Bft1033 2 meiosis_print
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Bft1033 2 meiosis_print

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Bft1033 2 meiosis_print Bft1033 2 meiosis_print Presentation Transcript

  • Meiosis
  • Reproduction Reproduction:  Asexual (Vegetative)  Many single-celled organisms reproduce by splitting, budding, parthenogenesis  Some multicellular organisms can reproduce asexually, produce clones (offspring genetically identical to parent).
  • Reproduction Reproduction:  Sexual (Generative)  Fusion of two gametes to produce a single zygote  Introduces greater genetic variation, allows genetic recombination  Zygote has gametes from two different parents.
  • Meiosis  Cell division to form the gametes:   sperm (male gamete) and egg (female gamete)  Characteristic of eukaryotes only  Normal cells are diploid: 2 copies of every gene  Gametes are haploid: 1 copy of every gene  Need to choose 1 copy of each gene randomly  Why have sexual reproduction?  Shuffling of alleles between parents and offspring leads to new combinations.
  • Overview  Start with a diploid cell (2n), with 2 copies of each chromosome, one form each parent  The two copies are called homologues  Each chromosome with 2 chromatids attached at the centromere  Meiosis consists of 2 cell divisions:  Meiosis I separate the homologues  Meiosis II separate the 2 chromatids  Meiosis I is unusual and needs a bit of study, but meiosis 2 is just like mitosis.
  • Meiotic Division Meiosis I • Prophase I • Leptotene  Meiosis I  • Zygotene  Leptotene  Zygotene  Pachitene  Diplotene  Diakinesis. • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II Prophase I  Metaphase I  Anaphase I  Telophase I  Meiosis II  Prophase II  Metaphase II  Anaphase II  Telophase II.
  • Prophase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene  Prophase I is virtually identical to prophase in mitosis, involving  the appearance of the chromosomes…  the breakdown of the nuclear membrane (envelope)  the development of the spindle apparatus. • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II
  • Prophase I Meiosis I Leptotene: • Prophase I  • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II initial phase of condensation  appears as thin threads with irregular dense granules (chromomeres)  chromomeres have a characteristic  size and number for a given chromosome.
  • Prophase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II Zygotene:  lateral pairing of homologous chromosomes (synapsis)  bivalent: synapsed homologous chromosome.
  • Prophase I Meiosis I • Prophase I • Leptotene Pachytene:  • Zygotene • Pachitene • Diplotene • Diakinesis  • Metaphase I  • Anaphase I  • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  further chromosome condensation tetrad: pairing of 4 chrommatids crossing over begins crossing over: event of genetic exchange between chromosomes chiasma: cross connection of two chromosomes caused by breakage and rejoining between chromatids
  • Prophase I Diplotene: Meiosis I • Prophase I  • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II Kiasma chromosomes begin to separate, crossing over visible.
  • Prophase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II Diakinesis:  the centromeres move away from each other  the chromosomes remain joined only at the tips of the chromatids.
  • Metaphase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  Homologous chromosomes align at the equatorial plate.
  • Anaphase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  Homologous pairs separate with sister chromatids remaining together.
  • Telophase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
  • (Interphase/Interkinesis) Meiosis I  Similar to interphase of mitosis  Without DNA replication. • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphse I • Anaphse I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II
  • Prophase II Meiosis I  the nuclear envelope is dissolved again  the spindle is set up again  Prophase II is identical to prophase of mitosis except that there is half the amount of chromosomes • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II
  • Metaphase II Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  Chromosomes line up individually on the equator plate.
  • Anaphase II Meiosis I  At anaphase the centromeres divide, splitting the 2 chromatids  The one-chromatid chromosomes are pulled to opposite poles. • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphse I • Anaphse I • Telophase I Meiosis II • Prophase II • Metaphse II • Anaphse II • Telophase II
  • Telophase II Meiosis I  • Prophase I  • Leptotene • Zygotene • Pachitene  • Diplotene  • Diakinesis • Metaphse I • Anaphse I • Telophase I  Meiosis II • Prophase II • Metaphse II • Anaphse II • Telophase II  Cell division is complete Four haploid daughter cells are obtained spindle fibers disappear nuclear membrane forms around chromosomes at each end of cell each nucleus has half the # of chromosomes as the original (haploid) now there are 4 sex cells (daughter cells).
  • Summary of Meiosis  2 cell divisions  Start with 2 copies of each chromosome (homologues), each with 2 chromatids  In meiosis I, crossing over in prophase mixes alleles between the homologues  In metaphase of meiosis I, homologues pair up, and in anaphase the homologues are separated into 2 cells  Meiosis II is just like mitosis  The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.
  • Summary of Meiosis
  • Mitosis vs meiosis Meiosis KM 23
  • Mitosis vs meiosis Meiosis KM 24
  • Meiosis creates genetic variation  During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)  Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over  No daughter cells formed during meiosis are genetically identical to either mother or father  During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.
  • Independent assortment Meiosis KM 26
  • Independent assortment Number of combinations: 2n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations
  • In humans 23 chromosomes in haploid 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations!
  • Crossing over Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes.
  • Meiosis and sexual life cycles  Life cycle = sequence of stages in organisms reproductive history; conception to reproduction  Somatic cells = any cell other than gametes, most of the cells in the body  Gametes produced by meiosis. Generalized animal life cycle