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Meiosis

Meiosis . Foundation for Sexual Reproduction. Chromosome Characteristics. Diploid set for humans; 2n=46. Autosomes; homologous chromosomes, one from each parent (humans=22 sets of 2). Sex chromosomes (humans have 1 set). Female-sex chromosomes are homologous (XX).

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Meiosis

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  1. Meiosis Foundation for Sexual Reproduction

  2. Chromosome Characteristics • Diploid set for humans; 2n=46. • Autosomes; homologous chromosomes, one from each parent (humans=22 sets of 2). • Sex chromosomes (humans have 1 set). • Female-sex chromosomes are homologous (XX). • Male-sex chromosomes are non-homologous (XY).

  3. Meiosis in Overview • Produces four genetically different, haploid (n) reproductive cells • Two cell divisions; eight steps • Reduces the number of chromosomes by half • When gametes unite during fertilization, the normal diploid (2n) number is restored • In testes- spermatogenesis  sperm • In ovaries – oogenesis  ova (egg cells)

  4. Interphase Meiosis Telophase II Cytokinesis II Prophase I Meiosis I Meiosis II Anaphase II Metaphase I Metaphase II Anaphase I Interkinesis Telophase I Cytokinesis I Prophase II No DNA Replication

  5. Each chromosome condenses, then pairs with its homologue. Spindle fibers appear Nucleolus and nuclear membrane disassemble Crossing over results in exchange of genetic material between homologous chromosomes – genetic recombination!! Pairing is called synapsis and each pair of homologous chromosomes is called a tetrad Crossing Over Prophase I

  6. Metaphase I • Spindle fibers from one pole attach to the centromere of one homologous chromosome; spindle fibers from the opposite pole attach to the other homologous chromosome of the pair. • Homologues align randomly at spindle equator.

  7. Each homologue is separated from its partner and moved to opposite poles. Random separation of the homologous chromosomes is called independent assortment and results in genetic variation Anaphase I

  8. A haploid number of chromosomes (still duplicated) ends up at each pole. During meiosis I, the original cellproduces two new cells, each containing one chromosome from each homologus pair Each new cell contains two copies (as chromatids) because the original cell copied its DNA before meiosis I Telophase I and Cytokinesis I

  9. There is no DNA replication between divisions. Sister chromatids of each chromosome are still attached at the centromere. Spindle fibers form and begin to move the chromosomes toward the midline of the dividing cell Prophase II

  10. Each chromosome is aligned at the spindle equator. Metaphase II

  11. Each chromosome splits. Chromatids are now chromosomes in their own right and are moved to opposite poles. Anaphase II

  12. Four daughter nuclei form. Following cytokinesis, each gamete has a haploid number of chromosomes, all in the unduplicated state. Telophase II and Cytokinesis II

  13. Animation

  14. Mitosis / Meiosis ~ Compared • Meiosis reduces the number of chromosomes by half. • Daughter cellsdiffer from parent, and each other. • Meiosis involves two divisions - Mitosis only one. • Meiosis I involves: • Synapsis– homologous chromosomes pair up. Chiasmata form (crossing over of non-sister chromatids). • In Metaphase I, homologous pairs line up at metaphase plate. • In Anaphase I, sister chromatids do NOT separate. • Overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome.

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