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meiosis and gene linkage

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  1. 1. MEIOSIS Pp275-278
  2. 2. What Mendel Knew… <ul><li>Each organism must inherit a single copy of every gene from each of its “parents” </li></ul><ul><li>Each of the organisms gametes must contain just one set genes </li></ul><ul><ul><li>When gametes are formed, there must be a process that separates the 2 sets of genes so each gamete gets one set </li></ul></ul>
  3. 3. Chromosome Number <ul><li>Homologous chromosomes </li></ul><ul><ul><li>Chromosome that has a corresponding chromosome from the opposite-sex parent </li></ul></ul><ul><li>Fruit fly has 8 chromosomes </li></ul><ul><ul><li>4 from mom </li></ul></ul><ul><ul><li>4 from dad </li></ul></ul>
  4. 4. Diploid <ul><li>Di= two sets </li></ul><ul><li>Cell that contains both sets of homologus chromosomes </li></ul><ul><ul><li>Cell contains </li></ul></ul><ul><ul><ul><li>2 complete sets of chromosome </li></ul></ul></ul><ul><ul><ul><li>2 complete sets of genes </li></ul></ul></ul><ul><li>Number of chrms in diploid cell represented by 2N </li></ul><ul><li>For Drosophilia (fruit fly) 2N=8 </li></ul><ul><li>Mendel said: </li></ul><ul><ul><li>Each adult cell contains two copies of each gene </li></ul></ul>
  5. 5. Haploid <ul><li>Means “one set” </li></ul><ul><li>Refers to cells that contain only one set of chromosomes </li></ul><ul><li>Gametes (sex cells) </li></ul><ul><li>Represented by N </li></ul><ul><li>Drosophilia fruit fly </li></ul><ul><ul><li>N=4 </li></ul></ul>
  6. 6. How are haploid (N) gametes made from diploid (2N) cells?
  7. 7. Meiosis <ul><li>Process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell </li></ul>
  8. 8. Meiosis <ul><li>2 distinct stages </li></ul><ul><li>Meiosis I </li></ul><ul><ul><li>A diploid cell enters here </li></ul></ul><ul><li>Meiosis II </li></ul><ul><ul><li>At the end of this, the diploid cell that entered meiosis has become 4 haploid cells </li></ul></ul>
  9. 11. Meiosis I <ul><li>Before meiosis 1, each chromosome is replicate </li></ul><ul><li>Then they divide like in mitosis </li></ul><ul><li>What happened in mitosis? </li></ul><ul><ul><li>PMAT </li></ul></ul><ul><li>Tetrad </li></ul><ul><ul><li>STRUCTURE MADE WHEN EACH CHROMOSOME PAIRS UP WITH ITS HOMOLOGOUS CHROMOSOME </li></ul></ul><ul><ul><li>4 CHROMATIDS IN A TETRAD </li></ul></ul>
  10. 12. Prophase 1 <ul><li>Each chromosome pairs with its homologous chromosome making a tetrad </li></ul><ul><li>As they pair up in tetrads, chromosomes exchange portions of their chromatids in the process …. CROSSING OVER </li></ul>
  11. 13. Crossing Over
  12. 15. Metaphase1 <ul><li>Spindle fibers attach to chromosomes </li></ul>Anaphase 1 <ul><li>The spindles pull homologous chromosomes apart to opposite poles/ends </li></ul>Telophase 1 <ul><li>Nuclear membranes form and cell separates into two new cells </li></ul>
  13. 17. Now what do we have? <ul><li>2 new daughter cells </li></ul><ul><li>Are they identical to the parents? </li></ul><ul><ul><li>No </li></ul></ul><ul><ul><li>The parent has 4 chromosomes </li></ul></ul><ul><ul><li>Each daughter cell only has 2 chromosomes (4 chromatids, but they are different because of crossing-over) </li></ul></ul><ul><li>Each daughter cell has a set of chromosomes and alleles different from each other and different from the parent diploid cell </li></ul>
  14. 18. Meiosis II <ul><li>Unlike Mitosis, Neither cell goes through a round of chromosome replication </li></ul><ul><li>Each cell’s chromosome has 2 chromatids </li></ul>
  15. 19. Prophase II <ul><li>Meiosis resulted in 2 haploid (N) cells, each with ½ the number of chromosomes in the original cell </li></ul>
  16. 20. Metaphase 2 <ul><li>Chromosomes line up in middle </li></ul>Anaphase 2 <ul><li>Sister chromatids separate and move to opposite poles </li></ul>Telophase 2 <ul><li>Meiosis II results in 4 haploid (N) daughter cells </li></ul><ul><li>4 daughter cells contain haploid number of chromosomes, just 2 each </li></ul>
  17. 23. Gamete Formation <ul><li>Male </li></ul><ul><ul><li>Haploid gametes produced by meiosis are called sperm </li></ul></ul><ul><li>Female </li></ul><ul><ul><li>Haploid gamete produced by meiosis is called an egg </li></ul></ul><ul><ul><li>Cell divisions at the end of meiosis one and two are uneven so one cell gets most of the cytoplasm (the EGG) and the other three are called polar bodies (don’t participate in reproduction) </li></ul></ul>
  18. 29. Mitosis vs. Meiosis <ul><li>Mitosis </li></ul><ul><ul><li>Results in the production of two genetically identical DIPLOID cells </li></ul></ul><ul><ul><li>Daughter cells have sets of chromosomes identical to each other and to parent cell </li></ul></ul><ul><ul><li>MITOSIS allows body to grow and replace other cells </li></ul></ul><ul><ul><li>Asexual reproduction </li></ul></ul><ul><li>Meiosis </li></ul><ul><ul><li>Results in four genetically different HAPLOID cells </li></ul></ul><ul><ul><li>MEIOSIS is how sexually reproducing organisms make gametes </li></ul></ul>
  19. 32. Genes
  20. 33. Gene Linkage <ul><li>Would genes on the same chromosome be inherited together? </li></ul><ul><li>Thomas Hunt Morgan gave us the answer </li></ul><ul><ul><li>1910 PRINCIPLE of LINKAGE </li></ul></ul><ul><ul><li>50 Drosophilia genes </li></ul></ul><ul><ul><li>Seemed to contradict Principle of Independent Assortment b/c certain genes were always inherited together </li></ul></ul><ul><ul><li>He grouped the fly’s genes into linkage groups </li></ul></ul>
  21. 34. <ul><li>Linkage groups are made up of genes that seem to be inherited together </li></ul><ul><li>Linkage groups assort independently but all genes in one group are inherited together </li></ul><ul><li>Drosophilia </li></ul><ul><ul><li>4 linkage groups </li></ul></ul><ul><ul><li>4 chromosomes </li></ul></ul><ul><li>What can be concluded? </li></ul>
  22. 36. Conclusions <ul><li>Each chromosome is actually a group of linked genes </li></ul><ul><li>Mendel’s Principle of Independent Assortment holds true </li></ul><ul><li>IT IS THE CHROMOSOMES THAT ASSORT INDEPENDENTLY, NOT THE INDIVIDUAL GENES </li></ul>
  23. 37. How Mendel missed it… <ul><li>What 7 genes did he study? </li></ul><ul><li>6 of the 7 genes were on different chromosomes </li></ul><ul><li>Two genes were on the same chromosome but they were so far apart on the same chromosome that they assorted independently </li></ul>
  24. 38. <ul><li>If two genes are on the same chromosome, does that mean they are linked for ever? </li></ul><ul><ul><li>No, chromosomes cross over during meiosis so they can separate </li></ul></ul><ul><ul><li>Crossing over produces new combinations of alleles </li></ul></ul><ul><ul><li>Important for genetic diversity </li></ul></ul>
  25. 39. Lucky Student <ul><li>Alfred Sturtevant 1911 </li></ul><ul><li>Columbia University </li></ul><ul><li>Worked in Morgan’s Lab </li></ul><ul><li>Hypothesis </li></ul><ul><ul><li>If two genes are farther apart on a chromosome, the they are more likely to be separated during meiosis </li></ul></ul><ul><li>Experiment </li></ul><ul><ul><li>Measured the rate at which linked genes were separated and recombined to make a “map” of distances between genes </li></ul></ul><ul><li>Conclusion </li></ul><ul><ul><li>Recombination rates could be used to make gene maps </li></ul></ul><ul><ul><li>Gene maps showed the location of a gene on a chromosome </li></ul></ul>