Lecture 5 – Meiosis I. A few important points about meiosis A. cell division that reduces chromosome number by 1/2 2n four 1n cells B. crossing over used to increase genetic diversity of gametes produced II. Overview of meiosis A. homologues pair B. 1st division: 2n cell divides to give two 1n cells C. 2nd division: each 1n cell divides to give total of four 1n cells meiosis I Two stages: meiosis I and meiosis II 2n 1n 1n meiosis II 1n III. Meiosis in detail A. meiosis I 1. prophase I a. leptotene b. zygotene synapsis = c. pachytene bivalent = tetrad = 1 1n 1n 1n d. diplotene chiasmata = crossing over = e. diakinesis 2. metaphase I 3. anaphase I 4. telophase I IV. Meiosis II A. between meiosis I and II 1. no DNA replication occurs 2. nuclear envelope may reform transiently 3. chromosomes may decondense briefly 4. meiosis II resembles mitosis, but: 2 B. steps of meiosis II 1. prophase II 2. metaphase II 3. anaphase II 4. telophase II V. Differences in gametogenesis A. spermatogenesis vs. B. oogenesis 2n 2n 1n 1n 1n 1n 1n 1n 1n 1n 1n 3 1n 1n 1n VI. Mitosis versus meiosis A. side-by-side comparison B. final thoughts on mitosis vs. meiosis reduces ploidy mitosis meiosis # cells produced homologues pair VII. Principle of segregation – Hereditary determinants separate in such a way that each gamete is equally likely to inherit either member of the pair. A A a a A A A 4 a a A a a VIII. Independent assortment – unlinked genes assort independently from one another. A. Products depend on metaphase I alignment of chromosomes. W Ww W Ww w w Wg W WG W g G G Gg g g w wg gG wG G w G g B. How metaphase I alignment determines gamete genotype. w w w W W g G g g G G w W W G w G G WG WG g g wg w g w w W g G W W g G W w W W W g wg Wg 5 G g W g w G w w g Wg G wG G wG
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