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Meiosis

Meiosis. Meiosis. Cell division in which the nucleus of a cell completes two successive divisions that produces 4 cells each with a chromosome number that has been reduced by half. Gametes , sperm (male gamete) and egg (female gamete). Gametes are haploid : 1 copy of every gene

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Meiosis

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  1. Meiosis

  2. Meiosis • Cell division in which the nucleus of a cell completes two successive divisions that produces 4 cells each with a chromosome number that has been reduced by half. • Gametes, sperm (male gamete) and egg (female gamete). • Gametes are haploid: 1 copy of every gene • Characteristic of eukaryotes only: not in prokaryotes.

  3. Why Sex? • Why have sexual reproduction? • Shuffling of alleles between parents and offspring leads to new combinations. Bad combinations die without reproducing; good combinations survive and reproduce more offspring.

  4. Overview • Use 2 cell divisions: • Meiosis 1. First separate the homologues • Meiosis 2. Then separate the 2 chromatids. • Each division of meiosis is subdivided into: 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 1 • Prophase 1 – Chromosomes condense, nuclear envelope dissolves, homologous chromosomes pair and cross over. • Synapsis – the pairing of homologous chromosomes. • Tetrad – Each pair of homologous chromosomes. • Crossing over – portions of one chromosome are broken and exchanged with the corresponding portions on one of the other chromosomes • Homologues break at identical locations, then rejoin opposite partners. This creates new combinations of the alleles on each chromosome. Occurs randomly several times on every chromosome. Results in mixing of the genes you inherited from your parents.

  6. More Meiosis 1 Metaphase 1, the pairs of homologous chromosomes line up side by side and are moved to the equator of the cell. Anaphase 1, the pairs of homologues are pulled to opposite poles by the spindle. • Note: the centromeres do NOT divide; the chromosomes remain in the 2-chromatid X-shaped state.

  7. Result of Meiosis 1 Telophase 1, individual chromosomes gather at each of the poles and spindle fibers disappear. Cytokinesis takes place. • Go from 1 cell to 2 cells. Each daughter cell contains 1 copy of each chromosome: they are haploid, with the chromosomes still having 2 chromatids each.

  8. Meiosis 2 • Meiosis 2 is just like mitosis • No replication of DNA between meiosis 1 and meiosis 2. Prophase 2, new spindle fibers form and attach to centromere Metaphase 2, Chromosomes line up individually on the equator of the cell. Anaphase 2, the centromeres divide, splitting the 2 chromatids. • The chromatids are pulled to opposite poles.

  9. More Meiosis 2 • Telophase 2, nuclear envelope forms around each set of chromosomes, spindle fibers break down, cytokinesis begins. • The result is four haploid cells.

  10. Summary of Meiosis • 2 cell divisions. • Start with 2 copies of each chromosome (homologues), each with 2 chromatids. • In meiosis 1, crossing over in prophase mixes alleles between the homologues. • In metaphase of meiosis 1, homologues pair up, and in anaphase the homologues are separated into 2 cells. • Meiosis 2 is just like mitosis. The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.

  11. Gamete Formation in Animals • Differences between male and female gametes. • Spermatogenesis - Males • 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).

  12. Oogenesis - Females • Females produce 1 egg and 3 polar bodies. • After meiosis 1, the cytoplasm is divided unequally: most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg. The other 3 meiotic cells are small “polar bodies”, which degenerate. • Ovum – female gamete - egg

  13. Meiosis Animation • http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__how_meiosis_works.html • http://digitalfrog.com/resources/archives/meiosis.mov

  14. Genetic Variation • Independent Assortment – random distribution of homologous chromosomes during meiosis. No set pattern for which chromosomes move into which cell. • Crossing Over – DNA exchange of genes • Random Fertilization – random fertilization squares the number of possible outcomes making about 64 trillion possible outcomes.

  15. Reproduction • Asexual Reproduction – reproduction that involves only one parent and results in genetically identical offspring. • Sexual reproduction – reproduction in which gametes from opposite sexes or mating types unite to form a zygote. • Zygote – a fertilized egg cell, first cell of new organism.

  16. Asexual Reproduction • The simplest and most primitive type of reproduction that can produce many offspring in a short period of time. • 3 types • Fission – (amoebas) separation of the parent into two or more individuals of about equal size. • Fragmentation – (multicellular eukaryotes) the parent body breaks into severalpieces and the pieces grow into adults. • Budding – (hydra) new individuals split off from existing parent.

  17. My Little “Bud”dy

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