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  1. 1. Meiosis The process of creating sex cells Ie: Eggs (female) Sperm (males)
  2. 2. What is Meiosis ? <ul><li>Meiosis involves two successive nuclear divisions that produce four haploid cells. The first division (meiosis I) is the reduction division; the second division (meiosis II) separates the chromatids. </li></ul>
  3. 3. Advantages of Meiosis <ul><li>Mitotic cell division produces new cells genetically identical to the parent cell. </li></ul><ul><li>VS </li></ul><ul><li>Meiosis increases genetic variation in the population. – exchange of information can occur during meiosis I while mutations can also occur </li></ul>
  4. 4. Differences <ul><li>Mitosis happens in somatic cells </li></ul><ul><li>Somatic: any cell in the body that is not a sex cell </li></ul><ul><li>Location? __everywhere except gonadal cells_ </li></ul><ul><li>Meiosis happens in sex cells </li></ul><ul><li>Sex cells: the reproductive cells of living things_ </li></ul><ul><li>Location?__ovaries and testicle of mammals…plants ovum and anther as pollen_ </li></ul>
  5. 5. Interphase <ul><li>Interphase is an important stage preceding meiosis. Without this stage meiosis would not occur. </li></ul><ul><li>During this stage, each individual chromatid replicates, similar to mitosis. </li></ul><ul><li>B replicates B and b replicates b </li></ul><ul><li>At this stage, the chromosomes are long and stringy and are not visible. </li></ul><ul><li>**Remember: All somatic cells are diploid in number (2n), therefore for each chromatid there also exists its homolog, which also replicates during interphase . </li></ul>
  6. 6. The graphical representation here shows that each chromotid pair BB is connected by a centromere (the gold structure). Each pair of the chromosome are referred to as a sister chromatids
  7. 7. Prophase I <ul><li>Prophase I is one of the most important stages of meiosis. </li></ul><ul><li>The chromatids twist and condense , creating chromosomal structures which are visible to the microscope. </li></ul><ul><li>Each chromosome then actively seeks out its homologous chromosome (Double stranded chromosomes that code for the same factors: but one was from your mom and one was from your dad originally) </li></ul><ul><li>After the homologous chromosomes pair, the structure is referred to as a tetrad (four chromatids) BB and bb. The point at which two non-sister chromatids intertwine is known as a chiasma. </li></ul><ul><li>Sometimes a process known as crossing over occurs at this point. This is where two non-sister chromatids exchange genetic material. This exchange does not become evident, however, until the two homologous pairs separate. </li></ul><ul><li>NOTE: A pair of non-sister chromatids are chromosomes that code for the same factors </li></ul>
  8. 8. Prophase I Cell Photographs of Prophase I
  9. 9. Metaphase I <ul><li>At metaphase, each chromosome has reached its maximum density (condensing). </li></ul><ul><li>The homologous pairs and their sister chromatids also prepare for separation </li></ul><ul><li>They interact with spindle fibers which form from either side of the nuclear envelope of the cell. </li></ul><ul><li>The spindle fibers are produced from a structure called a centriole. </li></ul><ul><li>There is a centriole at opposite ends of the cell, which is referred to as poles . Analogous to North and South or East and West </li></ul>
  10. 10. Metaphase I During metaphase, the chromosomes BB and bb are lined by the spindle fibers at what is known as the metaphase plate which is located in the middle of the cell As in the cell picture, it looks like a line through the center of the cell.
  11. 11. Metaphase I <ul><li>During this time information (sequences of ATCG) from BB can switch with sequences of ATCG from bb ). </li></ul><ul><li>This chiasma and crossing over is what gives the gametes their variations. </li></ul><ul><li>This is why you will never look exactly like your siblings. </li></ul>
  12. 12. Chiasma and Crossing Over
  13. 13. Anaphase I <ul><li>Anaphase I pulls apart the tetrad, separating each homologous chromosome. BB goes to one pole and bb goes to the other pole </li></ul><ul><li>The affects of crossing over are evident when anaphase I occurs. </li></ul>
  14. 14. Telophase I <ul><li>Telophase I varies from species to species. </li></ul><ul><li>Sometimes Telophase I is skipped and meiosis starts its second division immediately. </li></ul><ul><li>In general, however, two nuclear envelopes begin to surround the separate chromosomes and cytokinesis (splitting of the cytoplasm into two separate entities) will sometimes occur. Then a phase called interkinesis will follow, which essentially is a resting period from Telophase I to Prophase II. </li></ul><ul><li>This differs from mitosis because DNA replication of the newly divided genetic information does not occur. </li></ul>
  15. 15. Telophase I Now cells are 1n or n meaning that B and b , chromosomes that code for the same factor have been separated from each other. 1n or n  B and b separated, NOT 2n  B b
  16. 16. Prophase II <ul><li>During Prophase II, each dyad (1/2 a tetrad) is composed of a pair of sister chromatids and they are connected by a centromere. BB and bb now in separate cells </li></ul><ul><li>The centrioles (replicated during Telophase I) which produce the spindle fibers also start to move toward the poles of the cell. </li></ul><ul><li>Spindle Fibres :___________________________________________________________________ </li></ul>
  17. 17. Prophase II Evidence of crossover Between BB and bb
  18. 18. Metaphase II <ul><li>Metaphase II is similar to Metaphase I in that the dyads are lined up at a metaphase plate by the spindle fibers. </li></ul><ul><li>In this image BB only </li></ul>
  19. 19. Anaphase II <ul><li>Anaphase II separates the dyads into individual chromatids. </li></ul><ul><li>Each sister chromatid ends up on one side of the cell. </li></ul>
  20. 20. Telophase II <ul><li>At the end of Telophase II, the nuclear envelopes forms around each set of DNA and the cytoplasm divides once again. </li></ul><ul><li>As a result, four haploid cells have formed from one diploid cell. </li></ul><ul><li>The chromosomal content of a haploid cell in one-half the chromosomal content of a diploid cell ( n as opposed to 2n ) </li></ul>B B b b
  21. 21. Telophase II summary                   Telophase II The chromosomal material has be halved. This happens in a Human reproductive cell (sperm or egg) 2n = 46 chromosomes  1n = 23 chromosomes B B b b
  22. 22. Meiosis I Flowchart
  23. 23. Meiosis II Flowchart
  24. 24. Males: all 4 hapoid cells become sperm Females: in oogenesis, only 1 of the haploid cells becomes an egg, and the other 3 are reabsorbed by the body.