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Sexual Reproduction

Sexual Reproduction. SBI3U Ms. De Sousa. Meiosis allows : . 1. The formation of gametes 2. Genetic reduction : reduces the number of chromosomes to half (23)

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Sexual Reproduction

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  1. Sexual Reproduction SBI3U Ms. De Sousa

  2. Meiosisallows: 1. The formation of gametes 2. Geneticreduction: reduces the number of chromosomes to half (23) 3. GeneticRecombination: differentcombinations of alleles are formed. Contributes to genetic variation in society

  3. MEIOSIS • Meiosis occurs in somatic diploid cells(cells with 46 chromosomes)and produces 4 gametes, haploid cells. • Unlike mitosis, these gametes do not divide further, but instead fuse with another gamete to make a diploid zygote. (i.e. sperm fuses with the egg) • There are two main phases: Meiosis I and Meiosis II

  4. Begins as a diploidcell (2n) and becomes a haploidclel) Meiosisissubdividedinto 2 main phases: Meiosis I, Meiosis II

  5. Phases of meiosis • Meiosi I: Involves the separation of homologous chromosomes intotwo new daughtercells. • Meiosis II: Involves the seeparation of sisterchromatidsinto 4 new daughtercells. (Each of the previousdaughtercellsseparateinto 2)

  6. Stage 1: Interphase • The cellgoesthrough the G1, S and G2 stages. (similarly to Mitosis) • DNA replicates and is in the form of chromatin • Centrosomereplicates (2 centrosomes) • The cellisdiploid(46 chromosomes)

  7. Meiosis i Stage 1: Prophase I • Nuclearenvelopedisapears and chromatincondenses and becomes chromosomes • Homologous chromosomes pair (tetrads) • Synapsisoccurs, a proteinholdshomologous chromosomes together • Crossing Over, non sisterchromatids exchange genetic information. • Centrosomes move to the poles and spindlefibersform

  8. Crossing over Nonsisterchromatids Eachgene of one homologue isalignedwith the other homologue The non sisterchromatids – one maternal and one paternalchromatid of a homologous pair – are brokenat the same place thenrejoined to eachothers DNA Genetic information isexchanged

  9. Meiosis i Stage 2: Metaphase I • Homologous chromosomes (tetrads) line up on the metaphase plate of the cell • One chromosome of each pair isfacingeach pole • Spindlefibersattach to each homologue at the centromere

  10. Meiosis i Stage 3: Anaphase I • Homologous chromosomes (tetrads) separate and go to opposite poles • Cenromeresstay in tact because the sisterchromatids do not separate • Chromosome numberisnowhaploid (n)

  11. Meiosis i Stage 4: Telophase I • Homologous chromosomes uncoil (formingchromatin) • Spindlefibersdisappear • Nuclear membrane formsaroundeachnuclei • Cytokenesisoccurssimultaneously • Formation of haploiddaughtercells(eachwith 23 chromosomes)

  12. Meiosis II

  13. Meiosis ii Stage 6: Prophase II • Nuclear enveloppe disappears • Spindleapparatusforms

  14. Meiosis ii Stage 7: Metaphase II • Chromosomes are positioned on metaphase plate • Due to crossing over in meiosis I, the sisterchromatids are no longer identical • The spindlefibersare attached to the centromeres

  15. Meiosis ii Stage 8: Anaphase II • The centromere of each chromosome separates • Sisterchromatids come apart • Individual chromosomes move to opposite poles of the cell

  16. Meiosis ii Stage 9: Telophase II • Chromosomes decondenseintochromatin • Nucleiforms • Cytokenesisoccurssimultaneouslyamdseparates the cytoplasm and formstwo new haploiddaughtercells

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