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Transcript
Meiosis Gamete Formation Prophase I What’s happening in Prophase I? • Nuclear envelope begins to dissolve • Chromatin condenses into chromosomes – Homologous chromosomes pair (“synapsis”) – Already duplicated in S phase of Interphase “tetrads” • Centrosomes begin migrating to opposite poles – Centrosome = amorphous material around centrioles • Spindle fibers begin to form from centrosomes – Spindle fibers made of microtubules & proteins • Create “mitotic spindle” – “Kinetochore fibers” attach to centromere – “Polar fibers” link centrosomes but don’t attach to chromosomes Metaphase I What’s happening in Metaphase I? • Tetrads (4 sister chromatids/ 2 homologous chromosomes) are pulled to middle of cell by kinetochore fibers • Chromosomes’ kinetochores are ½ way between the 2 poles & kept in place there • Homologues align so same genes are adjacent • Crossing over (sister chromatids) occurs • Karyotype from photomicrographs made here Anaphase I What’s happening in Anaphase I? • Homologues (2 sister chromatids) are pulled to opposite poles by kinetochores • “Independent Assortment” = random separation of homologous chromosomes • Independent assortment genetic variation • Earlier crossing over genetic recombination • Genetic diversity improves species’ survival Telophase I What’s happening in Telophase I? • Chromosomes reach opposite poles of cell – 1 chromosome (½ of the homologue pair) to each – Each chromosome still is paired sister chromatids • Cytokinesis begins – Splits cytoplasm between the cells – Half of organelles will go to each cell • Nuclear envelope reassembles in some cells but not in others – This is species-dependent From Metaphase I to Cytokinesis Meiosis II: Telophase II Meiosis II in Lily Anthers Summary of Meiosis • Chromosomes replicated in Interphase • Meiosis I separates homologues • Meiosis II separates sister chromatids of each homologue • Four haploid cells form – Each has only 1 copy of the chromosomes (1 n) – Human ovum/ sperm has 23 chromosomes each – Haploids join in fertilization 23 pairs in diploid