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MEIOSIS

MEIOSIS. Meiosis. The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n) Meiosis is sexual reproduction. Meiosis. SEX CELLS divide to produce gametes (sperm or egg) .

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MEIOSIS

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  1. MEIOSIS

  2. Meiosis • The form of cell divisionby which gametes, with half the number of chromosomes,are produced. • Diploid (2n)  haploid (n) • Meiosis is sexual reproduction.

  3. Meiosis • SEX CELLS divide to produce gametes(sperm or egg). • GAMETEShave half the # of chromosomes. • Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis Two divisions (meiosis I and meiosis II). • (Meiosis is similar to mitosis with some chromosomal differences.)

  4. Overview • Start with a diploid cell, with 2 copies of each chromosome, one form each parent. The two copies are called homologues. Chromosomes each with 2 chromatids attached at the centromere. • Use 2 cell divisions: • Meiosis 1. First separate the homologues • Meiosis 2. Then separate the 2 chromatids. • The stages of meiosis have the same names as in mitosis: prophase, metaphase, anaphase, telophase. Each of the 2 cell divisions has all of these stages. • Meiosis 1 is unusual and needs a bit of study, but meiosis 2 is just like mitosis

  5. Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. • four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

  6. Prophase I • Longest and most complex phase (90%). • Chromosomes condense. • SYNAPSIS occurs:homologous chromosomes come together to form a tetrad. • TETRAD is two chromosomes or four chromatids(sister and nonsisterchromatids).

  7. Homologous chromosomes sister chromatids sister chromatids Tetrad Prophase I - Synapsis

  8. Metaphase I • Shortest phase • Tetrads align on the metaphase plate. • INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations How many combinations are there for humans????

  9. OR metaphase plate metaphase plate Metaphase I

  10. Answer • Formula: 2n • Human chromosomes: 2n = 46 n = 23 • 223 = ~8 million combinations

  11. Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres.

  12. Anaphase I

  13. Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed.

  14. Telophase I

  15. Meiosis II • No interphase II (or very short - no more DNA replication) • Remember: Meiosis II is similar to mitosis

  16. Prophase II • same as prophase in mitosis

  17. metaphase plate metaphase plate Metaphase II • same as metaphase in mitosis

  18. Anaphase II • same as anaphase in mitosis • sister chromatids separate

  19. Telophase II • Same as telophase in mitosis. • Nuclei form. • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg

  20. MEIOSIS II

  21. SPERMATOGENESIS • Differences between male and female gametes. • In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail).

  22. OOGENESIS • In females, the cell divisions of meiosis are asymmetric: most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg. Eggs also develop large amounts of yolk proteins, which are used to feed the developing embryo. The other 3 meiotic cells are small “polar bodies”, which degenerate.

  23. sperm n=23 n=23 egg 2n=46 zygote Fertilization • The fusion of a sperm and egg to form a ZYGOTE. • A zygote is a fertilized egg

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