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Understanding Sexual Reproduction and Meiosis: Key Processes and Variations

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Sexual reproduction involves the fusion of two haploid gametes to form a diploid embryo, with meiosis playing a crucial role in reducing chromosome number and increasing genetic diversity. The process includes two divisions: Meiosis I and Meiosis II, each with distinct phases similar to mitosis, but with key differences, such as crossing over and independent assortment. Variations in meiosis occur in oogenesis (egg formation) and spermatogenesis (sperm formation). While sexual reproduction promotes genetic diversity, asexual reproduction offers quick and complete gene transfer, each with its pros and cons.

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Understanding Sexual Reproduction and Meiosis: Key Processes and Variations

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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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