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Meiosis & Sexual Reproduction

Explore the process of meiosis and sexual reproduction, including the benefits of genetic variation, the steps of meiosis, and the production of sperm and eggs.

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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 amount of DNA • same number of chromosomes • same genetic information • What is Binary Fission? Aaaargh!I’m seeingdouble!

  3. Okay then….? • Why can’t we just do mitosis to reproduce? • Why produce sperm and egg and even go through the hassle of dating? • If we are going to do it can we make egg & sperm by mitosis? No! What if we did, then…. + 46 92 46 egg sperm zygote Doesn’t work!

  4. JonasBrothers Baldwin brothers Martin & Charlie Sheen, Emilio Estevez Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences.

  5. metaphase1 The value of sexual reproduction • Sexual reproduction introduces genetic variation • genetic recombination • independent assortment of chromosomes • random alignment of homologous chromosomes in Metaphase 1 • crossing over • mixing of alleles across homologous chromosomes • random fertilization • which sperm fertilizes which egg? • Driving evolution • providing variation for natural selection

  6. Human female karyotype 46 chromosomes 23 pairs

  7. Human male karyotype 46 chromosomes 23 pairs

  8. Homologous chromosomes • Paired chromosomes • both chromosomes of a pair carry “matching” genes • control same inherited characters • homologous = sameinformation single stranded homologouschromosomes diploid2n 2n = 4 double strandedhomologous chromosomes

  9. Meiosis: production of gametes • chromosome number must be reduced • diploid haploid • 2n n • humans: 46  23 • meiosis reduces chromosome number • makes gametes • fertilization restores chromosome number • haploid  diploid • n  2n haploid diploid

  10. Double divisionof meiosis DNA replication Meiosis 1 1st division of meiosis separateshomologous pairs Meiosis 2 2nd division of meiosis separatessister chromatids

  11. 2n = 4 double stranded prophase 1 2n = 4 double stranded metaphase 1 telophase 1 1n = 2 double stranded 2n = 4 single stranded Meiosis 1 • 1st division of meiosis separateshomologous pairs synapsis tetrad reduction

  12. Trading pieces of DNA • Crossing over • during Prophase 1, sister chromatids intertwine • homologous pairs swappieces of chromosome • DNA breaks & re-attaches prophase 1 synapsis tetrad

  13. 4 1n = 2 double stranded metaphase 2 1n = 2 single stranded telophase 2 Meiosis 2 • 2nd division of meiosis separatessisterchromatids 1n = 2 double stranded prophase 2 What doesthis divisionlook like?

  14. Steps of meiosis • Meiosis 1 • interphase • prophase 1 • metaphase 1 • anaphase 1 • telophase 1 • Meiosis 2 • prophase 2 • metaphase 2 • anaphase 2 • telophase 2 1st division of meiosis separateshomologous pairs (2n  1n) “reduction division” 2nd division of meiosis separatessister chromatids (1n  1n) * just like mitosis *

  15. Mitosis 1 division daughter cells genetically identical to parent cell produces 2 cells 2n  2n produces cells for growth & repair no crossing over Meiosis 2 divisions daughter cells genetically different from parent produces 4 cells 2n  1n produces gametes crossing over Mitosis vs. Meiosis

  16. 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 fertilization mitosis sperm development

  17. Sperm production • Spermatogenesis • continuous & prolific process • each ejaculation = 100-600 million sperm Epididymis Testis germ cell (diploid) Coiled seminiferous tubules primary spermatocyte (diploid) MEIOSIS I secondary spermatocytes (haploid) MEIOSIS II Vas deferens spermatids (haploid) spermatozoa Cross-section of seminiferous tubule

  18. Egg production: Oogenesis • Begins in the ovaries of the female fetus before birth • Pauses during first meiotic division • Final development occurs in the ovaries of the adult female • Each month one egg matures as cued by hormones • Completes the first meiotic division and starts the second meiotic division • Last bit of meiosis is finalized at time of fertilization

  19. primary follicles germinal cell (diploid) fallopian tube fertilization developing follicle primary oocyte (diploid) MEIOSIS I mature follicle with secondary oocyte secondary oocyte (haploid) first polar body ruptured follicle(ovulation) MEIOSIS II after fertilization ovum (haploid) second polar body corpus luteum Putting allyour eggin one basket! Oogenesis

  20. sperm vs. egg production • Similarities • Both produce haploid cells by meiosis • Both take place in the gonads • both are controlled by hormones • Differences • Spermatogenesis produces 4 sperm each time while oogenesis produces only 1 egg • Formation of mature sperm continually occurs while eggs only mature once a month (on average) • Sperm formation never stops, egg formation ends at menopause • Sperm can be released at anytime while eggs are released only once a month

  21. Differences across kingdoms • Not all organisms use haploid & diploid stages in same way • which one is dominant (2n or n) differs • but still alternate between haploid & diploid • must for sexual reproduction

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