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Meiosis and Sexual Reproduction Ch. 11.1-11.3

Meiosis and Sexual Reproduction Ch. 11.1-11.3. 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 Ch. 11.1-11.3

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  1. Meiosis and Sexual ReproductionCh. 11.1-11.3

  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 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 • Leading to Meiosis I normal cell cycle (G1, S, G2) • Leading to Meiosis II short interphase but no S phase (interkinesis)

  5. Meiosis I • Prophase I • Chromosomes condense • Homologous pairs form tetrads through pairing (synapsis) • Crossing-over occurs • Prometaphase 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. Meiosis II • Follows mitosis: • Prophase II • Chromosomes condense • Spindles form • Prometaphase 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

  7. Producing Variation • Where in meiosis does variation come from? • Random separation of paternal and maternal DNA • 8,388,608 possible combinations with our 23 pairs • Switching of genes during cross-over • Random recombination of haploid material • Creates enough variation that no two humans ever look alike (expect to twins) • Even in the same family, the chance of offspring being genetically the same is 1:7.0x1013

  8. Crossing-Over • Protein complex (synaptonemal 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

  9. Producing Errors • Sometimes during Anaphase I/II or after crossing over, chromosomes get stuck together • Nondisjunction failure to separate homologous chromosomes/ sister chromatids • One cell receives extra material and one cell misses a whole chromosome • Mostly fetal in humans • Trisomy 21 (Downs Syndrome) • Produces large amounts of variation in plants

  10. Alternative Life Cycles through Meiosis • Diploid Dominate Cycle • Meiosis cells used for fertilization • No mitosis for gametes • Animals • Haploid/Diploid Alternation Cycle • Sporophytes (diploid) produce spores (haploid) which under go mitosis to form gametophytes • Eventually gametophytes form sperm and eggs cells used to make sporophytes • Bushes, trees, and flowers • Haploid Dominate Cycle • Diploid only at fertilization, then cell produces haploid (+/-) spore cells • Spores produce gametophytes • Fungi and algae

  11. Homework • Read Ch. 12 and do Ch. 12 Vocab • Ch. 11 vocab • Test Your Knowledge and Interpret the Data for Ch. 11 • Test on Ch. 11, 12, and 13 next week

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