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  1. 1. MeiosisReproduction of sex cells
  2. 2. I. History In 1882, a scientist observed that the sex cells of roundworms contained different numbers of chromosomes in comparison to the body (somatic) cells of the individuals.
  3. 3. These observations led scientists to believe that a different type of cell division occurred that reduced the number of chromosomes by half
  4. 4. o In human and other eukaryotic organisms there are two types of cells1. Somatic cells or basic BODY cells that have 46 chromosomes or 23 PAIRS -Diploid cells contain a FULL SET of chromosomes
  5. 5. o In human and other eukaryotic organisms there are two types of cells2. Gametes or SEX cells that have 23 chromosomes -Haploid cells have HALF the number of chromosomes Sperm Egg
  6. 6. This was important because when 2 sex cells fused together during FERTILIZATION they produce a new DIPLOID cell called a ZYGOTE
  7. 7. II. Human Chromosome Types:Within a diploid cell there are two different types of chromosomes 1. _AUTOSOMES (non sex chromosomes): 44 chromosomes (22 sets) -most are HOMOLOGOUS CHROMOSOMES or pairs of chromosomes from mom and dadthat are alike and carry the same genetic traits
  8. 8. • Each pair have genes in the SAME location• _MALE sex chromosomes are NOT homologous
  9. 9. Are the following homologous or nonhomologous?
  10. 10. The other type of chromosomes are…2. SEX chromosomes: these determine the gendera. _XX – female (these are homologous)b. _XY – male(these are non homologous)
  11. 11. Chromosomes can be seen during different stages of the cell cycle. A KARYOTYPE is a photograph that shows these chromosomes.
  12. 12. III. The Sexual Life CycleA. MEIOSIS: DIVISION of a diploid cell toproduce SEX CELLS with 23 chromosomes
  13. 13. KEY POINTS:1. _TOGETHER meiosis and fertilization represent SEXUAL reproduction2. _GAMETE cells produces are haploid sperm and egg cells3. Results in 4 genetically NON-IDENTICAL daughter cells4. Ensures GENETIC VARIABILITY by mixing of genes
  14. 14. RESULT:1. Through the process of sexual reproduction PARENTS will pass genetic information onto the next generation2. The offspring will INHERIT a great variety of gene combinations
  15. 15. IV. A Closer Look at Meiosis• Two rounds of cell division called MEIOSIS I and MEIOSIS IIDNA replication only occurs 1 time between the nuclear divisions to reduce the original chromosome number by half
  16. 16. Draw this flow map across your paper…then add details under each squareOvary/Teste Interphase Meiosis I Meiosis II Production of Gametes
  17. 17. Ovary/Teste• Site of Meiosis
  18. 18. Interphase• DNA copies• (just like in Mitosis)
  19. 19. P M Meiosis I A T• Nuclear division• Crossing over of homologous chromosomes• Independent assortment of chromosomes
  20. 20. P M Meiosis II A T• No interphase, DNA does NOT copy• Otherwise, looks like mitosis• Nuclear division• Chromosomes separate
  21. 21. Production of Gametes• 4 Haploid cells• Sperm/egg
  22. 22. V. Phases of Meiosis I• To understand why haploid cells are genetically different, Meiosis I needs to be explainedA. Interphase: chromosomes copy (s-phase) (in humans, 46 chromosomes copy, making 92)
  23. 23. B. During Prophase:o _HOMOLOGOUS CHROMOSOMES PAIR and CROSSOVERo Crossing over: ensures genetic variability in the offspring because of RECOMBINATION or MIXING of genetic material
  24. 24. Color code your chromosomes…
  25. 25. D. During Anaphase:o _INDEPENDENT ASSORTMENT occurso After metaphase, pairs of chromosomes SEPARATE and SORT randomly during cell division to produce gametes containing one chromosome of each type
  26. 26. E. Telophase and Cytokinesis:• Pairs are separated into two new cells
  27. 27. VI. Phases of Meiosis II:• _NO INTERPHASE occurs between two divisions, the new cells produces will have a REDUCTION in chromosome number and only have half the number of chromosomes• PMAT, cytokinesis occurs: looks like mitosis• _4 NEW cells will RESULT and will be non identical due to crossing over and random separation of chromosomes