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Meiosis

Meiosis. 11.4. Homologous Chromosomes. Homologous chromosomes : pairs of chromosomes with genes for the same traits, but they can have different information about those traits. Humans. Humans have 46 chromosomes or 23 pairs of homologous chromosomes.

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Meiosis

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  1. Meiosis 11.4

  2. Homologous Chromosomes • Homologous chromosomes: pairs of chromosomes with genes for the same traits, but they can have different information about those traits

  3. Humans • Humans have 46 chromosomes or 23 pairs of homologous chromosomes. • Somatic(body) cells contain 46 chromosomes. • 46 is the diploid number for humans. - Diploid means that a cell has both homologs for each chromosome • Somatic cells reproduce by mitosis

  4. ½ the chromosomes in a cell come from Mom, ½ the chromosomes in a cell come from Dad

  5. The Homolog’s

  6. Reproductive Cells • Gametes(eggs and sperm) are called haploid because they contain one set of chromosomes. • Eggs also called Ovum and sperm contain 23 chromosomes. • @Eggs and sperm are produced by meiosis- a process that reduces chromosomes by half@

  7. What is Meiosis? • A type of cell division where ONE diploid cell (46 chromosomes) become FOUR haploid cells(23 chromosomes each). • The cells produced by meiosis are gametes (sex cells)

  8. One diploid cell gives rise to 4 haploid cells 46 23 23 23 23

  9. Who does it? • Plants • Fungi • Animals (including you) • Males start meiosis at puberty. • Females start meiosis in utero.

  10. Where does it occur? • In your gonads • Females- the ovary • Males-the testicles

  11. Why does it occur? • Genetic Variation • Mixes genes from Mom and Dad to create a unique baby • @Reduces the chromosome number by half@ • Ensures that each cell produced has one copy from each homologous pair of chromosomes • 23 in sperm + 23 in egg unite (fertilization) leads to a 46 zygote(baby)

  12. Meiosis creates the cells needed for sexual reproduction • Two haploid gametes join to form one diploid cell. - the new diploid cell (fertilized egg) is called a zygote

  13. How does it occur? • In two stages called Meiosis I and Meiosis II. • Each stage contains 4 distinct phases prophase metaphase anaphase teleophase telophase is followed by the division of cytoplasm known as cytokinesis.

  14. Prophase 1 • Homologouschromosomes join to form a tetrad -chromosomes in the tetrad cross over and exchange genes

  15. @Crossing over produces new genetic combinations@ - It is another way to increase genetic diversity

  16. Metaphase 1 • During Metaphase 1, the tetrads (groups of 4 homologous chromosomes) line up down the center of the cell - they assort independently -Chromosomes from each parent line up independently of each other

  17. Anaphase I • During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell. • When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell.

  18. Telophase I and Cytokinesis • During telophase I, a nuclear membrane forms around each cluster of chromosomes. • Cytokinesis follows telophase I, forming two new cells.

  19. Meiosis II • @The two cells produced by meiosis I now enter a second meiotic division. @ • Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.

  20. Prophase II • As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible. • The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I.

  21. Metaphase II • During metaphase of meiosis II, chromosomes line up in the center of each cell.

  22. Anaphase II • As the cell enters anaphase, the paired chromatids separate.

  23. Telophase II, and Cytokinesis • These four daughter cells now contain the haploid number (N)—just two chromosomes each.

  24. Independent Assortment • @Independent assortment increases genetic diversity@ - There are 23 pairs of homologous chromosomes in humans - independent assortment ensures that there are 223 possible combinations 223= 8,388,608 combinations - When two gametes join to form the zygote in fertilization the possible number of combinations is 223 X 223 = 64 trillion

  25. Independent Assortment

  26. What happens in prophase I that produces new genetic combinations? • Crossing over

  27. How many chromosomes do human sperm/eggs have? • 23 chromosomes

  28. How is mitosis different from meiosis? • Mitosis creates 2 daughter cells with same amount of chromosomes • Meiosis creates 4 daughter cells with half amount of chromosomes • Meiosis goes through 2 cycles

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