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Next time…

Next time…. pp. 250 - 264. Sexual reproduction. Embryos are formed by the fusion of two gametes Each gamete must have ½ a set of DNA (Each gamete is haploid/1n, while embryos/adults are diploid/2n). Meiosis. Purposes: Reduce chromosome number to form gametes

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Next time…

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  1. Next time… • pp. 250 - 264

  2. Sexual reproduction • Embryos are formed by the fusion of two gametes • Each gamete must have ½ a set of DNA • (Each gamete is haploid/1n, while embryos/adults are diploid/2n)

  3. Meiosis • Purposes: • Reduce chromosome number to form gametes • Shuffle genes between homologous chromosomes to increase genetic diversity • This only happens in gonads • Testes/ovaries

  4. Meiosis overview… • Two divisions • Each goes through the same steps as mitosis • Interphase, PMAT • Crucial differences between mitosis and each meiotic division • Crucial difference between each meiotic division

  5. Meiosis • Interphase I • DNA replication • Prep for division • Prophase I • Chromosomes condense, spindle formed • Nuclear membrane dissolves • Crossing over…

  6. Meiosis • Metaphase I • Homologous chromosomes line up in center • Different from Mitotic metaphase • Anaphase I • Homologous chromosomes pulled to the poles

  7. Meiosis • Telophase I • Chromosomes arrive • Nuclear membranes form • Chromosomes partially decondense • Cytokinesis • Cells pinch apart

  8. Meiosis • Interphase II • Very brief, pretty much straight to Prophase II • Prophase II • Chromosomes condense • Nuclear membranes dissolve

  9. Meiosis • Metaphase II • Sister chromatids line up at the center of the cell • Anaphase II • Sister chromatids are pulled apart and to opposite poles

  10. Meiosis • Telophase II • Nuclear membranes form • Cytokinesis • Cells pinched apart

  11. A video • http://www.youtube.com/watch?v=iCL6d0OwKt8&feature=related

  12. Different types of meiosis • Oogenesis – making eggs • Two divisions result in only one cell • Each division is unequal • Results in a large cell and a tiny one

  13. Different types of meiosis • Spermatogenesis • Divisions are equal • Result in four equally sized cells

  14. Variation from Meiosis • Crossing over • Independent assortment • The destination of each of mom and dad’s chromosomes in anaphase II is random • More than 8,000,000 possibilities

  15. So why sex? • Advantages: • More gene combinations in a population • Disadvantages: • Your genes only 50% represented in your offspring • Mating takes a lot of work

  16. How about asexual reproduction? • Advantages: • Fast, easy • Offspring get 100% of your genes • Disadvantages: • Less shuffling of genes in a population

  17. Sex determination • Humans have 22 pairs of autosomes and 1 pair of sex chromosomes • X and Y • Female is XX, male is XY • So dad determines the sex of the child

  18. Sex determination • Different in other animals: • Birds: WZ = Female, ZZ = Male • Ants, bees, wasps: Chromosome number • Turtles: temperature of the eggs • In some animals, it’s unknown: zebrafish

  19. Chromosome number • Visualize with a karyotype http://www.biology.iupui.edu/biocourses/N100/2k2humancsomaldisorders.html

  20. Differences in Number of chromosomes - aneuploidy • Caused by nondisjunction • Most differences result in fetus death • Some survive: • Trisomy 21 = Down’s syndrome • Trisomies 13 & 18 can make it to 1 year

  21. Sex chromosome aneuploidy • They happen a lot • Many have only minor effects on phenotype • Check it out

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