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MEIOSIS makes gametes

MEIOSIS makes gametes. Meiosis produces gametes. Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887 60 years before we knew that DNA contained the genetic material Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells

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MEIOSIS makes gametes

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  1. MEIOSIS makes gametes

  2. Meiosis produces gametes • Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887 • 60 years before we knew that DNA contained the genetic material • Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells • The fusion of gametes is called fertilization • It creates the zygote, which contains two copies of each chromosome • Meiosis only occurs in the gonads (ovaries and testes). • All other cell divisions are mitotic

  3. The Sexual Life Cycle • The life cycles of all sexually-reproducing organisms follows the same basic pattern • Haploid cells or organisms alternate with diploid cells or organisms Figure 9.4

  4. The Stages of Meiosis • Meiosis consists of two successive divisions, but only one DNA replication • Meiosis I • Separates the two homologs • Meiosis II • Separates the two sister chromatids • Meiosis halves the number of chromosomes see Figure 9.5

  5. Prophase I • Chromosomes condense (coil) • Nuclear membrane breaks down • Homologous chromosomes undergo synapsis (Pair up ) • Cross over occurs (Recombination) and chromosomes exchange segments newly forming microtubules

  6. Metaphase I spindle equator one pair of homologous chromosomes • Homologous chromosome pairsline up at random at the equatorial midline

  7. Anaphase I • Centromeres do not divide • Homologs separate and move to opposite poles

  8. Telophase I • Nuclear membrane reforms • Chromosomes uncoil After Cytokinesis • Two haploid cells are produced

  9. Meiosis II • After meiosis I there is a brief interphase • No DNA synthesis occurs • Meiosis II is similar to mitosis, but with two main differences • 1.Haploid set of chromosomes • 2. Sister chromatids are not identical because of cross over

  10. Prophase IIbegins with haploid cells • Chromosomes coil • Spindle forms • Nuclear membrane breaks down • Each chromosome is composed of two sister chromatids attached at the centromere

  11. Metaphase II • Chromosomes line up on the midline and attach to spindle fibers

  12. Anaphase II • Centromeres divide • Sister chromatids move to opposite poles

  13. Telophase II • Nuclear membrane reforms • Chromosomes uncoil After Cytokinesis • Four unique haploid cells are produced

  14. see Figure 9.6

  15. see Figure 9.6

  16. Meiosis I has two unique features 1. Synapsis • Homologous chromosomes pair all along their lengths in prophase I • While paired homologs cross over 2. Reduction division • Homologs separate in Anaphase I reducing the chromosome number in 1/2 (2n to 1n) • This produces haploid gametes

  17. Sexual reproduction increases genetic diversity through three key mechanisms 1. Crossing over 2. Independent assortment 3. Random fertilization Evolutionary Consequences of Sex

  18. Crossing over • DNA exchanges between maternal and paternal chromatid pairs see Figure 9.7

  19. Independent Assortment Label these SEE process!

  20. Independent assortment Three chromosome pairs • In humans, a gamete receives one homolog of each of the 23 chromosomes • Humans have 23 pairs of chromosomes • Independent assortment gives 223 combinations in an egg or sperm • 8,388,608 possible kinds of gametes • Random fertilization of two independently-produced gametes • Therefore, the possible combinations in an offspring • 8,388,608 X 8,388,608 =70,368,744,177,664 • More than 70 trillion! • And this number does not count crossing-over 23 combinations

  21. Compare and Contrast Mitosis and Meiosis* Practice this with colored pens so that you can SEE the difference *guaranteed to be on quiz and exam

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