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Meiosis & Sexual Reproduction

Explore the process of meiosis and sexual reproduction, including the advantages and disadvantages compared to asexual reproduction. Learn how meiosis helps create genetic variation and why offspring resemble but are not exact replicas of their parents.

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Meiosis & Sexual Reproduction

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  1. Meiosis & Sexual Reproduction

  2. Cell division / Asexual reproduction • Mitosis • produce cells with same information • identical daughter cells • exact copies • clones • same number of chromosomes • same genetic information Aaaargh!I’m seeingdouble!

  3. Asexual reproduction • Single-celled eukaryotes • yeast • Paramecium • Amoeba • Simple multicellular eukaryotes • Hydra • budding What are thedisadvantages ofasexual reproduction? What are the advantages?

  4. How about the rest of us? • What if a complex multicellular organism (like us) wants to reproduce? • joining of egg + sperm • Do we make egg & sperm by mitosis? No! What if we did, then….    + 46 92 46 egg sperm zygote Doesn’t work!

  5. 2n diploid = 2 copies Human female karyotype 46 chromosomes 23 pairs XX

  6. 2n diploid = 2 copies Human male karyotype 46 chromosomes 23 pairs XY

  7. 46 46 23 23 46 23 23 How do we make sperm & eggs? • Must reduce 46 chromosomes  23 • must half the number of chromosomes • haploid zygote egg meiosis fertilization sperm gametes

  8. 46 23 46 23 Meiosis makes sperm & eggs • 46 chromosomes to 23 chromosomes • half the number of chromosomes egg meiosis diploid haploid sperm

  9. Paired chromosomes • Homologous chromosomes • both chromosomes of a pair carry “matching” genes • control same inherited characters • homologous = sameinformation diploid2n 2n = 4 eye color (brown?) eye color (blue?) homologouschromosomes double strandedhomologous chromosomes

  10. doublestranded Meiosis 1 overview Divide 1 • 1st division of meiosis Copy DNA Line Up 1 prophase 1 metaphase 1 • 4 chromosomes • diploid • 2n telophase 1 • 2 chromosomes • haploid • 1n gamete

  11. 4 Bye Bye 2 Meiosis 2 overview • 2nd division of meiosis • looks like mitosis telophase 2 telophase 1 Line Up 2 metaphase 2 • 2 chromosomes • haploid • 1n gametes

  12. Meiosis = reduction division • Meiosis • special cell division in sexually reproducing organisms • reduce number of chromosomes • 2n  1n • diploid  haploid • half • makes gametes • sperm, eggs

  13. Meiosis & mitosis • Meiosis to make gametes • sperm & egg • Mitosis to make copies of cells • growth • repair • development

  14. Sexual reproduction lifecycle • 2 copies • diploid • 2n • 1 copy • haploid • 1n • 1 copy • haploid • 1n fertilization meiosis We’re mixing things up here! A good thing?

  15. 46 46 46 46 46 46 46 46 46 46 23 23 46 23 23 Putting it all together… meiosis  fertilization  mitosis + development gametes meiosis egg zygote mitosis &development fertilization mitosis sperm

  16. Dad Mom The value of meiosis 1 • Consistency over time • meiosis keeps chromosome number same from generation to generation from Mom offspring from Dad

  17. We’re mixing things up here! The value of meiosis 2 • Change over time • meiosis introduces genetic variation • gametes of offspring do not have same genes as gametes from parents • new combinations of traits variation from Dad from Mom offspring new gametes made by offspring

  18. How does this explain:family resemblance & differences! Why are the kids so similar to the parents but not exact? Michael & KirkDouglas Baldwin brothers Martin & Charlie Sheen, Emilio Estevez

  19. What are theadvantages ofasexual reproduction? What are theDISadvantages ofasexual reproduction? Any Questions?? What are theadvantages ofsexual reproduction? What are theDISadvantages ofsexual reproduction?

  20. Meiosis in males is called spermatogenesis and produces sperm. Meiosis in females is called oogenesis and produces ova.

  21. n=23 human sex cell sperm n=23 n=23 2n=46 haploid (n) n=23 n=23 diploid (2n) n=23 meiosis I meiosis II Spermatogenesis Secondary Spermatocyte Primary Spermatocyte Secondary Spermatocyte 4 sperm cells are produced from each primary spermatocyte.

  22. Oogenesis *** The polar bodies die… only one ovum (egg) is produced from each primary oocyte.

  23. Interphase I • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. • Centriole pairs also replicate.

  24. Prophase I • Longest and most complex phase. • 90% of the meiotic process is spent in Prophase I • Chromosomes condense. • Synapsis occurs: homologous chromosomescome together to form a tetrad. • Tetrad is two chromosomes or four chromatids(sister and nonsister chromatids).

  25. Homologous chromosomes sister chromatids sister chromatids Tetrad Prophase I - Synapsis

  26. During Prophase I “Crossing Over” occurs. • During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over. Crossing Over is one of the Two major occurrences of Meiosis (The other is Non-disjunction)

  27. Tetrad nonsister chromatids chiasmata: site of crossing over Crossing Overcreates variation (diversity) in the offspring’s traits. variation

  28. Question: • A cell containing 20 chromosomes(diploid)at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  29. Answer: • 10 chromosomes (haploid)

  30. Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  31. Answer: • 10 chromosomes

  32. Non-disjunction Non-disjunction is one of the Two major occurrences of Meiosis (The other is Crossing Over) • Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. • Non-disjunction results with the production of zygotes with abnormal chromosome numbers…… remember…. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring.

  33. Non-disjunctions usually occur in one of two fashions. • The first is called Monosomy, the second is called Trisomy. If an organism has Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21…. Three chromosomes in the 21st set. If an organism has Monosomy 23 it has only one chromosome in the 23rd set.

  34. Common Non-disjunction Disorders • Down’s Syndrome – Trisomy 21 • Turner’s Syndrome – Monosomy 23 (X) • Kleinfelter’s Syndrome – Trisomy 23 (XXY) • Edward’s Syndrome – Trisomy 18

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