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  1. 1. MEIOSIS
  2. 2. Mendel: Forgotten? Gregor Mendel published his findings on  genetics in 1865 but his work didn’t get much attention In the early 1900’s scientists did experiments  similar to what Mendel had done They searched for papers detailing results  similar to theirs and realized that their discoveries were not new. Genes were still a mystery though. Scientists  didn’t know where they were located or how they passed information from once cell to
  3. 3. One Makes Two Asexual reproduction: only one parent cell is  needed for reproduction Parent cell goes through mitosis and divides  into two daughter cells Most single-celled organisms reproduce in this  way as do most of the cells in your body
  4. 4. Two Make One Sexual reproduction: two parent cells join  together to form a new individual Parent cells are known as sex cells, different  from ordinary body cells Human body cells have 46 chromosomes or  23 pairs of homologous chromosomes
  5. 5. Human sex cells have only 23 chromosomes  Male sex cells are called sperm  Female sex cells are called eggs, or ova  Each sperm and each egg has only one of the  chromosomes from each homologous pair
  6. 6. Less is More When an egg and sperm join to form a new  individual, each parent donates ½ of a homologous pair of chromosomes This ensures the offspring will have a normal  number of chromosomes in each body cell
  7. 7. Meiosis to the Rescue Sex cells are made during meiosis  Meiosis produces new cells with half the usual  number of chromosomes When sex cells are made, the chromosomes  are copied once, and then the nucleus divides twice. Resulting sperm and eggs have ½ the number 
  8. 8. Back at the Lab Walter Sutton read Mendel’s studies which  showed that the egg and sperm must contribute the same amount of genetic information to the offspring Using some of his own observations, he came  up with the idea that: Genes are located on chromosomes
  9. 9. What was that again? Mitosis:  P  M  A  T/C 
  10. 10. Meiosis: First Time Through Prophase I  Metaphase I  Anaphase I: Chromosomes separate from their  homologous partners and move to opposite sides of cell Telophase/Cytokinesis I: Two cells are  formed, each with the same amount of chromosomes as the parent cell
  11. 11. Chromatids vs. Homologous Chromosomes
  12. 12. Meiosis: Going Again Prophase II: Nuclear membranes dissolve  again, meiotic spindle fibers form Metaphase II: Spindle fibers attach and line  paired chromatids up at equator Anaphase II: Chromatids pull apart and move  to opposite poles Telophase/Cytokinesis II: Four new cells have  formed from original cell, each with half the number of chromosomes present
  13. 13. Differences Between Mitosis and Meiosis Mitosis Meiosis One cell divisision Two cell divisions Two daughter cells Four daughter cells Daughter cells have Daughter cells have same number of half the number of chromosomes as parent chromosomes as parent cells cells
  14. 14. Male or Female? Sex chromosomes carry genes that determine  whether the offspring is male or female. In humans, females have two X chromosomes  while males have one X chromosome and one Y chromosome. Each egg cell produced by a female contains  one X chromosome. A sperm cell produced by a male can contain either an X or Y chromosome.
  15. 15. Male or Female?
  16. 16. Male or Female?
  17. 17. Think/Pair/Share Explain the difference between sex cells and  sex chromosomes. If there are 14 chromosomes in pea plant  cells, how many chromosomes are present in a sex cell of a pea? How many times do the chromosomes make  copies of themselves during meiosis? How many times do cells divide during meiosis?