Anaphase Telophase Telophase & C...
Locate the Four Mitotic Stages in Plants Metaphase Prophase Anaphase Telophase copyright cmassengale
Uncontrolled Mitosis <ul><li>If mitosis is not controlled , unlimited cell division occurs causing cancerous tumors </li><...
Meiosis Formation of Gametes   (Eggs & Sperm) copyright cmassengale
Facts About Meiosis <ul><li>Preceded by interphase which includes   chromosome replication </li></ul><ul><li>Two   meiotic...
Facts About Meiosis <ul><li>Daughter cells   contain half  the number of chromosomes   as the original cell </li></ul><ul>...
More Meiosis Facts <ul><li>Start with  46 double stranded chromosomes (2n) </li></ul><ul><li>After 1 division -  23 double...
Why Do we Need Meiosis? <ul><li>It is the fundamental basis of  sexual reproduction </li></ul><ul><li>Two haploid (1n) gam...
Fertilization – “Putting it all together” 1n =3 2n = 6 copyright cmassengale
Replication of Chromosomes <ul><li>Replication  is the process of duplicating a chromosome </li></ul><ul><li>Occurs  prior...
A Replicated Chromosome Gene  X Homologs separate in meiosis I and therefore different alleles separate. copyright cmassen...
Meiosis Forms Haploid Gametes <ul><li>Meiosis must  reduce the chromosome number by half </li></ul><ul><li>Fertilization t...
Meiosis: Two Part Cell Division Homologs separate Sister  chromatids separate Diploid Diploid Haploid copyright cmassengal...
Meiosis I: Reduction Division Early Prophase I (Chromosome number doubled ) Late Prophase I Metaphase I Anaphase I Telopha...
Prophase I   <ul><li>Early prophase </li></ul><ul><li>Homologs pair. </li></ul><ul><li>Crossing over occurs . </li></ul><u...
Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids)   Join to form a   TETRAD Called Synapsis ...
Crossing-Over <ul><li>Homologous chromosomes  in a tetrad  cross over each other </li></ul><ul><li>Pieces of chromosomes o...
Homologous Chromosomes During Crossing-Over copyright cmassengale
Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortmen...
Metaphase I Homologous pairs  of chromosomes align along the  equator  of the cell copyright cmassengale
Anaphase I Homologs separate  and move to opposite poles. Sister chromatids  remain  attached  at their  centromeres . cop...
Telophase I Nuclear envelopes  reassemble. Spindle disappears. Cytokinesis  divides cell into two. copyright cmassengale
Meiosis II <ul><li>Only one homolog of each chromosome is present in the cell . </li></ul>Gene  X copyright cmassengale Me...
Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Anaphase II Telophase II 4 Genetically Different haploid c...
Prophase II Nuclear envelope fragments. Spindle forms. copyright cmassengale
Metaphase II Chromosomes align  along  equator  of cell. copyright cmassengale
Anaphase II Sister chromatids  separate and move to  opposite poles . Equator Pole copyright cmassengale
Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two. co...
Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome  One allele of each gene...
Meiosis Animation copyright cmassengale
Gametogenesis Oogenesis  or  Spermatogenesis copyright cmassengale
Spermatogenesis <ul><li>Occurs in the  testes </li></ul><ul><li>Two divisions produce 4 spermatids </li></ul><ul><li>Sperm...
Spermatogenesis in the Testes Spermatid copyright cmassengale
Spermatogenesis copyright cmassengale
Oogenesis <ul><li>Occurs in the  ovaries </li></ul><ul><li>Two divisions produce  3 polar bodies  that die and  1 egg </li...
Oogenesis in the Ovaries copyright cmassengale
Oogenesis copyright cmassengale Oogonium (diploid) Mitosis Primary oocyte (diploid) Meiosis I Secondary oocyte (haploid) M...
Comparing Mitosis and Meiosis copyright cmassengale
Comparison of Divisions copyright cmassengale Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 2 4 Genetic...
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Biology lecture 3

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  • Replication is the process of duplicating chromosome. The new copy of a chromosome is formed by DNA synthesis during S-phase. The chromosome copies are called sister chromatids. Sister chromatids are held together at the centromere.
  • Biology lecture 3

    1. 1. Cellular Division copyright cmassengale
    2. 2. Cell Division <ul><li>All cells are derived from pre-existing cells </li></ul><ul><li>New cells are produced for growth and to replace damaged or old cells </li></ul><ul><li>Differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals) </li></ul>copyright cmassengale
    3. 3. Keeping Cells Identical <ul><li>The instructions for making cell parts are encoded in the DNA , so each new cell must get a complete set of the DNA molecules </li></ul>copyright cmassengale
    4. 4. DNA Replication <ul><li>DNA must be copied or replicated before cell division </li></ul><ul><li>Each new cell will then have an identical copy of the DNA </li></ul>Original DNA strand Two new, identical DNA strands copyright cmassengale
    5. 5. Identical Daughter Cells Parent Cell Two identical daughter cells copyright cmassengale
    6. 6. Chromosomes copyright cmassengale
    7. 7. Prokaryotic Chromosome <ul><li>The DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane </li></ul><ul><li>Plasmid </li></ul>copyright cmassengale
    8. 8. Eukaryotic Chromosomes <ul><li>All eukaryotic cells store genetic information in chromosomes </li></ul><ul><li>Most eukaryotes have between 10 and 50 chromosomes in their body cells </li></ul><ul><li>Human body cells have 46 chromosomes or 23 identical pairs </li></ul>copyright cmassengale
    9. 9. Eukaryotic Chromosomes <ul><li>Each chromosome is composed of a single, tightly coiled DNA molecule </li></ul><ul><li>Chromosomes can’t be seen when cells aren’t dividing   and are called chromatin </li></ul>copyright cmassengale
    10. 10. Compacting DNA into Chromosomes <ul><li>DNA is tightly coiled around proteins called histones </li></ul>copyright cmassengale
    11. 11. Chromosomes in Dividing Cells <ul><li>Duplicated chromosomes are called chromatids & are held together by the centromere </li></ul>Called Sister Chromatids copyright cmassengale
    12. 12. Karyotype <ul><li>A picture of the chromosomes from a human cell arranged in pairs by size </li></ul><ul><li>First 22 pairs are called autosomes </li></ul><ul><li>Last pair are the sex chromosomes that are called genosome </li></ul><ul><li>XX female or XY male </li></ul>copyright cmassengale
    13. 13. Boy or Girl? Y - Chromosome X - Chromosome The Y Chromosome Decides copyright cmassengale
    14. 14. Cell Reproduction copyright cmassengale
    15. 15. Types of Cell Reproduction <ul><li>Asexual reproduction involves a single cell dividing to make 2 new, identical daughter cells </li></ul><ul><li>Mitosis & binary fission are examples of asexual reproduction </li></ul><ul><li>Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells </li></ul><ul><li>Meiosis is an example </li></ul>copyright cmassengale
    16. 16. Cell Division in Prokaryotes copyright cmassengale
    17. 17. Cell Division in Prokaryotes <ul><li>Prokaryotes such as bacteria divide into 2 identical cells by the process of binary fission </li></ul><ul><li>Single chromosome makes a copy of itself </li></ul><ul><li>Cell wall forms between the chromosomes dividing the cell </li></ul>Parent cell 2 identical daughter cells Chromosome replicates Cell splits copyright cmassengale
    18. 18. Prokaryotic Cell Undergoing Binary Fission copyright cmassengale
    19. 19. Animation of Binary Fission copyright cmassengale
    20. 20. The Cell Cycle copyright cmassengale
    21. 21. Five Phases of the Cell Cycle <ul><li>G 1 - primary growth phase </li></ul><ul><li>S – synthesis; DNA replicated </li></ul><ul><li>G 2 - secondary growth phase </li></ul><ul><ul><li>collectively these 3 stages are called interphase </li></ul></ul><ul><li>M - mitosis </li></ul><ul><li>C - cytokinesis </li></ul>copyright cmassengale
    22. 22. Cell Cycle copyright cmassengale
    23. 23. Interphase - G 1 Stage <ul><li>1 st growth stage after cell division </li></ul><ul><li>Cells mature by making more cytoplasm & organelles </li></ul><ul><li>Cell carries on its normal metabolic activities </li></ul>copyright cmassengale
    24. 24. Interphase – S Stage <ul><li>Synthesis stage </li></ul><ul><li>DNA is copied or replicated </li></ul>Two identical copies of DNA Original DNA copyright cmassengale
    25. 25. Interphase – G 2 Stage <ul><li>2 nd Growth Stage </li></ul><ul><li>Occurs after DNA has been copied </li></ul><ul><li>All cell structures needed for division are made (e.g. centrioles) </li></ul><ul><li>Both organelles & proteins are synthesized </li></ul>copyright cmassengale
    26. 26. What’s Happening in Interphase? What the cell looks like Animal Cell What’s occurring copyright cmassengale
    27. 27. Sketch the Cell Cycle Daughter Cells DNA Copied Cells Mature Cells prepare for Division Cell Divides into Identical cells copyright cmassengale
    28. 28. Mitosis copyright cmassengale
    29. 29. Mitosis <ul><li>Division of the nucleus </li></ul><ul><li>Also called karyokinesis </li></ul><ul><li>Only occurs in eukaryotes </li></ul><ul><li>Has four stages </li></ul><ul><li>Doesn’t occur in some cells such as brain cells </li></ul>copyright cmassengale
    30. 30. Four Mitotic Stages <ul><li>Pro phase </li></ul><ul><li>Meta phase </li></ul><ul><li>Ana phase </li></ul><ul><li>Telo phase </li></ul>copyright cmassengale
    31. 31. Early Prophase <ul><li>Chromatin in nucleus condenses to form visible chromosomes </li></ul><ul><li>Mitotic spindle forms from fibers in cytoskeleton or centrioles (animal) </li></ul>Chromosomes Nucleolus Cytoplasm Nuclear Membrane copyright cmassengale
    32. 32. Late Prophase <ul><li>Nuclear membrane & nucleolus are broken down </li></ul><ul><li>Chromosomes continue condensing & are clearly visible </li></ul><ul><li>Spindle fibers called kinetochores attach to the centromere of each chromosome </li></ul><ul><li>Spindle finishes forming between the poles of the cell </li></ul>copyright cmassengale
    33. 33. Late Prophase Nucleus & Nucleolus have disintegrated Chromosomes copyright cmassengale
    34. 34. Spindle Fiber attached to Chromosome Kinetochore Fiber Chromosome copyright cmassengale
    35. 35. Review of Prophase What the cell looks like What’s happening copyright cmassengale
    36. 36. Spindle Fibers <ul><li>The mitotic spindle form from the microtubules in plants and centrioles in animal cells </li></ul><ul><li>Polar fibers extend from one pole of the cell to the opposite pole </li></ul><ul><li>Kinetochore fibers extend from the pole to the centromere of the chromosome to which they attach </li></ul><ul><li>Asters are short fibers radiating from centrioles </li></ul>copyright cmassengale
    37. 37. Sketch The Spindle copyright cmassengale
    38. 38. Metaphase <ul><ul><li>Chromosomes, attached to the kinetochore fibers , move to the center of the cell </li></ul></ul><ul><ul><li>Chromosomes are now lined up at the equator </li></ul></ul>Pole of the Cell Equator of Cell copyright cmassengale
    39. 39. Metaphase Chromosomes lined at the Equator Asters at the poles Spindle Fibers copyright cmassengale
    40. 40. Metaphase Aster Chromosomes at Equator copyright cmassengale
    41. 41. Review of Metaphase What the cell looks like What’s occurring copyright cmassengale
    42. 42. Anaphase <ul><li>Occurs rapidly </li></ul><ul><li>Sister chromatids are pulled apart to opposite poles of the cell by kinetochore fibers </li></ul>copyright cmassengale
    43. 43. Anaphase Sister Chromatids being separated copyright cmassengale
    44. 44. Anaphase Review What the cell looks like What’s occurring copyright cmassengale
    45. 45. Telophase <ul><ul><li>Sister chromatids at opposite poles </li></ul></ul><ul><ul><li>Spindle disassembles </li></ul></ul><ul><ul><li>Nuclear envelope forms around each set of sister chromatids </li></ul></ul><ul><ul><li>Nucleolus reappears </li></ul></ul><ul><ul><li>CYTOKINESIS occurs </li></ul></ul><ul><ul><li>Chromosomes reappear as chromatin </li></ul></ul>copyright cmassengale
    46. 46. Comparison of Anaphase & Telophase copyright cmassengale
    47. 47. Cytokinesis <ul><li>Means division of the cytoplasm </li></ul><ul><li>Division of cell into two, identical halves called daughter cells </li></ul><ul><li>In plant cells, cell plate forms at the equator to divide cell </li></ul><ul><li>In animal cells, cleavage furrow forms to split cell </li></ul>copyright cmassengale
    48. 48. Cytokinesis Cleavage furrow in animal cell Cell plate in plant cell copyright cmassengale
    49. 49. Mitotic Stages copyright cmassengale
    50. 50. Daughter Cells of Mitosis <ul><li>Have the same number of chromosomes as each other and as the parent cell from which they were formed </li></ul><ul><li>Identical to each other, but smaller than parent cell </li></ul><ul><li>Must grow in size to become mature cells (G 1 of Interphase) </li></ul>copyright cmassengale
    51. 51. Identical Daughter Cells Chromosome number the same , but cells smaller than parent cell What is the 2n or diploid number? 2 copyright cmassengale
    52. 52. Review of Mitosis copyright cmassengale
    53. 53. Draw & Learn these Stages copyright cmassengale
    54. 54. Draw & Learn these Stages copyright cmassengale
    55. 55. Name the Mitotic Stages: Interphase Prophase Metaphase Anaphase Telophase Name this? Name this? copyright cmassengale
    56. 56. Eukaryotic Cell Division <ul><li>Used for growth and repair </li></ul><ul><li>Produce two new cells identical to the original cell </li></ul><ul><li>Cells are diploid (2n) </li></ul>Chromosomes during Metaphase of mitosis Prophase Metaphase Anaphase Telophase Cytokinesis copyright cmassengale
    57. 57. Mitosis Animation Name each stage as you see it occur? copyright cmassengale
    58. 58. Mitosis in Onion Root Tips Do you see any stages of mitosis? copyright cmassengale
    59. 59. Test Yourself over Mitosis copyright cmassengale
    60. 60. Mitosis Quiz copyright cmassengale
    61. 61. Mitosis Quiz copyright cmassengale
    62. 62. Name the Stages of Mitosis: Interphase Early prophase Mid-Prophase Late Prophase Metaphase Late Anaphase Early Anaphase Early Telophase, Begin cytokinesis Late telophase, Advanced cytokinesis copyright cmassengale
    63. 63. Identify the Stages Early, Middle, & Late Prophase Late Prophase Metaphase Anaphase Late Anaphase Telophase Telophase & Cytokinesis ? ? ? ? ? ? ? copyright cmassengale
    64. 64. Locate the Four Mitotic Stages in Plants Metaphase Prophase Anaphase Telophase copyright cmassengale
    65. 65. Uncontrolled Mitosis <ul><li>If mitosis is not controlled , unlimited cell division occurs causing cancerous tumors </li></ul><ul><li>Oncogenes are special proteins that increase the chance that a normal cell develops into a tumor cell </li></ul>Cancer cells copyright cmassengale
    66. 66. Meiosis Formation of Gametes (Eggs & Sperm) copyright cmassengale
    67. 67. Facts About Meiosis <ul><li>Preceded by interphase which includes chromosome replication </li></ul><ul><li>Two meiotic divisions --- Meiosis I and Meiosis II </li></ul><ul><li>Called Reduction- division </li></ul><ul><li>Original cell is diploid (2n) </li></ul><ul><li>Four daughter cells produced that are monoploid (1n) </li></ul>copyright cmassengale
    68. 68. Facts About Meiosis <ul><li>Daughter cells contain half the number of chromosomes as the original cell </li></ul><ul><li>Produces gametes (eggs & sperm ) </li></ul><ul><li>Occurs in the testes in males ( Spermatogenesis ) </li></ul><ul><li>Occurs in the ovaries in females ( Oogenesis ) </li></ul>copyright cmassengale
    69. 69. More Meiosis Facts <ul><li>Start with 46 double stranded chromosomes (2n) </li></ul><ul><li>After 1 division - 23 double stranded chromosomes (n) </li></ul><ul><li>After 2nd division - 23 single stranded chromosomes (n) </li></ul><ul><li>  Occurs in our germ cells that produce gametes </li></ul>copyright cmassengale
    70. 70. Why Do we Need Meiosis? <ul><li>It is the fundamental basis of sexual reproduction </li></ul><ul><li>Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote </li></ul>copyright cmassengale
    71. 71. Fertilization – “Putting it all together” 1n =3 2n = 6 copyright cmassengale
    72. 72. Replication of Chromosomes <ul><li>Replication is the process of duplicating a chromosome </li></ul><ul><li>Occurs prior to division </li></ul><ul><li>Replicated copies are called sister chromatids </li></ul><ul><li>Held together at centromere </li></ul>Occurs in Interphase copyright cmassengale
    73. 73. A Replicated Chromosome Gene X Homologs separate in meiosis I and therefore different alleles separate. copyright cmassengale Homologs (same genes, different alleles) Sister Chromatids (same genes, same alleles)
    74. 74. Meiosis Forms Haploid Gametes <ul><li>Meiosis must reduce the chromosome number by half </li></ul><ul><li>Fertilization then restores the 2n number </li></ul>The right number! copyright cmassengale from mom from dad child meiosis reduces genetic content too much!
    75. 75. Meiosis: Two Part Cell Division Homologs separate Sister chromatids separate Diploid Diploid Haploid copyright cmassengale Meiosis I Meiosis II
    76. 76. Meiosis I: Reduction Division Early Prophase I (Chromosome number doubled ) Late Prophase I Metaphase I Anaphase I Telophase I (diploid) copyright cmassengale Nucleus Spindle fibers Nuclear envelope
    77. 77. Prophase I <ul><li>Early prophase </li></ul><ul><li>Homologs pair. </li></ul><ul><li>Crossing over occurs . </li></ul><ul><li>Late prophase </li></ul><ul><li>Chromosomes condense. </li></ul><ul><li>Spindle forms. </li></ul><ul><li>Nuclear envelope fragments. </li></ul>copyright cmassengale
    78. 78. Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids)   Join to form a TETRAD Called Synapsis copyright cmassengale
    79. 79. Crossing-Over <ul><li>Homologous chromosomes in a tetrad cross over each other </li></ul><ul><li>Pieces of chromosomes or genes are exchanged </li></ul><ul><li>Produces Genetic recombination in the offspring </li></ul>copyright cmassengale
    80. 80. Homologous Chromosomes During Crossing-Over copyright cmassengale
    81. 81. Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment copyright cmassengale
    82. 82. Metaphase I Homologous pairs of chromosomes align along the equator of the cell copyright cmassengale
    83. 83. Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres . copyright cmassengale
    84. 84. Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. copyright cmassengale
    85. 85. Meiosis II <ul><li>Only one homolog of each chromosome is present in the cell . </li></ul>Gene X copyright cmassengale Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Sister chromatids carry identical genetic information .
    86. 86. Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Anaphase II Telophase II 4 Genetically Different haploid cells copyright cmassengale
    87. 87. Prophase II Nuclear envelope fragments. Spindle forms. copyright cmassengale
    88. 88. Metaphase II Chromosomes align along equator of cell. copyright cmassengale
    89. 89. Anaphase II Sister chromatids separate and move to opposite poles . Equator Pole copyright cmassengale
    90. 90. Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two. copyright cmassengale
    91. 91. Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome copyright cmassengale
    92. 92. Meiosis Animation copyright cmassengale
    93. 93. Gametogenesis Oogenesis or Spermatogenesis copyright cmassengale
    94. 94. Spermatogenesis <ul><li>Occurs in the testes </li></ul><ul><li>Two divisions produce 4 spermatids </li></ul><ul><li>Spermatids mature into sperm </li></ul><ul><li>Men produce about 250,000,000 sperm per day </li></ul>copyright cmassengale
    95. 95. Spermatogenesis in the Testes Spermatid copyright cmassengale
    96. 96. Spermatogenesis copyright cmassengale
    97. 97. Oogenesis <ul><li>Occurs in the ovaries </li></ul><ul><li>Two divisions produce 3 polar bodies that die and 1 egg </li></ul><ul><li>Polar bodies die because of unequal division of cytoplasm </li></ul><ul><li>Immature egg called oocyte </li></ul><ul><li>Starting at puberty, one oocyte matures into an ovum (egg) every 28 days </li></ul>copyright cmassengale
    98. 98. Oogenesis in the Ovaries copyright cmassengale
    99. 99. Oogenesis copyright cmassengale Oogonium (diploid) Mitosis Primary oocyte (diploid) Meiosis I Secondary oocyte (haploid) Meiosis II (if fertilization occurs) First polar body may divide (haploid) Polar bodies die Ovum (egg) Second polar body (haploid) a A X X a X A X a X a X Mature egg A X A X
    100. 100. Comparing Mitosis and Meiosis copyright cmassengale
    101. 101. Comparison of Divisions copyright cmassengale Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 2 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction

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