Meiosis
Essential terms <ul><li>Gametogenesis </li></ul><ul><li>Synapsis </li></ul><ul><li>Crossing over </li></ul><ul><li>Disjunc...
Key points <ul><li>Mitosis –  diploid.   Identical.   Stability. </li></ul><ul><li>Meiosis –  haploid. Combination. Variat...
Meiosis <ul><li>Purpose : </li></ul><ul><li>production of sex cells (gametes) </li></ul><ul><li>Outcome  </li></ul><ul><li...
<ul><li>1. Chromosomes found in  homologous pairs </li></ul><ul><ul><li>identical but may have varying gene messages </li>...
<ul><li>Homologous chromosomes form pairs in form of tetrads (4 chromatids) </li></ul><ul><li>Involves 2 divisions = Meios...
Interphase <ul><li>chromosomes  replicate </li></ul><ul><li>Consists of 2  identical sister chromatids attached at their c...
Prophase I <ul><li>Condenstaion. Chromatin thickens and coils. Become visible . </li></ul><ul><li>Synapsis occurs  Homolog...
Metaphase I <ul><li>Chromosomes have thickened. </li></ul><ul><li>Each tetrad interacts with spindle fibers. </li></ul><ul...
Anaphase I <ul><li>sister chromatids remain attached while homologues move towards the opposite pole. (Mitosis =sister chr...
Telophase I + Cytokinesis <ul><li>Very short phase compared to mitosis </li></ul><ul><li>May not always occurs </li></ul><...
2nd division (II) <ul><li>Prophase II  = spindle apparatus forms </li></ul><ul><li>Chromosomes move towards the metaphase ...
<ul><li>Anaphase II =  centromeres of sister chromatids separate </li></ul><ul><li>sister chromatids are pulled towards op...
Cytokinesis <ul><li>4 haploid gametes results. </li></ul><ul><li>Haploid state has been achieved. </li></ul><ul><li>If cro...
 
Independent assortment <ul><li>Mechanism that allows for genetic variation. </li></ul><ul><li>Reduction of diploid – diplo...
Nondisjunction and Translocation <ul><li>Too few or too many chromosomes in gametes </li></ul><ul><li>Translocation: attac...
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Meiosis

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Meiosis

  1. 1. Meiosis
  2. 2. Essential terms <ul><li>Gametogenesis </li></ul><ul><li>Synapsis </li></ul><ul><li>Crossing over </li></ul><ul><li>Disjunction / Non disjunction </li></ul><ul><li>Chiasmata </li></ul>
  3. 3. Key points <ul><li>Mitosis – diploid. Identical. Stability. </li></ul><ul><li>Meiosis – haploid. Combination. Variation. </li></ul><ul><li>Sexual reproduction ensures genetic continuity and genetic variety, producing offspring's that often differ greatly from patients. </li></ul><ul><li>Associated with gametogenesis. </li></ul><ul><li>Counterbalances fertilization - makes sure # of chromosomes remains constant </li></ul>
  4. 4. Meiosis <ul><li>Purpose : </li></ul><ul><li>production of sex cells (gametes) </li></ul><ul><li>Outcome </li></ul><ul><li>2 divisions </li></ul><ul><li>4 daughter cells with 1/2 (haploid) sets of chromosomes </li></ul>
  5. 5. <ul><li>1. Chromosomes found in homologous pairs </li></ul><ul><ul><li>identical but may have varying gene messages </li></ul></ul><ul><li>b. Humans have 46 (23 pairs) </li></ul><ul><ul><li>22 homologous pairs (autosomes) </li></ul></ul><ul><ul><li>X and Y (23rd pair) sex chromosomes </li></ul></ul><ul><ul><li>Females (XX); Males (XY) </li></ul></ul><ul><li>c. Pairs separate during reproduction </li></ul><ul><ul><li>Offspring receives info from each parent </li></ul></ul><ul><ul><li>Exchanging genetic material during crossing over (Prophase I) </li></ul></ul><ul><li>d. Separated during meiosis (Anaphase I) </li></ul>
  6. 6. <ul><li>Homologous chromosomes form pairs in form of tetrads (4 chromatids) </li></ul><ul><li>Involves 2 divisions = Meiosis I and II </li></ul>
  7. 7. Interphase <ul><li>chromosomes replicate </li></ul><ul><li>Consists of 2 identical sister chromatids attached at their centromeres </li></ul><ul><li>centriole pairs also replicate into two pairs </li></ul>
  8. 8. Prophase I <ul><li>Condenstaion. Chromatin thickens and coils. Become visible . </li></ul><ul><li>Synapsis occurs Homologous chr. pair up forming tetrads. </li></ul><ul><li>Sister chromatids are attached at centromeres. </li></ul><ul><li>Each pair = bivalent. </li></ul><ul><li>Nonsister chromatids features crossing over resulting in chiasmata. = More than 1 can form = genetic variation) </li></ul><ul><li>• spindle forms from microtubules </li></ul><ul><li>• nuclear envelope/nucleoli disperse </li></ul><ul><li>5 stages: Leptonema, zygonema, pachynema, diplonema and diakenesis. </li></ul>
  9. 9. Metaphase I <ul><li>Chromosomes have thickened. </li></ul><ul><li>Each tetrad interacts with spindle fibers. </li></ul><ul><li>Movement to the equitorial plate. </li></ul><ul><li>Homologues are destined to separate towards opposite poles </li></ul>
  10. 10. Anaphase I <ul><li>sister chromatids remain attached while homologues move towards the opposite pole. (Mitosis =sister chromatids are moved apart) </li></ul><ul><li>One half of each tetrad ( one pair of sister chromatid) is pulled toward each pole of the dividing cell. </li></ul><ul><li>No attraction of the sister chromatids – reduction in the number of chromosomes. </li></ul>
  11. 11. Telophase I + Cytokinesis <ul><li>Very short phase compared to mitosis </li></ul><ul><li>May not always occurs </li></ul><ul><li>Each pole now has a haploid set of chromosomes composed of two sister chromatids attached at the centromere </li></ul><ul><li>cytokinesis occurs producing two daughter cells </li></ul><ul><li>Nuclear membrane forms. </li></ul><ul><li>No interphase – No DNA replictaion. Already 2 chromatids. </li></ul>
  12. 12. 2nd division (II) <ul><li>Prophase II = spindle apparatus forms </li></ul><ul><li>Chromosomes move towards the metaphase II plate </li></ul><ul><li>Metaphase II = chromosomes align on metaphase plate </li></ul><ul><li>kinetochores of sister chromatids point towards opposite poles </li></ul>
  13. 13. <ul><li>Anaphase II = centromeres of sister chromatids separate </li></ul><ul><li>sister chromatids are pulled towards opposite poles </li></ul><ul><li>Telophase II = I member of each homologue pair is present at each pole. </li></ul><ul><li>Each chromosome is referred to as monads. </li></ul>
  14. 14. Cytokinesis <ul><li>4 haploid gametes results. </li></ul><ul><li>Haploid state has been achieved. </li></ul><ul><li>If crossing over has occurred = each monad is a combination of maternal and paternal genetic information ie. Receives info from grandparents. = increases genetic variation. </li></ul>
  15. 16. Independent assortment <ul><li>Mechanism that allows for genetic variation. </li></ul><ul><li>Reduction of diploid – diploid means that each gamete will only carry one form of gene for a particular characteristics. </li></ul><ul><li>Crossing over results in the exchange of genetic info from maternal to paternal chromosomes = possibility of new combination of genes. </li></ul><ul><li>Random assortment = mixture of maternal and paternal chromosomes = more combination. </li></ul>
  16. 17. Nondisjunction and Translocation <ul><li>Too few or too many chromosomes in gametes </li></ul><ul><li>Translocation: attachment of all or part of chromosome to another chromosome </li></ul><ul><li>Nondisjunction: failure of chromosomes to separate </li></ul><ul><li>Down Syndrome </li></ul><ul><li>1. Translocation of chromosome #15 to #21, or </li></ul><ul><li>2. Nondisjunction with extra #21 </li></ul><ul><li>Nondisjunction of sex chromosomes </li></ul><ul><li>1. XO- Turner Syndrome; sterile female </li></ul><ul><li>2. XXY- Kleinfelter Syndrome; sterile male </li></ul><ul><li>3. XXX- Metafemale; limited fertility </li></ul>

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