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Meiosis and Sexual Reproduction

Meiosis and Sexual Reproduction. Why Meiosis?. Advantages : Dramatic increase in genetic variation Keeps genetic material from multiplying ( ploidy ) R eplication done with somatic cells ( body cells ) would double the amount of DNA each time Disadvantages :

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Meiosis and Sexual Reproduction

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  1. Meiosis and Sexual Reproduction

  2. Why Meiosis? • Advantages: • Dramatic increase in genetic variation • Keeps genetic material from multiplying (ploidy) • Replication done with somatic cells (body cells) would double the amount of DNA each time • Disadvantages: • Reproduction is more complex • Meiosis requires more energy • Must find other half of genetic material to reproduce • Sexual Reproduction fusion of two haploid male and female cells (gametes) • Fertilization union of male and female nuclei; produces a zygote

  3. The Cells of Meiosis • What type of cells do meiosis? • Eukaryotes • Types of meiotic cells: • Spermatozoa (sperm) produced by male gonads (testes); contain paternal chromosomes • Ova (eggs) produces by female gonads (ovaries); contain maternal chromosomes • Source of variation though chromosomes are the same (homologous) they have different DNA for the same genes (alleles)

  4. Phases of Meiosis • Two Phase Groups: • Meiosis I • Prophase I Telophase I • Homologous chromosomes separate • Meiosis II • Prophase II  Telophase II • Sister chromatids separate (like mitosis) • Both phases have cytokinesis

  5. Meiosis I • Early Prophase I • Chromosomes condense • Homologous pairs form tetrads through pairing (synapsis) • Crossing-over occurs • Late Prophase I • Nucleus breaks down • Spindle fibers attach to kinetochores (one for each sister chromatids) • Metaphase I • Homologous pairs line up at metaphase plate • Anaphase I • Homologous pairs separate to opposite poles • Telophase I • Nucleus reforms, spindles break down, and short interkinesis

  6. Crossing-Over • Protein complex carefully exchanges alleles on homologous chromosomes • Event is random and happens in 2-3 places on every chromosome • 2 (of 4) chromatids exchange alleles results in two recombinant chromosomes and two parental chromosomes • Double crosses possible? • Yes, depending on other alleles and distance

  7. Meiosis II • Follows mitosis: • Early Prophase II • Chromosomes condense • Spindles form • Late Prophase II • Nucleus breaks down • Spindles attach to kinetochores • Metaphase II • Chromosomes move to metaphase plate • Anaphase II • Sister chromatids separate to opposite poles • Telophase II • Nucleus reforms, chromosomes decondense, spindles disappear • End result is 4 haploid cells

  8. Mendel’s Dihybrid Crosses The Law of Segregation Gametes will get only 1 copy of 1 allele All alleles separate when sister chromatidsseparate The Law of Independent Assortment Genes for different traits are inherited independently of each other Shown in meiosis when homologous chromosomes separate Genes on chromosome 1 move different than genes on chromosome 2, 3, 4, etc…

  9. Producing Variation • Where in meiosis does variation come from?- Law of Independent Assortment • Random separation of paternal and maternal DNA • 8,388,608 possible combinations with our 23 pairs • Random recombination of haploid material • 1:7.0x1013possible you • Switching of genes during cross-over • Too random to measure • Creates enough variation that no two humans ever be the same…ever (expect for twins)

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